95 Nautique stumble/stall under load

[waterskidude]

My 95 Nautique GT 40 EFI is having a problem that is difficult to diagnose. When it's cold, you can pull about 4-6 slalom passes perfectly at 34 mph. After that, in the middle phase of picking up a skier, it stumbles for about 1 second then returns to normal operation for the rest of the pass. After 2 or three skiers, the problem then starts happening late in the course or just outside of the course. So basically you can run for about 30 seconds clean, then it will start to stumble again. The heavier the skier, the more prevalent the problem. So far we changed the low pressure fuel pump and checked the relays. Anybody had this before?

Thanks.

[Blamey]

Do you have an FCC? Assuming you don't as it's a 95 but they were added later to some engines (like mine).

Without FCC I would think it may be vapor lock.

Have you tested your fuel pressure at the rail?

[waterskidude]

That's next. Here is a video of it happening. https://vimeo.com/187287087/00d17b2ae1

[IMScarlet]

Pull the fuel filter and check then if you remove the high pressure pump check the inlet as they usually have a strainer that sometimes starts blocking up. From your description it sounds like the harder you load the engine the worse it gets. EFI will run a different map when cold so the timing may not allow it to run under full load.

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[Quinner]

For sure check/clean out the anti siphon valve, free and easy to do. There is a good thread about this problem on 95' GT-40's on CCFan, don't recall what the outcome was, you should check it out.

[waterskidude]

Thanks, I'll try these things and post my results.

[bdub]

Might need to have the injectors evaluated. I was having a similar issue and my injectors were the problem.

[glassywaters]

Mine was doing something similar and changing the ECM relay solved my problem.

[waterskidude]

I haven't had a chance to use this boat since I last posted. I did swap out the relays, that didn't help. I'm skiing tomorrow hopefully the new fuel filter made a differemce.