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mercruiser 4.3 lx not starting

Do you have a wiring diagram? It would troubleshooting alot easier. The disconnected wires: the black wire is a ground and the purpe one should be 12 VDC. You can test w/a meter. On the OD is the trim position sender wiring disconnected? I would tape over any bare wires to prevent an accidental elec. short.

I believe the oil pressure switch only supplies 12 VDC to the elec. fuel pump after the engine starts. It's a safety issue so that the pump won't run if the engine stalls and/or loses oil pressure. The pump gets gets 12 VDC from the starter circuit only when the ignition key is in the start position.
 
I know you said you are sure timing is correct but how did you verify that. Going by the mark on the harmonic balancer might not be enough. As some one earlier pointed out you might have a loose timing chain. You might want to pull your valve cover off and crank the motor by hand and watch the valves go through the 4 strokes to visually line it up TDC. Then check if the harmonic balancer is true to that. It definitely sounds like timing or you have a large leak to ground in your ignition.
 
on the wiring diagram it goes from the oil pressure switch to the fuel pump out the black wire to the ignition cut out switch and im assuming if its is bad then it wont have a good circuit being its operated off grounds I think
 
I verified that the mechanical timing is good pulled the number one spark plug and with the number one on tdc on compression stroke verified that the rotor on the dist is pointing to number one position on the dist
 
disconnect the battery when cleaning connections. Follow the neg cable to the main ground.

I have a feeling you just got this boat. The oil pressure switch is hooked to another switch ? That switch could be bad won't give juice to the fuel pump if it is a electric fuel pump.

Tach wire is hooked to the coil low.
 
ok i dont know why but since i hooked a jumper wire to the oil pressure switch the boat runs fine besides timing because I turned the dist (did not pull it off the gear tooth) just rotated it a bit so how do you figure out the timing there is only a v notch no degrees
 
Think i found out the answer to my question myself. I read online that the thunderbolt IV ignition has a mbt (mean best timing circuit) and that purple wire i was asking what it was is called wire 13 which needs to be grounded out to disable the mean best timing circuit to get a baseline timing of 8 deg btdc. does this sound correct to you guys?
 
Maybe I`m missing something here but if you have power in the on position of the ignition switch but not when cranking, would the most obvious thing not be the ignition switch itself ? I had an old truck a few years back that would refuse to start as long as you were cranking it on the starter and key was in the start position but sometimes as you released the key and the motor was still flipping over, the damn thing would fire up. Your video reminds me of that situation , the way it fires when you release the key.
 
You don't do anything with any wires on a Tbolt IV to set timing. You do on a Tbolt V. As a follow up to a hunch I mentioned earlier, pull your distributor cap off, turn the engine over by hand and watch the rotor, it should start moving as soon as you start turning the engine, then try moving back a bit and it should start moving back at the same time. If not, it's a timing chain issue. While you're at it, grab your rotor and rotate it clockwise then let go. It should return by itself. Easy things to try in order to eliminate critical possibilities.
 
The oil pressure switch has nothing to do with Ignition. The oil pressure switch only closes the power circuit to the electric fuel pump (if you have an electric fuel pump) after the engine is running and had built up enough oil pressure. While cranking the starter is putting power directly to the electric fuel pump(if you have an electric fuel pump) to start the engine until the oil pressure switch closes and powers the pump.

I would remove the ignition switch from the helm and see if you are losing power to the purple wire while cranking the engine. If so replace the Ignition switch. Also sounds like you burned out your ignition trigger.
 
according to the wiring diagram in my service manual the oil pressure switch ground side goes into the dist after the fuel pump if im remebering correct good chance im wrong though I have a horrible memory lol. anyway I finally figured out the issue with not starting it turns out the altinator was bad.... somehow pulling current away from the coil so the spark was really week. that was really strange
 
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removed the alternator and jumped the coil so i would have power to it and was able to get the engine in time :) who would of thought a bad alternator could kill a boat ignition system
 
ok still not right but not sure if its cause the alternator is not there, causing not quite enough power to get to the coil (its night and hard to diagnose lol) but I got it so It will run at 8 degrees btdc is that the right timing for a thunderbolt IV ignition? and when I try to rev it up it wants to crap out on me and pop back through the carb it seems to idle ok missing here and there but still idling, it will not start at 8 degrees but it will start at about 10 degrees, and idle better and rev better but still a pop here and there through the carb. My thoughts are I dont have the alternator and it might be the reason, but I also was wondering if it could be a bad coil? and maybe the optic trigger inside the dist? or should I just hang tight untill I get a new alternator? any thoughts? Let me go over one more time what I have done so far


New dist cap
New rotor
new plug wires
new plugs gaped to .040
coil is about 2 months old
checked mechanical timing and its perfect dist has no play so that rules out loose timing chain
I have gone over plug wires to corisponding cylinders on dist cap
rebuilt carb
new fuel pump
fuel tank vent not clogged
fuel check valve replaced
fuel lines all are good shape
fuel tank has been drained and cleaned fresh fuel put in
cant remember what else

thanks :)
 
The alternator has nothing to do with the ignition system. I have a feeling that you have a bad connection on the purple wire in the harness. When you removed the Alternator you reduced the electrical load on the purple wire allowing more voltage and amperage to travel to the coil. The whole problem with your engine is going to be a bad connection or a loose or bad wire. I never recommend running a new wire. In my 20 years exp as a Diagnostic Marine Mechanic, nine out of ten times when you find the actual problem there is more than one damaged or broken wire. Just take your time.

Have you used a digital volt/ohm meter to check for actual voltage at the purple wire on the coil. You should have battery voltage. (Key on Engine off). Once you have this electrical problem repaired, you should check the distributor shaft for any wear. IE excessive up and down movement.
 
I have checked the dist shaft for wear or excessive play, its good to go. as for a bad wire im still trying to trace with my multimeter to see if there are any broken wires or bad wires I was getting 12 volts to my coil when ignition on engine off. right now Im waiting for the weather to clear up and my new alternator to get in. when I took the old alternator to the repair shop they said that when they rev up the alternator it was drawing 8 volts instead of putting out juice.
 
...and idle better and rev better but still a pop here and there through the carb.
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Did you ever run a compression test? A burned valve will cause missfiring along w/cross firing plug wires or carbon tracking inside the dist. cap even if the parts are new. Running the engine in the dark will show defective plug wires. Finally the spark amplifier that tells the coil to fire could be failing after 15 years. The plug wires should be routed in parallel so that they do not cross over each other. What brand plug wires did you install?
 
I posted what i did for me i don't think i recommended anyone else to do it. What people do on their boat is their bus.

I would just like to see guy guy on the water having fun and not in the driveway messing with the boat forever, and if a safe alt. is available coast guard approved well ?
 
I dont know I thought it said .040 could be mistaken though but anyway The full diagnoses was. The alternator main lead off the rectifier inside it was broken off and grounding itself to the case causing I guess a massive short therefore killed my ignition sensor or optic sensor inside the distributer. what got me about this is a whole lot of people were telling me it either works or it doesn't so I just kept overlooking it because i had spark. Well after replacing that and bought a new coil for the heck of it, (last thing on the ignition system that wasnt new figured might as well be new lol) and it fixed my issue. I took it out to the lake today and really tuned the carb and dist timing. boat now runs faster and cooler temps!!!! ran a full tank with no problems at all. I am glad this issue is over. Thank you everyone for the help and input!!!!!
 
Ok for everyone still having an issue with my ignition system after replacing the alternator,
ignition sensor in distributer
coil
dist cap
rotor
spark plug wires
spark plugs
rebuilt carb
new fuel pump
checked shift interupt switch
checked power to coil was good

yesterday I replaced the coil and ignition sensor in the dist cap
boat ran great used a half of tank of fuel. ran it for about 2 hours no issues
today went out and about 30 mins into it the boat started loosing spark it would hesitate and start to stall out untill i let off the throttle then the boat died would not start again. I think it fried another ignion sensor in the dist cap? noooo clue help please ohhh forgot somthing when I started the post. my tach started acting weird when I was wot my tack went from reading 4700rpms to read 4000 maybe 3800 but my engine was still turning 4700 rpms it would go back up then back down. If i let off the throttle to about 3500 rpms it would read correct down to idle. then the motor started doing its hesitating thing I disconnected the tach to see if that was the issue from the coil and from the back of the guage didnt fix the issue, hooked it back up and boat was still hesitating and acting weird then about 10 mins later engine died and wouldnt start up. soo that leads me to believe something fried my igntion sensor idk what do you think?

my other tread was cant time 4.3
 
moondrifter said:
Disconnect the tach. Wire from the dist.
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which wire on the dist is the tach wire? I thought the tach wire goes to the coil? and when i disconnected the wire from the coil the tach did not work and I still had the problem. I read post number 27 several times cleaned all my connections but this is not a bad connection issue traced the wire from the helm and there is no breaks or bad wires with my mulimeter even ran a wire like they said and it did not fix the issue took the wire back out and reconnected it how it is supposed to be. like i said in post 54 I am almost positive my ignition module fried again. what could cause it to go besides the tach I have 12 volts at my coil not losing power there.
 
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