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I have a 95 MerCruiser 5.7, Thunderbolt IV, Serial #OF367661,

I have a '95 MerCruiser...
I have a '95 MerCruiser 5.7, Thunderbolt IV, Serial #OF367661, and Weber carb.
This motor has a serious issue.
It will start and run fine for a little while, then start missing out bad. It usually starts missing on #8 cylinder at about 1600-1800 rpm, and the faster I try to go, the more cylinders start missing out.
Miss verified with a timing light on the plug wires at the distributor.
I have rebuilt the carb (other issues), replaced the ignition module, ignition pickup sensor, and tested the coil. When I did my spring tune up, isntalled new plugs gapped to .035", rotor and distributor cap.
The distributor shaft feels fine, maybe .040" of vertical movement, and no detectable horizontal movement.
HELP!
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Answered in 1 hour by:
6/27/2010
K. Kergan
K. Kergan, Marine Mechanic
Category: Boat
Satisfied Customers: 1,031
Experience: 24 yrs exp. Inboard/Outboard mechanic Mercury/ Mercruiser, Volvo Penta and OMC, Johnson/Evin.
Verified
Can you see the miss with the timing light on the coil wire?
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Customer reply replied 8 years ago

No,

I had an inline spark tester on the coil wire at the same time. I could not detect a miss at all there.

And it hurt like hell when we got hit by a wave from a 34' cabin cruiser while I was reaching for the timing light clamp and accidentally grabbed the spark checker.

 

Rotor, distributor cap and wires and maybe plugs are the only things left. I have seen a situation where the engine ran fine for about fifteen or twenty minutes and started missing and crapping out. Shut it off for ten minutes and it would do it again. That one was a bad distributor cap leaking spark to the wrong cylinders, the rotor can leak spark away to the distributor shaft and wires can leak through the sides to ground. Sometimes you can see bad wires, if you run the engine in the dark, you'll see any stray sparks. Hope this helps
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Customer reply replied 8 years ago

It has new rotor, distributor cap and plugs.

I had originally thought about the wires, but it would seem very strange for more than one to go bad at once, and even then only short out at higher rpm's.

I am concerned about the distributor itself, but as I said, it seems good and tight.

 

The really funny thing is, if it sits overnight, it will start and run fine for up to 2 hours the next day, then start acting up. Even when it is acting up, it will not necessarily do it all the time, just 99% of the time.

 

I also forgot to mention that oil pressure is good, about 38 at idle and 42 at 3000 rpm, temperature never goes over 140-145. Don't know if that makes a difference.

The boat is in storage about 50 miles away, but am going to pick it up in the morning.

 

I'll check on the plug wires and get back to you tomorrow. If you come up with any other ideas, let me know.

 

Customer reply replied 8 years ago

The plug wires checked out fine. No apparent arcing at all.

 

Ok, you have consistent spark coming from the coil so the coil and the distributor module have to be working unless we are mistaken about the consistent spark from the coil. If it does have good spark from the coil you are losing some of it between the coil wire and the spark plugs, this would mean cap, rotor, wires or plugs, new or not it has to be one of these. If the ignition module is breaking down when hot the misses will show up on the coil wire.

Edited by K.Kergan on 6/28/2010 at 3:53 AM EST
K. Kergan
K. Kergan, Marine Mechanic
Category: Boat
Satisfied Customers: 1,031
Experience: 24 yrs exp. Inboard/Outboard mechanic Mercury/ Mercruiser, Volvo Penta and OMC, Johnson/Evin.
Verified
K. Kergan and 87 other Boat Specialists are ready to help you
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Customer reply replied 7 years ago

It did turn out to be a bad coil.

For some reason, the miss did not show up with the inline spark tester, but with the timing light, it was readily apparent.

Thanks for the help.

 

Cheers, XXXXX XXXXX found it.
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