On Sun, 18 Jan 2009 22:46:24 -0500, Jim7492383432
wrote:

wrote:
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
Check suction side of water pump for air leak, obstruction, collapsing
hose. Check exhaust for blockage. Gaskets wear plates and seals in water
pump may need replacing.

That's what I was thinking to check first.
No sense assuming a head gasket and doing unnecessary work when
there's cooling system stuff to check out.
That "blipping" of the throttle cooling it down might be a clue
pointing at a collapsed hose, or some change in boat attitude
affecting the inlet stream.
Can you get pressure gage on these raw water systems?

--Vic