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4.3 Mercruiser - randomly dies

RBN_TUL said:
THE FIX; After multiple trips to several different boat repair shop, it was determined that it was a stuck poppet valve in the exhaust manifold that was overheating the electronics. All he had to do was get the valve unstuck and it has seemed to work good ever since.
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were the electronics mounted to the exhaust elbow? if so you can certainly see how that could happen!
Note to Mercruiser, exhaust elbows get hot, D'oh!
 
My module is mounted to the distributor, so maybe my problem is different. But the symptoms are exactly the same. Neither the engine or the engine bay seemed to be getting particularly hot, but leaving it to cool down for 1/2 hour seemed to be the only way to get it to run again, albeit only briefly. I'd suspect a vapor lock, but the accelerator squirters were pumping well when trying to restart, and starter spray didn't help. It wasn't flooded either, as cranking with full throttle wouldn't reliably get it to start, and if it did it would only run for seconds. Still a mystery.... Thanks to both of you for your help, though.
 
One way to try to find which component is causing the problem is to use an IR temp gun. When coils go bad, sometimes they start getting really hot, and that can cause problems similar to what the OP experienced.
 
Thank you. that's a good idea, and I have a temp gun. I also plan to temporarily mount a spare coil and ignition module that can be rapidly plugged in and out when it dies. Perhaps a fuel pump too.
 
What tipped me off to this was a hard to find problem with my boat that seemed more like a fuel problem, after going through the whole fuel system including the tank, it turned out to be a gradually failing ignition coil. It would start OK when cold then lack power, stumble, misfire and stall when hot.
The coil passed both primary and secondary resistance tests. When hooking up my tach/dwell meter to it one time I noticed that it was REALLY HOT like 140* hot on the IR temp gun. Replaced coil and problems seem gone now. The only other way to find out would be to use an ignition oscilloscope to look at peak voltage readings. It was making spark on a simple spark tester.
 
Great info. When it started to stumble it felt like a fuel issue. But the accel squirters were still pumping well, and it wouldn't restart, do I think that rules out fuel. Only time (=temperature falling) would fix it. So I think mine is electrical, like yours. I was planning on using a timing light to check for spark, but from what you said that probably wouldn't be sensitive enough to tell a good from bad coil. I'll try another coil.
 
tedthebrit said:
Great info. When it started to stumble it felt like a fuel issue. But the accel squirters were still pumping well, and it wouldn't restart, do I think that rules out fuel. Only time (=temperature falling) would fix it. So I think mine is electrical, like yours. I was planning on using a timing light to check for spark, but from what you said that probably wouldn't be sensitive enough to tell a good from bad coil. I'll try another coil.
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Drizzle some fuel in when it doesn't want to start- that will verify the fuel as a cause. If you have spark after adding fuel, it should start.
 
jimn said:
Drizzle some fuel in when it doesn't want to start- that will verify the fuel as a cause. If you have spark after adding fuel, it should start.
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The accel squirters were spraying fuel OK, but a few squirts and/and starter fluid didn't reliably help it start any better. So I think it must be electrical. The trouble is it's so intermittent that it's hard to do any conclusive tests. I can't tell if a change I made (like a spray of gas or starter fluid) helped it start, or if it just decided to that time anyway! One remote possibility is that a needle valve in the carbs is sticking open, and it's flooding. But cranking with the throttle fully open didn't reliably restart it either. All I do know for sure is that it'll start after waiting ~30 minutes. Overheating module? Sensor? Coil?
 
tedthebrit said:
The accel squirters were spraying fuel OK, but a few squirts and/and starter fluid didn't reliably help it start any better. So I think it must be electrical. The trouble is it's so intermittent that it's hard to do any conclusive tests. I can't tell if a change I made (like a spray of gas or starter fluid) helped it start, or if it just decided to that time anyway! One remote possibility is that a needle valve in the carbs is sticking open, and it's flooding. But cranking with the throttle fully open didn't reliably restart it either. All I do know for sure is that it'll start after waiting ~30 minutes. Overheating module? Sensor? Coil?
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If it's flooding, open the throttle completely and crankit- there won't be enough vacuum to draw gas out and if some fuel is still in the cylinders, it shlould start- if it does, you'll obviously need to slam it back to idle.

If you don't have a manual, find a way to access that info- it contains a lot of troubleshooting info.

Have you tested voltage from the stator? It has a pair of wires for low speed and high speed- if the low speed winding is heat-affected, it can cause these problems.
 
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