Are you certain the engine really is overheating? I'd probably also try to
figure out if I could check the efi operation. After a hard run with it
overheating pull the plugs and take a look at them. Lean fuel could be
causing it if it's not a cooling system problem. That would also explain
why it doesn't happen when the system is stuck in cold start mode.

If it is the cooling system it sounds like maybe an obstruction.

The fact that you can run it at wot for extended periods also suggests
something is happening when you pull up a skier or launch hard. A long wot
run is just as much load as pulling up a skier.

Unfortunately the hard start could be causing your temporary obstruction or
detonation from a fuel management issue though.

I would not expect it to be head gaskets "temporarily" failing. My
experience with head gaskets is that once they fail that's it they are shot.
Once a cylinder breaks thru a gasket it burns it thru. There is a steel
ring around the cylinder. Behind that it's pretty much regular gasket
material.

The system should be able to handle air. I'm assuming it's a raw water
system?

You can test you air pocket theory by drilling a small hole in the
thermostat thru the plate where it will always pass a little air/water.

wrote in message
...
Hello
New to the group, hoping to find someone with some knowledge to help
me fix an issue i am having with my boats motor. Apologies for the
long post, but for those who like a problem, i am sure you will enjoy!
Details as follows:

INDMAR LT1 V8 Corvette motor - 1997 make, 500 running hours.

SYMPTOMS:
- Once the engine has been operating for at least 20 minutes, when
brought up to cruising speed from a standing start (such as pulling up
a skier) when the engine is under load, the engine temp rises rapidly
from 150F (normal operating temp) to ~200F when the management takes
over and shuts down the motor.
- The temp can be brought back to normal (equally as rapidly - nearly
instantaneously! which i know is not good for anything) by a quick
"blip" of the throttle, which appears to restart the cooling.
- The problem does not happen at any speed if the engine is not put
under excessive load, ie, i can run it at WOT with no problem so long
as i take it there slowly.

WHAT IT IS NOT!
I have been over the cooling system and checked / replaced the
following parts, none of these have fixed the problem.
- Raw water impeller,replaced (was due anyway), not the problem, good
water flow
- Raw water thermostat, replaced also ran without, not the problem
- Cooling circuit, does not appear to have any blockages
- Engine side thermostat, replaced, not the problem, Ran without, NOT
OVERHEATING but runs too cold, as expected (~110F), and the EFI
management stays in cold start calibration
- Water pump, replaced, was squeaking, thought it could be sucking air
through the bearing (cam driven not belt), did not fix the problem
although the engine doesn't squeak anymore! (have to take the good
with the bad!)

WHAT I THINK THE PROBLEM MAY BE
I think i may have done a head gasket. But i am open to other
suggestions from anyone who may have a bit more knowledge on the
engines. I figure water is pouring into the exhaust (or exhaust into
the water) when the engine is under load, creating a hot air pocket
under the engine side thermostat which is preventing it from opening
and causing the problem. Potentially it is only when under load that
this seal is "blowing out", which is why i can run all day so long as
i take her to speed slowly. There are no other leaks, could the
cooling circuit be sucking air from anywhere else?

CAN YOU HELP?? (PLEASE!)
Before heading down the track of tearing the engine apart, i am hoping
that i could get some comment on my "diagnosis" and any other
potential problems i may have missed. Also keen for some advice on the
following:
- Is there any way to confirm this (given that it is an intermittent)
prior to pulling the engine apart? Especially with relation to which
cylinder bank has the problem.
- Although i do know the common sense answer to this question, am i
likely to do the engine any harm running without a thermostat (other
than running a bit rich on cool start calibration) to keep the engine
cool and not putting it under load. For various reasons, ideally i
need to get a few hours out of the engine before i can get this
maintenance done.

Apologies for the long post. This problem has stumped at least half a
dozen marine mechs over here (Australia) and i am keen for resolution.

Cheers
Michael S