GT40 Overheating at Idle

[INDpilotP]

My ‘99 Super Sport (EFI GT40) is overheating at idle. Once you get it above 1250rpm or so (even sitting still), it draws enough water to cool back down and hold 160. Risers are too hot to touch for more than a couple seconds. It has fresh impeller, new thermostat, can see water moving in strainer, can’t feel any obstructions in tranny cooler, etc. I’m about stumped and don’t want to start aimlessly throwing increasingly expensive parts at it. Any ideas?

TIA!

[Jonny Quest]

That is a typical symptom of air leaks into the raw water system. Do not tighten your hose clamps with a screwdriver. Use a nut driver instead. Get your hose clamps tight and make sure that you have no air leak at your raw water strainer. The O-ring is an air leak suspect. Look for cracks in the plastic fittings and in the hoses.

JQ

[JayG80]

All good points by J Q. It is possible to overtighten the clamps on the transmission cooler and crush the point where the hoses attach.

Debris can become lodged in the screen inside the transmission cooler.

Check your raw water impeller for damaged vanes and be sure the hub does spin inside the rubber.

[Bevostein]

Check your water strainer. If its cross threaded it can let a little air in and cause this exact symptom, overheat at idle but cool at speed

[INDpilotP]

Jonny Quest I’m also suspecting an air leak somewhere. All connections seem tight and can’t see any air in the strainer bowl. Either way going to replace the bowl itself, gasket and put in new impeller.

If that doesn’t help, I’m thinking about temporarily putting some clear water line on at various points along the cooling system to try to find the culprit. Thoughts?

[INDpilotP]

Bevostein I hoping it’s something simple like that. I’ve ordered a new strainer bowl and gasket to count those off the list of 20 or so possible causes.

[Jonny Quest]

Use a little Vaseline on the strainer O-ring and bowl threads. Helps a bunch.

JQ

[NautiqueJeff]

If the above suggestions don't work, it might be time to pull the exhaust risers and see if the water passages in the exhaust are corroded to the point that they are blocking water. You may end up having to replace the manifolds and risers. Don't do that until the other possibilities are ruled out though.

You might also check the transmission cooler. If you have ever had an impeller fall apart, that is likely where the broken blades are stuck, and that can block significant water flow.

[INDpilotP]

Use a little Vaseline on the strainer O-ring and bowl threads. Helps a bunch.

JQ

Thanks, I'll give it a try!

[INDpilotP]

NautiqueJeff I had thought about the risers and manifolds as well, hoping that's not the case. Would really prefer to avoid that expense and the headache of potential broken bolts and removal issues.

The transmission cooler on mine is pre-fresh water pump and feels clean. I think it runs to the v-drive after the water pump, but I may be wrong. Was kind of hoping that I might find an impeller chunk when I changed the thermostat, but that wasn't the case.

[INDpilotP]

I ended up buying a rebuilt raw water pump to replace my original and after about an hour of running it seems to have solved my issue. I suspect there was a tiny air leak somewhere in the original. I never saw it leak any water, but there was a bit of play in the bearings that I noticed when changing impellers.

[SilentSeven]

It's great when people post follow-ups for what fixed their problem!

[INDpilotP]

SilentSeven Agree! I’m all for trying to help someone else avoid the headaches of those before us!

Found someone on eBay that sells rebuilt Sherwood pumps for around $275 shipped. That includes shipping to send your old pump back as a core. I was much happier to pay that than $670 on NautiqueParts for a new one. Rebuilt one looked like new and seller was quick and easy to work with.