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  1. #1

    Default Chasing a misfire

    My apologies for the novel, just trying to make sure I explain the issue in detailÖ

    Notes: I'm helping some friends here. They are not the most mechanically minded and here in SW FL we have A LOT of shallow waterways. Considering both of these items, I suggested they get an outboard deck boat but that if they got a I/O that I personally only have Merc's. I don't have much experience with VP engines or drives, I don't have the diagnostic adapters to be sure I'm fixing the right issue and therefore if they need my help, they should stick with my recommendations but in any case, get a survey. What do they do... Come home with an 05 Chaparral 204ssi with a VP 5.0gxi-f and did not get a survey.

    So here's the issue and the info I got from them on what happened leading up to the issue was apparently not the full details.
    - Pre 1st outing, boat is 2 quarts low on oil and the battery is garbage. They put the oil and a new battery in it however they hooked the battery up backwards and got some smoke from the alternator. Corrected it and they say the boat started right up. Hold that info...


    - First time out, they bottom it out, stuck in the sand, suck up a bunch of sand and the VP safety system puts the motor into limp mode when it reaches 200 degrees. They limp it back in, flush it, and it fires back up.


    - 2nd time out, they're hearing an alarm but aren't sure if it's the depth or engine alarm yet they didn't check the gauges or least remember checking the gauges but the boat dies and will not start back up. They paddle it back in and then ask for my help... Grrrrrr
    At that time they were keeping it on a lift so i went over to see if i could get it to run enough to get it to trailer. I did my own flush on the raw water system, checked the impeller, took the thermostat out just in case and got it to fire up but that required full throttle and it was misfiring pretty bad. However i idled it to the ramp and got it out of the water.


    - At home, I pulled the plugs and they were soaked with carbon and gas as well as were the original ac delco's with the red overspray still on them! After looking at some things, like noticing the Fram oil filter, not knowing what oil they put in it, and the drive oil was stinky black as well as low. So i did basic maintenance (oil, drive oil, impeller, thermostat, plugs, distributor cap, rotor, fuel filter) just for peace of mind before I start chasing this misfire. (Plug wires weren't in stock so they weren't done)
    After startup, it's still misfiring and soaking the new plugs. It's was pulling air like a MUG through the flame arrestor, tachometer was jumping all around from 0 to 5600 to 3500, like all over the place but the motor wasn't. So i slapped a quick IAC in it, checked the fuel pressure, 10psi at low side, 50psi at high side, and had a clean fuel pressure regulator too. Took a fuel sample, found 1 or 2 little pieces of VP's famous paint but nothing holding it back, after all, i have too much fuel or at least too much fuel for the spark I'm getting anyway.


    At this point, i found a local shop to run a scan on it, it showed low voltage to the TPS and a misfire on cylinders, 3,6 & 8. They recommended i do the plug wires and a new TPS.
    (wait 3 days for the parts)
    Get the parts, put them on, battery dead. !! Jumped it off and it's still idling rough, still required full throttle to crank it, it's smoking black and tach is still all over the place. At this point i notice the trim gauge isn't working. Temp, oil pressure and fuel gauge seem to be fine but the battery gauge shows way low.
    My battery charger also shows bad battery so I took it to be checked and the store says its just low and needs a full charge.
    So you know you get so wrapped up in a problem that the simple S**t just blows right by you?? So why is the battery low? It's brand new, store says its good, trickle charged to 100% just fine even though it's been fried once... But that thought rings my little bell. "The smoking alternator"!!
    Because this is a VP with a bunch of sensors and wires that my carbed Merc's don't have, i don't know where to start.
    1. Could the battery hooked up backwards fried the alternator?
    2. If so, could that be a culprit to the misfire?
    3. Any suggestions on what to check for and what to rule out would be greatly appreciated. Like possible wire shorts, fuses, ECM possible issues etc... Again, i have all BBC Merc's with carbs so these massive wiring harnesses aren't my cup of tea, although i can work on them if i know what to look for...
    Thx in advance!

  2. #2
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    Default Re: Chasing a misfire

    1. yes, very possible - a volt meter will tell you if the alternator is producing sufficient voltage.
    2. yes, very possible as well...low voltage to the ECU will cause all kinds of erratic behaviors from the engine.
    3 seriously, get a Rinda scanner or their software with the adapter/cables. Trying to troubleshoot a rich injected engine without one is like fishing with a hand line...

    I'd start by make sure the alternator is working and you get a nominal 14 VDC at the battery...then hook up the scanner and see where the sensor inputs are (nominal range) and go from there.

    Getting a copy of the MEFI-4/4B manual (do a www search) will also come in real handy

  3. #3

    Default Re: Chasing a misfire

    Quote Originally Posted by makomark View Post
    1. yes, very possible - a volt meter will tell you if the alternator is producing sufficient voltage.
    2. yes, very possible as well...low voltage to the ECU will cause all kinds of erratic behaviors from the engine.
    3 seriously, get a Rinda scanner or their software with the adapter/cables. Trying to troubleshoot a rich injected engine without one is like fishing with a hand line...

    I'd start by make sure the alternator is working and you get a nominal 14 VDC at the battery...then hook up the scanner and see where the sensor inputs are (nominal range) and go from there.

    Getting a copy of the MEFI-4/4B manual (do a www search) will also come in real handy
    thx Mako. So here’s what I did yesterday.
    1. fully trickle charged the battery. Reading was 12.81 volts before installing.
    2. installed the battery, fired up the engine, that brought the battery down to 12.4 volts.
    3. At about mid throttle, checked voltage at the battery, still 12.4 volts. Should of gone up around 14 volts correct?
    4. noticed the ground wire on top of the alternator was melted, and the coils in the alternator were black. Tried to take the ground lead off the alternator but it apparently melted the screw in so it just snapped off. Cleaned up and sanded the paint off another ground point on the alternator, no help..


    questions:
    1. Is there a way to check the alternator itself without checking it at the battery to know if its the alternator itself shot or the leads going to / from it just melted?
    2. I know this is a rookie question but it’s hard to see where all the wires go to as they are in a bundle and go behind the motor into a tight space. But where does the ground wire from the alternator hook to on the other end?


    As far as the scanner goes, I am fully aware Of the hand fishing line bonanza lol. But because it’s a friends boat, and I’ll never use the Volvo adapter ever again, im trying not to spend the money on software for this motor. I did have a guy come out and he scanned it with the Diacom software. Like I said in my post, misfire on 3,6,8. Low voltage to TPS. I will say that on a fully charged battery, the engine fired right up and idled on its own, although slightly rough , but not as bad as it was, and without having to give it full throttle to start. Now the 3rd time I started it, it required full throttle.


    thank u for the manual tip, I actually have 3 separate service manuals I dug up for it but im not sure if the one you said is one of them. I’ll look shortly. I also really appreciate you reading my novel And replying in detail like that. It really helps me. I see you’re a Merc guy too, I wish I could set this thing on fire and have a marshmallow roast over it lol.. I have twin Gen IV 454 bluewaters in my 88 regal 360 commodore and I can almost walk around those motors, easy to get parts, easy to diagnose and knock on wood, they’ve never let me down! I’ve just kept them maintained and I put new quadrajets on them last year. They fire right up and growl through the warm up then simmer down to a very nice 750 rpm idle. Love those old beasts!


  4. #4
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    Default Re: Chasing a misfire

    if the voltage is that low running, there is no charge leaving the alternator...could be alternator or harness or both. the ground cable usually runs from the alternator frame to the block.

    if the alternator isn't working, the voltage the ECU sees will continue to drop as the engine runs...at some point, you will get misfires and other anomolies...so the misfire codes could have been due to the low voltage...the alternator function needs to be restored before further troubleshooting.

    I spent almost a decade working for a Merc Master Mechanic - thru college and then part time after getting a desk job...you can do it without the scan tool but it will be painful...that engine is a standard production GM, so not much to worry about due to the VP badging...

  5. #5

    Default Re: Chasing a misfire

    Quote Originally Posted by makomark View Post
    if the voltage is that low running, there is no charge leaving the alternator...could be alternator or harness or both. the ground cable usually runs from the alternator frame to the block.

    if the alternator isn't working, the voltage the ECU sees will continue to drop as the engine runs...at some point, you will get misfires and other anomolies...so the misfire codes could have been due to the low voltage...the alternator function needs to be restored before further troubleshooting.

    I spent almost a decade working for a Merc Master Mechanic - thru college and then part time after getting a desk job...you can do it without the scan tool but it will be painful...that engine is a standard production GM, so not much to worry about due to the VP badging...
    Quick question: Before I go digging through the harness, and because a new alternator will take a few days to get hereÖ ďCANĒ, I run temporary leads straight off the alternator +/- to the battery to check if the alternator is producing power, just not getting it to the battery? Or how could I quickly test to see if the alternator itself is fried, or if itís somewhere in the wiring?

    1 thing that just came to mind was that they said after correcting the backwards hookup of the battery, the boat started up, but after 10-15 minutes of travel, it died and would not start back up. (They only have 1 battery on it). So Iíd like to see if just replacing the alternator itself will correct it or if I need to start wiring huntingÖ

  6. #6
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    Default Re: Chasing a misfire

    Black wires internal to alternator never a good sign...
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  7. #7
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    Default Re: Chasing a misfire

    Quote Originally Posted by sandkicker View Post
    Black wires internal to alternator never a good sign...
    2X!!

    you can pull the alternator and take it to a shop to be tested....that will give you a place to start...

  8. #8
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    Default Re: Chasing a misfire

    The alternator has already been tested right there on the boat and it's toast. It's not making voltage which is why you are getting all these haywire performance glitches. EFI hates it when it doesn't get the proper voltage.
    I'm nearly certain that the new alternator will do the trick. And a good battery. I see people using the cheapest car batteries on their boats. Not so good.
    Seems that this owner is determined to keep you busy fixing their foul-ups. Better start charging for your services!
    Practicing the ancient art of ren-ching

  9. #9

    Cool Re: Chasing a misfire

    Quote Originally Posted by o2batsea View Post
    The alternator has already been tested right there on the boat and it's toast. It's not making voltage which is why you are getting all these haywire performance glitches. EFI hates it when it doesn't get the proper voltage.
    I'm nearly certain that the new alternator will do the trick. And a good battery. I see people using the cheapest car batteries on their boats. Not so good.
    Seems that this owner is determined to keep you busy fixing their foul-ups. Better start charging for your services!
    lol! I know right! So I had them order an alternator. I disconnected the melted ground, jumped a new ground to it, fully charged brand new battery, started it up, ran like crap, checked the output on the alternator with an amp clamp and had 2.5 amps! Lol

    Just a little funny point here, but when I first saw the boat, Iím standing there looking at the engine compartment as she was telling how meticulous the guy was with maintenance as her husband is adding the 2 quarts of oil to it to bring it to level. When sheís done talking I pointed at the Walmart Fram oil filter and holding the dipstick for the stinky black drive oil that was at only half level as I said ďI beg to differĒ! Hahaha

  10. #10
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    Default Re: Chasing a misfire

    i've fired clients before.... :-)
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  11. #11
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    Default Re: Chasing a misfire

    In the land of the blind, the one-eyed man is king
    Practicing the ancient art of ren-ching

  12. #12

    Default Re: Chasing a misfire

    Im backkkkk! Lol So I have some updates and more questions.

    I Finally received and changed the alternator. Getting full charge now so that’s great. It fired right up but was still kind of acting a little weird. (Tachometer jumping all around, not accelerating smoothly etc..)
    2. Changed cap, rotor, wires and plugs. (I did put some cheap copper plugs in for testing as it was fouling the expensive iridium plugs. Gapped them to .60)
    3. Changed TPS and IAC as it was still pulling loads of air at idle then checked the new plugs as they were already wet with gas. (Keep in mind, I’ve checked the fuel pressure. 10psi on the low side, 50psi on the high side, no garbage in regulator screen, no gas in vacuum line coming off the regulator)
    4. Here’s where it gets funky on me. (Also keep in mind, this was at first just supposed to be only a tune up). So, the oil was new, only a few less than 10-minute runs on it. I use Rotella T4 in everything I own. The boat holds 5qt, 6 with new filter. I checked the oil last night and it was Wayyyyy up the dipstick. So I pumped it out and got 4 gallons of black, thin, gas smelling oil out! 4 gallons! Because I had changed this prior to the part changes, I let it ride and changed it again. Cleaned up the plugs, checked the gasket ring seating on the IAC.
    5. Fired it up. Boom! It fired right up, no throttle and idled a smooth 900rpm cold with good acceleration. However, this didn't last long, it started to get rough, puffing smoke and dropping black soot so i shut it down.
    6. I then decided to swap the cap and rotor back to the old just in case the Sierra cap and/or rotor might be defective. Checked the oil for gas but it too early, and it would not fire up. probably would have if i gave it full throttle but I'm past getting it to start at whatever cost. So, I tried a few different combos (new cap w/ old rotor, old cap w/ new rotor, all new, all old etc) no change, won't fire up. I left it there for the night.

    Questions:

    1. Will this engine have any issues on the copper plugs? I wouldn't think this much of an issue but who am I? I use copper in every one of my old mercs.
    2. Shouldn't the rotor start/stop in the same position every time? During those swaps, i noticed 2 or 3 significantly different positions it was facing.
    3. Despite the fuel pressure checking out, can they still be the issue? If so, how would i know which one it is? How to check them?
    4. Despite no fuel in the regulator vacuum line, can it still be bad?
    5. How do correctly check the spark at each cylinder? I know the old touch the plug to the block but is there a more accurate check?
    6. The day before the battery was hooked up backwards, they had ran the drive in the ground twice in the shallow SW FL water. I changed the impeller, thermostat, cleaned the in and out lines but stopped at that. Could there possibly be a blockage somewhere else causing issues?


    And yes Mako, i am still fishing with a hand line! LOL

  13. #13
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    Default Re: Chasing a misfire

    Quote Originally Posted by RawkSteady View Post
    ...
    Questions:

    1. Will this engine have any issues on the copper plugs? I wouldn't think this much of an issue but who am I? I use copper in every one of my old mercs.
    2. Shouldn't the rotor start/stop in the same position every time? During those swaps, i noticed 2 or 3 significantly different positions it was facing.
    3. Despite the fuel pressure checking out, can they still be the issue? If so, how would i know which one it is? How to check them?
    4. Despite no fuel in the regulator vacuum line, can it still be bad?
    5. How do correctly check the spark at each cylinder? I know the old touch the plug to the block but is there a more accurate check?
    6. The day before the battery was hooked up backwards, they had ran the drive in the ground twice in the shallow SW FL water. I changed the impeller, thermostat, cleaned the in and out lines but stopped at that. Could there possibly be a blockage somewhere else causing issues?
    1. i doubt it, especially during your troubleshooting...

    2. no

    3. yes, based on the 3X excess in the oil pan, I'd be thinking one or more injectors are stuck open and diluting the oil among other non-desirables.

    4. yes, but your fuel pressure gauge will tell you if the regulator is functional.

    5. I normally use an inductive (clip on) timing light.

    6. I doubt anything in the drive is causing the engine performance issues you are describing...

  14. #14

    Default Re: Chasing a misfire

    Quote Originally Posted by makomark View Post
    1. i doubt it, especially during your troubleshooting...

    2. no

    3. yes, based on the 3X excess in the oil pan, I'd be thinking one or more injectors are stuck open and diluting the oil among other non-desirables.

    4. yes, but your fuel pressure gauge will tell you if the regulator is functional.

    5. I normally use an inductive (clip on) timing light.

    6. I doubt anything in the drive is causing the engine performance issues you are describing...
    ok so going off #3 above. If im Getting a misfire on 3,6,8 per the scan. Im wondering if those injectors are stuck? I had gas dripping off those plugs too after the 5th cleaning of the plugs. How do I check injectors? I feel like this is the one. But im also wondering if due to the battery hookup frying the alternator issue, is there a sensor or something controlling the injectors that could of also gotten fried by the battery issue?

    also, isnít there a method with the throttle and key to clear codes on a VP without a scanner or disconnecting the battery for the night?

    thx!

  15. #15

    Default Re: Chasing a misfire

    Quote Originally Posted by RawkSteady View Post
    ok so going off #3 above. If im Getting a misfire on 3,6,8 per the scan. Im wondering if those injectors are stuck? I had gas dripping off those plugs too after the 5th cleaning of the plugs. How do I check injectors? I feel like this is the one. But im also wondering if due to the battery hookup frying the alternator issue, is there a sensor or something controlling the injectors that could of also gotten fried by the battery issue?

    also, isn’t there a method with the throttle and key to clear codes on a VP without a scanner or disconnecting the battery for the night?

    thx!
    Re comment on the injectors - send them out for cleaning and flow testing. If they check out ok you know its a control issue if they comeback bad or clogged with paint you mentioned gives other issues to chase down.

    Previous comment about getting rhinda is probably spot on. Unless injectors are obviously screwed you'll be chasing your tail. The boat probably got no maintenance - it was my understanding the VP recommends oil gas mix before winterizing not to fog the engine but to lube the fuel system and injectors. Might be the culprit.

    Your a good friend for helping them out but these can be slippery slopes when repair costs get high and you're working for beer ....

  16. #16
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    Default Re: Chasing a misfire

    2X on sending the injectors out...if you have an experienced (reputable) shop close to you, it may be a wise investment to use them.

    "...3,6,8 per the scan...." - did you find a scan tool / software package??

    if you had the scan tool hooked up and the engine running, you should have checked the entire suite of input sensors...

    to flush the codes, if the ecu is the mefi-4 type, usually less than 5 minutes with the battery disconnected is enough to flush the memory. if its the VP specific ecu, couldn't tell you as I've never tried one....

  17. #17

    Default Re: Chasing a misfire

    Quote Originally Posted by makomark View Post
    2X on sending the injectors out...if you have an experienced (reputable) shop close to you, it may be a wise investment to use them.

    "...3,6,8 per the scan...." - did you find a scan tool / software package??

    if you had the scan tool hooked up and the engine running, you should have checked the entire suite of input sensors...

    to flush the codes, if the ecu is the mefi-4 type, usually less than 5 minutes with the battery disconnected is enough to flush the memory. if its the VP specific ecu, couldn't tell you as I've never tried one....
    I didnít get ahold of the tool / software to scan it again but in my original post, I put that I had a full scan done and it came back as a misfire on 3,6, & 8 and low voltage to the TPS. The TPS problem was probably due to the alternator at that time but I replaced it anyway. That part is resolved. Im referencing the misfire on 3,6,8 now because those are my wet plugs when I pull them out and I assume 3 stuck injectors is more than enough to cause the 2.5 gallons of fuel in the oil?

    this may be kind of ghetto but I have a master marine mechanic thatís a childhood best friend, however heís 10 hours away. He told me I ďcouldĒ check them this way since itís apparent by the amount of fuel in the oil, that if itís an injector(s), itíll be easy to see..Tell me if you think his suggestion would work.

    he said to pull the fuel rail, leave the injectors in and find a block of wood to drill holes in and place the fuel rail on with the injectors in the holes to hold them in and turn it over. Look for any that just soak the block instead of mist. I assume 3,6, and 8 are stuck. If so, he suggested just replacing them.

    I know itíll make a mess but I can handle that. Should that at least give me an idea if Iím on to it? There are no injector shops close to me so if I can just replace em, then I willÖ

    thoughts?

  18. #18

    Default Re: Chasing a misfire

    Quote Originally Posted by RawkSteady View Post
    I didnít get ahold of the tool / software to scan it again but in my original post, I put that I had a full scan done and it came back as a misfire on 3,6, & 8 and low voltage to the TPS. The TPS problem was probably due to the alternator at that time but I replaced it anyway. That part is resolved. Im referencing the misfire on 3,6,8 now because those are my wet plugs when I pull them out and I assume 3 stuck injectors is more than enough to cause the 2.5 gallons of fuel in the oil?

    this may be kind of ghetto but I have a master marine mechanic thatís a childhood best friend, however heís 10 hours away. He told me I ďcouldĒ check them this way since itís apparent by the amount of fuel in the oil, that if itís an injector(s), itíll be easy to see..Tell me if you think his suggestion would work.

    he said to pull the fuel rail, leave the injectors in and find a block of wood to drill holes in and place the fuel rail on with the injectors in the holes to hold them in and turn it over. Look for any that just soak the block instead of mist. I assume 3,6, and 8 are stuck. If so, he suggested just replacing them.

    I know itíll make a mess but I can handle that. Should that at least give me an idea if Iím on to it? There are no injector shops close to me so if I can just replace em, then I willÖ

    thoughts?
    yes you can do that but it doesn't really tell the condition of the others. Its your boat, but if it were me since you are taking them out have them all serviced at a place like this https://injectorrepair.com/ No sense doing all the work to pull them and not go back in with known good ones, especially when its December...

    I'm sure if you peruse google a bit you can find other shops and maybe some reviews on motorhead forums.

  19. #19

    Default Re: Chasing a misfire

    Quote Originally Posted by Dieter View Post
    yes you can do that but it doesn't really tell the condition of the others. Its your boat, but if it were me since you are taking them out have them all serviced at a place like this https://injectorrepair.com/ No sense doing all the work to pull them and not go back in with known good ones, especially when its December...

    I'm sure if you peruse google a bit you can find other shops and maybe some reviews on motorhead forums.
    and I would totally agree with you in the event it actually was my boat or a boat that was being kept. It’s a friend of mines boat and we are just trying to get it to a satisfactory level to sell it.


    i know my original post is way back and a novel to read but to summarize, these 2 friends know nothing about boating and just moved here to SW FL from Minneapolis. The waters here are shallow and so shallow combined with no experience or mechanical knowledge, before they bought a boat I suggested a couple things as best for “them”. 1. Get an outboard or at least a merc (I’m not a VP fan so if I’m gonna help I’d rather it be a motor I have knowledge of), 2. take a boating class, and 3. GET A SURVEY! So 2 days later, the wife didn’t consult anyone, not even the husband lol, and pulled in the driveway pulling (wait for it…. 1. An inboard VP, 2. Never got a survey (said the guy said he maintained it to a tee despite it being 2 quarts low on oil, had a fram oil filter on it, the drive oil was 1/2 full of burnt black drive oil, the trim gauge didn’t work and the trailer was falling apart ��������*♂️.


    2 days later they bottom it out twice, (no boating class, no sea strainer), hook up the battery backwards and claim it just died while some annoying alarm was going off. Hmmmm… So after all this bs to figure out all the issues so the boat “could” pass a survey, they’ve agreed, they’re selling it and will shop with me for the next.., ����

  20. #20

    Default Re: Chasing a misfire

    Do what ever you want, but Regardless of if you do three or eight injectors i think you will find it is cheaper to service the injectors vs replace.

  21. #21

    Default Re: Chasing a misfire

    Quote Originally Posted by Dieter View Post
    Do what ever you want, but Regardless of if you do three or eight injectors i think you will find it is cheaper to service the injectors vs replace.
    so I had a misfire on 3,6 and 8. Coincidentally, or not, #3,6 & 8, injectors are stuck wide open. Didnít even have to turn the engine over, Turned the key to on, and as soon as the fuel pump kicked on, 2 bursts of gas came from each of those injectors. They look original anyway. Ordered a new set of 8. Be here Wednesday..

  22. #22

    Default Re: Chasing a misfire

    It may run well enough that your friends keep it as long as they can find the deeper water..

  23. #23

    Default Re: Chasing a misfire

    So… I’m back with an update. It’s been a long road but I’m almost there. Following my last post I had done my redneck fuel injector test and 2 injectors were stuck open
    So I was waiting on the new injectors to get here. Since they arrived, here’s what I did and where I am.


    1. swapped the injectors
    2. Changed the oil since I knew it was flooded with gas.
    3. Reset all codes
    4. Fired it up. (No throttle required)
    5. It ran good for a minute then it started pulling a lot of air and smoking again.
    6. Found out that the “NEW” IAC was bad from the box.
    7. Waited 2 weeks for the replacement
    8. Put it in, fired right up, very minimal smoke until it cleared out then it idled beautifully (for about 10 minutes)


    Around the 10 minute mark it VERY slowly started to idle up a bit. Around 200-300 rpm at most. I then looked at the tach and it showed it at like 3500! Def not right! Maybe 900-1000 at most. But the tach would go from the 3500 to 2000 then to 1000 then 3000 etc., not exactly but you get the point. It was doing these drastic moves but not in sync with the engine idle climbing and declining 200-300 at a time. The idle up and down was almost too slow to notice if you aren’t paying attention.


    So here’s what I need to know, and I’m not sure how accurate this is or if it’s even a thing with my model engine. VP 5.0GXI-F.


    In my first post I talked about the initial problem stemming from the owner hooking the battery up backwards and frying the alternator. I replaced the alternator but now I’m hearing that any time someone hooks a battery up backwards that the reverse polarity will take out 1 of not both of the diodes? Then it will run but the diode will allow power back up the line and mess up other stuff like the alternator and fuel pump. Is this a real thing? And if so, I can’t seem to find these diodes. I assume they’re wrapped up in the mess of electrical tape VP wraps the harness in? Any help on how to find them, where to get them etc would be appreciated (if it’s a concern). I only have about 20-30 min total idle time since I swapped all this stuff so I’m hoping if it is, it hasn’t had enough time to ruin anything new…
    HELP! Lol

  24. #24

    Default Re: Chasing a misfire

    Quote Originally Posted by RawkSteady View Post
    So… I’m back with an update. It’s been a long road but I’m almost there. Following my last post I had done my redneck fuel injector test and 2 injectors were stuck open
    So I was waiting on the new injectors to get here. Since they arrived, here’s what I did and where I am.


    1. swapped the injectors
    2. Changed the oil since I knew it was flooded with gas.
    3. Reset all codes
    4. Fired it up. (No throttle required)
    5. It ran good for a minute then it started pulling a lot of air and smoking again.
    6. Found out that the “NEW” IAC was bad from the box.
    7. Waited 2 weeks for the replacement
    8. Put it in, fired right up, very minimal smoke until it cleared out then it idled beautifully (for about 10 minutes)


    Around the 10 minute mark it VERY slowly started to idle up a bit. Around 200-300 rpm at most. I then looked at the tach and it showed it at like 3500! Def not right! Maybe 900-1000 at most. But the tach would go from the 3500 to 2000 then to 1000 then 3000 etc., not exactly but you get the point. It was doing these drastic moves but not in sync with the engine idle climbing and declining 200-300 at a time. The idle up and down was almost too slow to notice if you aren’t paying attention.


    So here’s what I need to know, and I’m not sure how accurate this is or if it’s even a thing with my model engine. VP 5.0GXI-F.


    In my first post I talked about the initial problem stemming from the owner hooking the battery up backwards and frying the alternator. I replaced the alternator but now I’m hearing that any time someone hooks a battery up backwards that the reverse polarity will take out 1 of not both of the diodes? Then it will run but the diode will allow power back up the line and mess up other stuff like the alternator and fuel pump. Is this a real thing? And if so, I can’t seem to find these diodes. I assume they’re wrapped up in the mess of electrical tape VP wraps the harness in? Any help on how to find them, where to get them etc would be appreciated (if it’s a concern). I only have about 20-30 min total idle time since I swapped all this stuff so I’m hoping if it is, it hasn’t had enough time to ruin anything new…
    HELP! Lol

    I cant comment on the location of the diodes but I would try running these two ways :

    Tach disconnected - usually grey wire on neg coil terminal

    disconnect the alternator, if diodes are bad it wont back feed power.

    See if it runs better one way or other .

  25. #25

    Default Re: Chasing a misfire

    here's a couple vids but symptoms aren't the same as yours

    https://youtu.be/VzEPHa4J6p0

    https://youtu.be/KuiKw7eu7es

  26. #26

    Default Re: Chasing a misfire

    Quote Originally Posted by Dieter View Post
    here's a couple vids but symptoms aren't the same as yours

    https://youtu.be/VzEPHa4J6p0

    https://youtu.be/KuiKw7eu7es

    Awesome thx! The only issue i have with disconnecting the grey wire at the coil, is there is no grey wire at the coil! LOL

    So when you say disconnect the alternator. I assume you just mean the small plug on it and after i start the engine? Not the main power or ground lug right?

  27. #27

    Default Re: Chasing a misfire

    And i've actually watched both of those videos a couple times. Problem is that the one has a different harness than mine and the diagram one doesn't represent the same configuration either... I was so hoping that by the end of either of those that i would of gained some kind of info. BUT... I didn't so i posted here lol

  28. #28
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    Default Re: Chasing a misfire

    the 'diodes' that usually get wrecked with a reverse hookup on the battery are the ones inside the alternator...new alternator should eliminate that one.

    on the tach, it could just be as simple as the selector switch being corroded....note the position and rotate it several times and then return it to the original setting....some like to apply WD-40 before the procedure...

    the tach signal (gray wire) comes from the MEFI-4 ECU....

  29. #29

    Default Re: Chasing a misfire

    Quote Originally Posted by makomark View Post
    the 'diodes' that usually get wrecked with a reverse hookup on the battery are the ones inside the alternator...new alternator should eliminate that one.

    on the tach, it could just be as simple as the selector switch being corroded....note the position and rotate it several times and then return it to the original setting....some like to apply WD-40 before the procedure...

    the tach signal (gray wire) comes from the MEFI-4 ECU....

    so I just did the 2 things that Dieter said. I donít think unplugging the alternator did anything and since I couldnít find a gray wire on the coil, I just unplugged the tach. It still fired right up but after it was nearly at full temp so 20 minutes in to idling or so, it did start running rough. I plugged the tach back in and it all the sudden was reading correctly, about 600rpm. But as it idled more, it started to almost die and then the IAC would catch it. Then it would do it again in about a minute. I wanted to check if I could rev it and come back to neutral without it dying and it didnít die although it was smoking a bit as I gave it gas. I killed it, started it right back up and the tach was still good. Considering the surging is now a whole new thing, I closed it up and left lol.

  30. #30

    Default Re: Chasing a misfire

    On the alternator was thinking take off all the wires, if concern was about back feeding voltage make sure the 12 v hot coming off is removed. That's just my SWAG removing the plug may have 'turned it off'.

    It seems like it runs ok until engine is physically warmed up correct? Then runs rich? If that is the case thought would be ECU still thinks the engine is cold? Maybe engine temp sensor (not gauge sender)? You've checked IAC operation and filter is clean? You checked fuel pressure?

    Full disclosure I've never had an MPI boat so swagging here. You likely need to find a place to hook it up for diagnostics. I bought the soft ware for my Seadoo for like $400... all to reset the throttle position sensor. Fuel mapping is screwed with out it.

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