GT-40 1996 SS issues with idle after it has warmed up

[workky]

The boat runs perfect, but after about 20 minutes of running, it wont idle worth a crap. It idles very low stumbles and shuts off. If will fire right back up and run fine until you put it in gear and go and return to idle as if picking up a fallen rider. Boat cranks and idles perfect when cold. It Starts every time when its hot, just wont idle

[t.franscioni]

Assuming you have fuel injection? Could be any number of things. First thing I would do is clean your spark arrestor, IAC valve, and throttle body if it hasn't been done in a while. It can gunk up from oil mist because your crank case breather attaches directly to the spark arrestor. So when you are at idle and the throttle plate is closed there is very little flow of air through a gunked up spark arrestor and or broken IAC valve will cause a rich mixture. When cold the engine needs a rich mixture and it idles fine. When the engine heats up the air/fuel ratio needs to lean out and if your IAC valve is stuck closed or everything is gunked up then you won't be getting enough air and the engine will be running rich causing a bad idle at engine operating temperature. That could be reason why it starts running fine when you open the throttle because by doing so you allow the engine to get enough air to bring the fuel mixture into balance. So either gunked up intake and/or broken IAC valve maybe....

If its not a gunked up intake or broken IAC valve then also check your fuel pressure in your rails. If your fuel pressure regulator is bad you will have too much fuel pressure in your rails and that cause the same problem. Overly rich mixture and therefore bad idle until you open the throttle up to allow more air to lean out the mixture....

Could be an ECU issue or a bad MAP, Temp sensor maybe.... Not to sure about that though

[workky]

Ill check on those ideas, Thanks
Its is a GT-40, it sis fuel injected, not the throttle body kind

[Blamey]

CorrectCraftFan.com has thread called "poor man's gt40 diagnostic ” which details common gt40 issues. There is one that discusses a loping idle and the fix it to replace the IAC value. Read the thread, lots of good info.

[Jono]

i would check the fuel filter first..