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502EFI problem

Old 11-08-2004, 04:31 PM
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Default 502EFI problem

I have twin 1996 502EFI's with VST tanks. Motors were modified by quality performance marine engine shop. They have performed extremely well for 300 hrs. Top end valvetrain components replaced @ 250 hrs. Recently one engine has developed an intermittent miss/sputter. While cruising @ 3500 RPM, sometimes the motor will suddenly lose 300 or so RPM causing me to move that throttle forward to get back to 3500 RPM. It sounds a little different when it starts happening. Also, frequently just off idle when starting to plane, that motor RPM jumps around a little on the way to speed. It will sputter and cough. I have changed the distributor cap, plug wires, and plugs. I have experienced VST screen problems in the past but this does not appear to be that as VST problems usually stay once they arrive until the VST screen is replaced or cleaned.

Any suggestions on where to look ? Any ideas at all will be appreciated.
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Old 11-08-2004, 06:42 PM
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Default Re: 502EFI problem

Originally Posted by deboatmon
I have twin 1996 502EFI's with VST tanks. Motors were modified by quality performance marine engine shop. They have performed extremely well for 300 hrs. Top end valvetrain components replaced @ 250 hrs. Recently one engine has developed an intermittent miss/sputter. While cruising @ 3500 RPM, sometimes the motor will suddenly lose 300 or so RPM causing me to move that throttle forward to get back to 3500 RPM. It sounds a little different when it starts happening. Also, frequently just off idle when starting to plane, that motor RPM jumps around a little on the way to speed. It will sputter and cough. I have changed the distributor cap, plug wires, and plugs. I have experienced VST screen problems in the past but this does not appear to be that as VST problems usually stay once they arrive until the VST screen is replaced or cleaned.

Any suggestions on where to look ? Any ideas at all will be appreciated.
i had similar problems with one of my 502 ef's this past summer. intermitant loss of rpm's. rough idle when cold but not all the time. checked fuel pressure with a mechanical pump. pressure was right where it was supposed to be. couple of weeks later it wouldnt start. turned out to be the high pressure fuel pump inside the VST tank. i wish it had gone bad all at once instead of chasing it for weeks.

Last edited by Steve_H; 11-08-2004 at 06:47 PM.
 
Old 11-08-2004, 06:48 PM
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Default Re: 502EFI problem

I had something somewhat similar on 502 Mag MPI, it was a intermittent ground.
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Old 11-09-2004, 05:31 PM
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Default Re: 502EFI problem

Run a long hose with a fuel pressure gauge on it from the shrader sp? valve on the front of the rail to a place you or someone can watch it. If you have a fuel pressure problem that will show it. You indicated that you did not think it was the VST screen but have you checked it recently?

My old 502 EFI had a chafed wiring harness that would ground the pump inside the VST and shut me down intermittently. You can hot wire the pump directly to a hot 12 v source and to ground and ride. Look at the wiring diagram to determine the correct polarity and LIFT the stock wiring harness leads. If the problem goes away it is the fuel pump wiring.

If it is not fuel pressure and you have replaced coil/cap/wire/rotor, pull the distributor and look at the circular magnet on the distributor shaft, if it is cracked replace it. I also had that problem.

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