1996 GT40 Running Issues

[Kgodfrey3398]

New to the forum! I am having a running issue with my GT40 that is in a 1983 ski Nautique (engine added this year). The engine has 650 hours. (MAY) When I first got it back from the mechanic that added the engine, I tested it for about an hour and it ran great; the mechanic also tested it after dropping the engine in and had no issues. (JULY) This is when it started skipping and gradually bogging down. The fuel I put in had water in it so I drained it all out put new gas in and a new filter. Kept having the same issue. It runs fine for about 5-10 min then starts skipping and bogging down to the point of dying completely. There is NO water in the fuel system at this point; I have checked and rechecked dozens of times. Water doesn’t seem to be the issue. Fuel pressure is fine; running at 30-40psi with no drops. Then I noticed that it’s only starting to skip and die when the motor hatch is closed and things get heated up, so maybe electrical issue? Coil was the first thing I changed. It ran very smooth with a new coil, very noticeable difference but it started skipping and dying in the same amount of time. I really thought the coil would fix it. There is no “RPM Range” when it starts acting up, only when it gets nice and hot. I know this because I can lift the motor hatch up while stopped and it starts running smooth again.

When it starts acting up, it hesitates, skips, and there is a loud backfire pop. With the new coil not fixing it, I now think it could be either plug wires, distributor, or ECM(hopefully not). I would also like to note that the mechanic that put it in is very reputable and I’ve known him and skied with him for a long time. He does great work.

Any insight would be much appreciated.

[jpwhit]

If you don't have adequate airflow into the engine box, then the intake air temperature may just be getting too high. For a fuel injected engine, if the air going into the throttle body is too hot, it'll cause all kinds of issues. I can't remember off the top of my head if the GT40 has an intake air temperature sensor, but if it does then the intake air temp can likely be monitored while the engine is running with Diacom. Is the mechanic that installed the engine helping you diagnose the issue? I would think he would have a Rhinda Diacom system.

[Kgodfrey3398]

Yes he is helping. He still thinks it could be water in the system due to the high amounts previously mentioned. I want to bring it to him but it won’t be immediately since he’s backed up.

I suppose I could install vents on the motor hatch for more air flow but I run the blower at idle and turn it off at speed. As far as I know the front vents on the boat bring in plenty of air. I could see that being a possibility since the engine is sitting in a smaller area compared to the 1995-2000s Nautiques.

[jpwhit]

Stick a piece of 2x4 under the engine box to keep it from closing all the way. If the engine runs fine like that, then it’s most likely it is just getting too hot in the engine box and better airflow is needed. Putting extra vents, as you mentioned, may or may not be enough. It just depends on where the vents are placed and how much airflow comes in through the vents. You need to get cool air directed toward the engine's air intake.