GT40 High Idle Problems

With Spring coming soon, I've been thinking of a couple issues to be addressed when the season starts. In the last 2 years the engine has been out of my boat twice for non engine-related issues (long story). To be brief, since the engine went back in the boat last summer we've had issues with idle speed after the engine is warm.

Idle speed will consistently settle at 1100rpm once then engine is warm. When its cold, everything is fine. If the engine is shut down and restarted while at high idle speeds it seems to correct itself until the boat is put in gear and then put back in neutral. At this point it will usually return to the 1100rpm mark again. If you're lucky you may get another pass or 2 with normal idle speeds.

Has anyone else had similar issues? I've read of lots of people with the opposite problem with the GT40 (engine stalling, rough idle, etc) but not much on high idle problems. To the experts, what is the likely culprit? I am hoping its not a bad ECM, and don't think that it is since the problem started immediately after the engine was re-installed. Bad ground somewhere during the engine install? Vacuum leak? Trying to put some ideas together before the boat comes out of storage so I'll have an idea where to start.

Engine is a '97 GT40 EFI.

Makes sense, I'll be sure to try that when the boat comes out of storage.

Another coincidence is that we've had intermittent problems with the bilge pump not shutting off after getting the boat back last summer. Had to disconnect the battery just to get the pump to stop. Only noticed the problem one night when I went out to the driveway and could hear something running inside the boat as it sat on the trailer. I hate the idea of chasing electrical problems around, but I'm thinking that may be the case this year.

These motors have an idle air control as well. Another thing worth checking.

No PP right now...some day though

Baseline when running well is actually 6-800 rpm from memory...it only idles at 1100rpm once the engine is warm (usually after one or 2 sets if we're skiing, maybe 30mins cruising). I've had a couple other people mention the throttle cable also.

If it were a pinched throttle cable, or a cable not returning to normal idle position for whatever reason, wouldn't you see this problem regardless of whether the engine is warm or not? I'm hoping this isn't one of those annoying problems we chase forever and never really get to the source.

It is not really a IAC motor on the GT40 engine it is a Solenoid that opens a piston allowing extra air into intake. It might just need a clean with some carb cleaner. I would also make sure the throttle body is fully closed when the idle is high. Another cause could be a air leak somewhere on intake or throttle body after the engine warms up.

Here is a video of IAC operation on a GT40.