Gt-40 surging at idle

[dead pair]

I have a 2001 SAN with the GT-40 motor. When I start the motor, the rpms will surge from normal idle down to nearly stalling. On occasion, the motor will die when it surges to the low rpm. The oil pressure gauge will also surge when the motor dips to the low rpms. If I apply just a little throttle, I have no problems. At no other time do I have any issues with the motor missing or surging. It runs like a champ while accelerating and maintains a constant rpm underway.

This problem developed at the end of the season last year. I replaced all the fuel with fresh fuel at the start of the season this year. I had the intention of changing the spark plugs but after removing a few I found them to be nearly perfect.

I have read several post trying to diagnose the problem but my symptoms seem unique. The other posts, people have suggested problems with relays, map sensor, fuel problems, injector problems and several other solutions.

Please Help!

[Chexi]

I'd still start with the fuel relay. Cheap and easy. Bad fuel relay can cause surging and/or stalling.

[91xl]

i would look at the idle air control valve. ford vehicles do have surge and stall issues when the iac acts up. so i would think a boat could do the same thing. the ecm is adjusting the idle faster than the valve can move and gives the surging idle. can even try removing the iac and clean with carb cleaner.

[peteSki]

Does the surging happen when the engine is cold? The reason I ask is that I had a bad MAP sensor a while back and I would get rpm surges and almost stall when at idle but only after the engine had a chance to fully warm up. When starting cold this would not happen. Might give you a bit more evidence of a map sensor issue if yours is surging only when hot like mine did. I also read the trouble codes and a MAP sensor fault code had been stored, so I would suggest you do that as well.

Best of luck.

Pete

[siderider]

did you solve your surge problem?

i have the same issue & have replaced the iac motor but still surges after restarts when hot but runs perfect otherwise

[hyparks]

I get something similar with mine but mine happens when the boat has been going flat out. When you drop into neutral the revs seem to stay quite high for quite some time. If i kick it back into gear and then back into neutral it levels itself out to around 900rpm.
Not sure why it does this??

[jhiestand]

Any luck with this? I've had similar occur on a few occasions but it was highly sporadic and would go away once coming off idle. It seemed like it was having a hard time getting consistent fuel at lower RPM's. As part of my regular maintenance I changed the FCC filter and the problem went away.

[Chexi]

Interesting. This was the initial guess the dealer made on my problem with getting enough fuel at idle (my problem occurs only with a warm engine). Still being checked out... will update when I know more.