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Old 07-16-2005, 11:29 PM
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Rough idle on bbc502 efi boat engine, running out of options

Coming up on summer holiday, and the rough idle on our 502 efi bbc, in the boat is giving me a real headache.
Last summer I hauled the injectors out and had them cleaned, new filter screens seal etc. Put them back in same roughness, maybe a little better.

When it comes down to idle, it's as if someone slowly begins pulling plug wires, one by one, until it really has a nasty idle. When we throttle up again, it's the opposite, gradually the cylinders each seem to kick in again one by one, until it's running okay again.

Last year, I discovered that the IAC (idle air control) valve was rusty and sticking, so I replaced that. This helped a little.

I've done all the standard tune up run through. New rotor, cap, plug wire, plugs. Both fuel filters are new. I replaced the mechanical fuel pump this year.

The problem still exists. Any tips from you bbc, and fuel injection gurus, are welcome.

The new injectors are $200 ea plus $13 for the seal kit x 8 so I have been delaying the inevitable, possibly. It really seems like sticky or clogged injectors at low fuel pressure, though.

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Old 07-17-2005, 05:47 AM
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Have you checked all the ground connections?And the alternator output?I have never worked on a boat.On cars that are fuel injected, I have found a bad ground cause intermittent problems.
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Old 07-17-2005, 07:33 AM
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Is a stock Mercruiser system, or an aftermarket EFI system?
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Old 07-17-2005, 08:11 AM
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It sounds like it loading up on fuel and then clears up when you rev it. I would think a leaking injector. Some motor have a cold start injector that is on the end of the intake to give a little shot when the engine is cold. If yours has that one dont forget to check it also.
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Old 07-17-2005, 10:19 AM
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Yes it's a stock mercruiser efi system. I had to take apart the crossram intake, to service and there are just the 8 fuel injectors.

The rough running almost reminds me of the old 68 ford van that we had, when it was wet, water would leak onto the distributor cap, as the engine warmed up more and more wires would dry out and more cylinders would fire

They bench tested the injectors, when I had them cleaned last season, but I believe they only did the before and after flow and pattern test at one single pressure. The engine has two fuel pumps, a mechanical feeding an electric. Maybe at lower fuel pressures, the injectors are acting up?

The only other thing, that I thought was possible, was the advance mechanism in the distrubutor. Though it really doesn't act like the engine timing is retarded or too advanced.

If you idle the boat fast, it doesn't act up that much. If you let the boat idle right down, then slowly it begins to get rougher and rougher. If you shut off, to say load skiers, when you start up again, it idles exactly as it was (with the same roughness) just before it was shut off.

I hope to nail it down, this summer, over the years it's been a great running engine.
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Old 07-28-2013, 06:15 PM
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rough idle on a 502 marine

Ripped: I saw the message you put on the Forum in 2005. My 502 Marine engine is running rough at idle just like yours did. Did you ever find out how to fix the rough idle and stalling? If so, let me know. Thanks, Bill Large
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