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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.
 
jimn said:
""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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Disregard this sentence, It refers to an outboard engine not your V6 I/O............................
 
kghost said:
Disregard this sentence, It refers to an outboard engine not your V6 I/O............................
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Thanks; I did. Being a Chevy, the coil, distributor and module (which is mounted on it) are almost impossible to reach when on the water. Plan right now is to mount 2 ignition modules in an easy-to-access area, along with 2 coils, so I can swap each out within seconds if/when it does again. New distributor cap and wires. That just leaves the sensor as a single-point electrical failure, but I'll carry a spare. I'll install a fuel pressure gauge, and a spare fuel pump going to a changeover valve, though I cannot imagine this being a fuel problem, as the squirters are working, and starter fluid doesn't help. I hate to just throw new parts at it without determining the cause first, but since I cannot duplicate the problem except on the water under load I have little option. Breaking down on the water is no fun.
 
tedthebrit said:
Thanks; I did. Being a Chevy, the coil, distributor and module (which is mounted on it) are almost impossible to reach when on the water. Plan right now is to mount 2 ignition modules in an easy-to-access area, along with 2 coils, so I can swap each out within seconds if/when it does again. New distributor cap and wires. That just leaves the sensor as a single-point electrical failure, but I'll carry a spare. I'll install a fuel pressure gauge, and a spare fuel pump going to a changeover valve, though I cannot imagine this being a fuel problem, as the squirters are working, and starter fluid doesn't help. I hate to just throw new parts at it without determining the cause first, but since I cannot duplicate the problem except on the water under load I have little option. Breaking down on the water is no fun.
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Agreed went through this same business for the last 2 seasons, what fooled me was that the coil problem was somewhat intermittent, and it tested well on simple primary and secondary ohm testing. How hot it got was what made me change it with one of my spares and the improvement was immediate. This bothered me so much that I got a basic ignition oscilloscope and am now learning how to use it, if I had that, I'd have seen that the voltage output was way below par.
The good part, was I verified that the whole fuel system is in good shape, from the pickup tube/anti siphon valve to the Rochester Quadrajet 4bbl!
 
Way back in time, we used to use a 'freeze spray' when troubleshooting many electronic devices...haven't looked for any in years but it might be worth trying, especially it you think the part(s) in question have a repeatable 'fails hot then cools and behaves normal' behavior.
 
louc said:
Agreed went through this same business for the last 2 seasons, what fooled me was that the coil problem was somewhat intermittent, and it tested well on simple primary and secondary ohm testing. How hot it got was what made me change it with one of my spares and the improvement was immediate. This bothered me so much that I got a basic ignition oscilloscope and am now learning how to use it, if I had that, I'd have seen that the voltage output was way below par.
The good part, was I verified that the whole fuel system is in good shape, from the pickup tube/anti siphon valve to the Rochester Quadrajet 4bbl!
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Well, I know have THREE (!) coils mounted next to each other, and can swap the wiring to them in maybe 30 seconds. So IF it's coil related I should be able to isolate it easily. I want to be able to fix it, then reproduce the problem shortly afterwards to be sure. This should allow that.
 
makomark said:
Way back in time, we used to use a 'freeze spray' when troubleshooting many electronic devices...haven't looked for any in years but it might be worth trying, especially it you think the part(s) in question have a repeatable 'fails hot then cools and behaves normal' behavior.
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Good idea. Thanks. I've done the same thing. (40 years as an EE.) I still have a can of freezer, but I'm not sure there's enough in a can to cool a coil or ignition module very much. I've mounted 2 modules side by side, and can switch them over in seconds, so I should be able to substitute one for the other and back without the environmental conditions changing much. Fingers crossed!
 
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