High idle and stalling issues older GT40 engine

[mmallo]

This boat is a 1997 Supra Vision with the GM 5.7L GT-40 PCM EFI HO motor.

Case 1 (usually happens in a cold engine scenario):
Boat starts in high idle 1200 RPM and stays there - when put in gear, idle drops to 1K RPMs. Boat runs fine in this condition on the open water, easily goes 30 MPH shows no signs of any issues - no stalling.

Case 2 (usually happens in a warm engine scenario):
Boat starts in normal idle 650 RPM and runs fine in or out of gear. Easily goes 30 MPH on open water for hours. When idling back through the channel boat stalls out. Much harder to start - sometimes have to apply 3/4 throttle (out of gear) to get it to restart, and goes 10-15 seconds then dies again. Can repeat this cycle and stays running less each time. When in this condition after stalls, the oil light comes on and we hear the fuel control cell priming 4-5 times.

Spent a day troubleshooting (without a ton of tools):
Sanded and tightened all electrical to the batteries and the engine block/starter.
Pulled out and cleaned contacts to all the sensors on the throttle body.
Went to auto store and replaced the throttle position sensor as it looked worn and was indicated on the forums as a potential problem - however this had no effect.
Could not find an IAC (idle air control) like the ford engines have on the diagram - is there not one for the GM?

I have the engine diagrams from PCM - but just don't know where to go next. Looking for some advice here - should I just start swapping parts (low pressure pump, relays, more sensors?) or buy some diagnostic tools to get to the next step? (fuel pressure sensor, timing light, gm fault code reader)?

[lively]

What's the fuel pressure ?


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[cadunkle]

Just a wild guess but for the high idle issue I'd be looking at temperature sender(s). For the stalling and such I'd be looking at fuel filter and pump. If it were my boat I'd swap a proper intake and carb on it and call it a day.

[SquamInboards]

Are you sure this is a GM based engine? Where is the distributor? The only reason I ask is that I think the GT40 EFI engines were all Ford based. In any case, you might poke around on the correctcraftfan forum also - lots of engine troubleshooting knowledge over there, too. Not to say there aren't good minds here, but there are simply more Nautiques in the world than Supras.

I've got a GT40 Service Manual with tons of troubleshooting info in it; I can't attach it for some reason, but if you can confirm you've got a Ford engine I think you could make good use of the document.

Nevermind, here's a link to it: https://www.correctcraftfan.com/Down...ice_Manual.pdf

[cadunkle]

Indeed, you either have a GT40 Ford 351w or a Chevy 350, not both. This will affect what parts you may need but not significantly change your troubleshooting or suspect components.

[mmallo]

Thanks everyone for the replies. Glad to learn that there is no such thing as a "GM 5.7L GT-40 PCM EFI HO" engine, that helped me clarify that it is for sure an GM engine as that is what is stamped on the block - the previous seller told me it was a GT-40, and that is clearly not the case! Purchased a fuel pressure gauge and replacing the IAC as lately it just stays in high idle - will report back as I go....

[SquamInboards]

So, this boat is new to you? Did the seller mention any recent work done, and/or did they run it for you? Or did they say, "ran fine last season" or something like that?

I'm not necessarily saying that the seller was hiding anything (although I'm not saying he wasn't) - but if there was recent work done it could be a huge help to know what and why.

You ought to be able to find a service manual for the engine online... can you post a picture of it?

[mmallo]

Original poster here - problem was resolved. It ended up being a bad IAC controller, but a few of the replacements I found at O'Reilley autoparts that looked exactly like the one in the boat didn't work correctly - I still got high idle because there must have been air leaking around the wrong part. Got the correct part from a local dealership and now it idles perfect! Thanks for the help here!