gt 40 cooling issue

If none of the above solves your heating issue the engine water pump may be suspect. I have seen the impeller on some pumps rust away till it is ineffective and I have seen the impeller lose its clamp on the shaft and free spin rather than pump water. Rare and the last thing I would check but possible.

Yes it did, that was why i changed it last year. I was hoping it solved the issue.

John,

There's three different overheating at idle boats going concurrently in this thread, so it's easy to lose track.

The set screw indicates which way the pump is facing. Per Hollywood, in is correct for a Left Hand Standard (Most GT-40s are) engine. Also, you'd be overheating at every rpm if it was faced wrong.

1.) Yours if fine anywhere but idle, right?

2.) Was there anything changed etc. before the overheat?

3.) If yours is getting hot at idle, 190ish, are the exhaust manifolds too hot to put your hands on, or is this just a gauge indicated overheat?

4.) You replaced the impeller:
     A.) Did you correctly position the thin o-ring
     B.) Did the kit come with a paper gasket and if so did you put it in (shouldn't reallymatter, but just wondering
     C.) Did you use a PCM or Sherwood impeller or an off-brand?

5.) Do you have a quick flush kit installed anywhere

6.) Did you check your strainer for blockage and presence of gasket?

7.) Did you check your tranny cooler for blockage, end crushage? Hose clamp snuggness?

8.) Any other gauges giving wonky indications?

Nice!

Keep us posted.

I made a silicone gasket for my 78 Nautique, it worked for years. I filled the Oring slot with silicone, coated my cover with light grease where it would be touching the silicone. Installed the cover and let it dry. The cover still came off normally because the grease kept it from sticking closed. I used that gasket for years.
I think your lack of gasket created your issue. A minor air leak will give all of the symptoms you mentioned. Congratulations on a cheap fix although you had several headaches first.

Hollywood, would you happen to have a part number for the McMaster source on the seals? Maybe what years it would cover? Thanks

Thanks for the part number, my GT 40 has the gasket that looks like it came off an oil filter. Both my Nautiques have run a water filter/strainer. They do catch leaves and things all the time. My Malibu had no strainer on the intake water line. I guess all the leaves just end up in the block or go out the exhaust?

If I was fighting a frustrating issue for a long time and I could fix it for under ten bucks before a summer holiday weekend, I'd be very happy man, even if said part could be had a little cheaper elsewhere.

So, did the original poster get his gasket and go skiing or what?