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Mercruiser 350 MPI misfiring under load

S

scooter111

Member
"Hi, I am new to this forum an

"Hi, I am new to this forum and am hoping someone can help me out. Being in New Zealand I think our mechanics and Merc rep are lacking some knowledge compared to the USA.

I have a 24ft trailer boat which had a 1994 350 mercruiser which had water damage. I replaced it with a 2006 350 MPI engine with about 170 hours. we have it all installed and running. It runs great at idle and if you rev up slowly under load until we hit about 3000 rpm then it starts to misfire badly. if we give it heaps from stand still it will do it at lower revs. It can get up higher in the revs, but it runs badly. Seems to rev up ok in neutral.

We have checked it with a laptop and it is showing up ok on the computer.
We have tried replacing the coil and distributor cap and plugs and this hasn't done anything.
We checked it with a fuel pressure guage under load and that was showing up ok.

next step was a compression test all ok. have changes crank sensor. they tried changing the computor and also hooked it up to another key start wiring harness.

They are going to try replacing the distributor next and also the fuel filter.

It did have high olil level which has been reduced. Someone has mentioned that high oil could cause these symtoms

I hope someone can help as the costs are getting out of control and we are running out of ideas.

thanks

Chris"
 
"Chris

The only thing that

"Chris

The only thing that I can think of is that maybe the new engine has a different volumetric efficiency than the old one. Perhaps a different cam profile, different cylinder heads or something causing a degree of incompatibility with the fuel calibration in the ECM. Does this engine have O2 sensors? Was the old motor also MPI? Vortec? I would want to know if its lean misfiring under high speed/load conditions. You said it checked out ok on the computer but not sure what that really means."
 
"The new engine had all the pa

"The new engine had all the parts on it including fuel pump, computer, wiring etc. the old motor was a carb model.

The mechanic has had a laptop plugged in for checking on the computer for faults and no faults have shown up.

Have just spoken to the mechanic again and they said it felt like there was some play in the distributor. with any luck it will be that, when they change it.

I am very lucky as the Mercury rep is supplying parts free of charge for trial and error to find the source, but it it chewing up a lot of labour."
 
"I'd have them watch the t

"I'd have them watch the timing while its missing, to see if the timing is jumping, if it is then think about dist, if not then I would put a spark tester inline with each plug and see which one is missing, could be bad plugs, bad gap, bad plug wires, water getting in at higher rpms creating the miss."
 
"To add to what brian said...

"To add to what brian said... frequenly you can see a missing spark in a plug wire while running by useing a timing light. Watch the flash and you can see it studder if you are missing a spark.

Sounds to me like a lean missfire but not sure. If it has an O2 sensor, you can read the output to see if it is pegged lean or rich. I would be less concerned about faults set on the ECU. Hint: If it was a lean miss, opening a major vacuum leak would make it worse."
 
You might want to try running

You might want to try running it at night to observe spark leakage from the wires or anywhere else
 
"so another week or so later,

"so another week or so later, they have changed the whole distributor and that did nothing
I also checked it the other morning in the dark as someone mentioned, no visible spark
they are now looking at the fuel again as they seemed to have tried most of the electrical.

they tried changing the fuel pump and also blocking off the return. all the pressures are good

next they are going to try changing the injectors and if that doesn't do it, they are saying we may have to try mechanical. they have a guage where they can do a falling pressure test to see if it is maybe a sticky valve?? problem is they haven't been able to isolate the problem to one area

I took it out the other day with 4 people and some gear and it started to mis at about 2200rpm and so would not get on the plane. obvioulsy the higher the, the lower it causes problems?

I see someone mentioned about opening up the fuel cap to let air in? would that make any difference if the fuel pressure is all ok?

The mercury rep has been out 3 times this week and is obviously trying to help as much as possible but seriously starting to run out of ideas and cash"
 
"If you are running out of ide

"If you are running out of ideas, you might try resetting all the spark plug gaps to .020 and seeing what effect it has. The smaller gap requires less voltage and depending on the results, it may point you in the right direction as to spark vs fuel problems. Observing the engine at night for spark leakage should really be done under power when the missing occurs rather than at idle. Good luck...keep us posted"
 
"Thanks Bob, yes I did try it

"Thanks Bob, yes I did try it at night under load on the way out fishing. was a slow and short trip at 10mph to keep it running normal

Will changing the gap cause any damage to my spark plugs if it doesn't work as they are brand new plugs? can they be changed back?

thanks

Chris"
 
You can re-gap them without a

You can re-gap them without a problem or if you still have the old ones. gap those to .020 and use them for the test. Be careful not to cross thread them and use a torque wrench for tightening...Running "under load" would mean at 3000RPM or whatever speed causes the engine to start missing.
 
"So, we gapped the plugs today

"So, we gapped the plugs today at 0.025 but couldn't take it for a run as I found the mechanics have removed a water hose for something they are going to do on the injectors. cooler or something?

Somethings we did note though were

1. there were 3 spark leads with corrosion on the distributor cap end plugs. so we cleaned those up and lubricated them. they seemed quite loose after that so had to adjust them a bit. pretty annoyed that the mechanics did notinhg about those. Any chance that could be significant enough to cause the problem?

2. the spark leads were crossing and touching each other aswell as the coil lead so we tidied those up a bit. can this cause problems or are they that well insulated?

I also checked the earth on the engine and it felt pretty solid.

I will have to get them to replace the hose so we can test it out."
 
"On my '97 Tahoe's plu

"On my '97 Tahoe's plug wires, @ about 170k miles I finally changed them cuz when it rained, it didn't run 6 out of 8 wires had pit's like someone stabbed them with an AWL.. Oh and the Dist Cap had hugh pile of corrosion on the posts inside (alum though, not brass like the marine ones) though now I'm running a marine cap cuz I could get the brass posts."
 
"Ok we tried smaller gap plugs

"Ok we tried smaller gap plugs, alternative high octane fuel and looked for oil foam and nothing.

What we did notice that was that there is no plug to what I later discovered is the hall effect/camshaft sensor in the distributor. We had a look on the way home at a 2008 engine, same type in a boat yard. this one had a plug coming off it to the loom.

There is a plug we found which could fit it, but it has like another adaptor plug on it, but it does go to nowhere. we thought it could be just for pluggin computor in?? the plug that looks like it could fit had a purple coloured seal. and is a 3 pin.

After what I saw on the other engine, this looks like a serious issue. IS this so??

Before we knew all this , my brother who is an average back yard mechanic at best thought it seemed like a major distributor problem where the spark was not advancing as the load came on.

I don't want to talk to my mechanic about this yet until I know more.

1. could this be my problem or am I clutching at straws.
2. could I do damage if I try hooking that plug on

252291.jpg
sensor location"">
252292.jpg
plug to nowhere
 
chris that more than likely co

chris that more than likely could be your problem as i believe that is an electronic spark advance
1. does your motor have the required plug
2. if it does plug it in and see what the affect is
3.if your harnes doesnt have that plug then i would strongly consider haveing a shop do the install of that plug
 
P.S chris are them pictures of

P.S chris are them pictures of your engine or the one you looked at if thats your motor and plug then i would plug it in and see if it helps
these newer type motors have special designed plugs that only fit for there location if that helps
 
"Hi steve, yes pictures are of

"Hi steve, yes pictures are of my engine showing obvious plug missing from distributor. I am beginning to suspect major foul play.

The engine does have a plug as shown on the picture which looks like it would fit. This has another adpator plug on it with about 8 inches of cable and terminates in a different plug and goes nowhere, which maybe why no one has thought of it. Not sure why its like that. I am scared of actually plugging it in and doing damage to the engine however this could also be the problem.

Oh and I also noticed that on the computer one of the stickers has a date 2001!! not 2006. how do I tell actual date of this. can you tell from the scan tool or on the serial number??"
 
"I found this diagram of a mer

"I found this diagram of a mercruiser distributor which seems to be the same as mine. the sensor listed as No 6 if you can read it is the one I am talking about. This <u>must</u> have to be connected to something


252313.gif
distributor diagram
 
have now purchased an online l

have now purchased an online licence for a Seloc manual. A photo of the distributor on a similar one shows no connection to that plug and makes no real reference to it apart from that it should be capped. It labelled it in a similar diagram as being a module (I was really hoping that was my problem)

I now have the wiring schematics though so can now look over those earth wires everyone has mentioned. do I need to check with a volt meter or just visual to look for obvious loose/damaged areas?
 
my guess is the serial number

my guess is the serial number will give you the date but if the date is written on it then enough said take a volt meter and have your engine running and check them pins and see if they have any current to them. check it at idle then check it with a slight rev and let me know what kinda reading you get
 
""can now look over those

""can now look over those earth wires everyone has mentioned. do I need to check with a volt meter or just visual to look for obvious loose/damaged areas?"

I like to disconnect the battery and then remove each one, clean it and where it attaches and reinstall. Coat them w/a dab of grease."
 
"had a look over the wires and

"had a look over the wires and plugs today. didn't have a volt meter, but everything looked ok. I did note some minor corrosion on one of the pins in the plug to the coil, but this looked pretty minor. I sprayed it with some electrical lubricant.

I also looked at another engine just like mine and it also had the open plug from the dizzy, so I guess that idea is well out the window.

Something I did find up by the water pump was an earth wire with a ring attachment. it was taped up but still visible. The diagram I have suggests the Mercathode is in that area and the earth wiring diagram also shows an earth feed for this. It was with some other blank wires also, so I am assuming that I may not have one and this probably doesn't mean anything towards my problem.

Also found more evidence my engine may actually be 2001 not 2006. the wiring loom also has a dated tag as well as the computer. will need to the on the scan tool to confirm, I doubt this make any difference at this stage with trying to solve the problem
252409.jpg
earth plug - mercathode??
 
"I have spoken to the mechanic

"I have spoken to the mechanic again and they hae checked all the wires and earths already. they had also changed the leads and distributor in one hit to check that.

they are about to change the injectors but can't see that being it. They are thinking possibly hydraulic lifters?

have made contact with the person I brought it off, so will see what happens there. Also confirmed it is a 2002 motor not 2006!"
 
"definately fresh gas right ?

"definately fresh gas right ? we chased a power issue on a outboard and we thought the owner had fresh gas in his cans, we used his gas it would run crappy then our gas a couple times it would run great, got it running good, then he took it, ran crappy, used his gas then our gas 2-3 mechanic working on the thing we didn't realize till after many hours, and we chased tails a while. What a waste. All over bad gas that we thought was good, never checked it. Owner when he bought the boat never replaced the fuel and though the boat was sitting for sale in our yard, we didnt sell it to him and he thought the gas was OK.. and he works with small engines."
 
We tried a small tank with fre

We tried a small tank with fresh high octane gas. Ran it for about 10min out in the water. probably used about 1 litre or quarter gallon. this should have been sufficient to use up existing gas in lines. Made no difference at all. Also replaced the fuel filters.

The merc rep and my mechanic both said bad gas would not be the issue ages ago. I ended up trying it myself about a week ago.
 
"I have found in the past, a f

"I have found in the past, a faulty tacho will cause this problem. They can break down internally and pull the whole ignition system down as the rpm's rise. Just disconnect the grey wire on the back of the tacho and see how it goes. I had the same problem and a new tacho fixed it."
 
I think we ruled that out as t

I think we ruled that out as they disconnected the boat wiring loom and tried it with a short loom to run it separately
 
"Chris
If your mechanic is wo

"Chris
If your mechanic is worth his salt, he/she'll have a portable vacuum gauge in the tool box. Hook it up and then check this web site for a real time diagnosis of what your engine is doing, not what the computer is doing. Scroll down the page a bit for the scenarious. http://www.secondchancegarage.com/public/186.cfm
Good luck
Bert"
 
"Hi All

I have an update.

"Hi All

I have an update. I went back to the guy I brought the engine off, who is also a service mechanic for a reputable boat manufacturer here in new zealand. Within half a day he found they had wrapped one of the braided fuel lines on the bildge pump when they were installing it and it was so bad it had crimped the fuel line and it is showing signs of damage. He has replaced the hose and has checked it out in the water. Apparently he says it runs a lot better and also runs now at high revs but is still 'surging' at about 2300rpm.

so after 2 months with my mechanic we found a significant problem was that, but its still not fixed, althoug its better. The new guy, who seems to be very good, now has to go through and recheck all the stuff that the other guys did.

Can't help but think the other guys may have done something along the way that could have fixed it and the problem was not noticed due to the fuel line problem.

I haven't been in the boat yet, so can't comment too much on the difference, but will update soon, will hopefully try to see it this weekend.

My mechanic now wants payment which will probably be $2000-3000. not sure if I should be paying this though as their error has probably contributed to the problem not being found.

They have downplayed the fuel hose issue saying it was very minor and that it was not showing up in the faults on the computer. Would this show as a fault on the scan tool if it had insufficient fuel??

I am also pretty sure that when we first got the engine running the problem was up closer to high 2000 rpm and it was able to get on the plane slowly. most recently the problem was down closer to 2300rpm."
 
well... way back in the beginn

well... way back in the beginning you said the fuel pressure was ok... if this fuel line deal was the issue then the fuel pressure gage would go to near zero when the misfire occurs.

so i'm not sure youve found what you are looking for yet.
 
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