Overheating at Idle

[vomv]

I need help with my mercruiser 5.7. I changed the:

Outdrive water pump
Thermostat
Risers and manifold have 10 hours on them

The engine still runs hot at idle. I ran it for a few minutes today and the risers were cool but the engine was at 190 degrees. The Thermostat housing was also so hot you could not keep your hand on it.

When I rev the engine, the temp comes down to 165. Can anyone help. Thank you

[Ed Donnelly]

Did you replace the thermostat with a factory replacement??.Ed

[MOP]

An idea that may be the cause! If you are raw water cooled the ball valves in the thermostat housing can cause it to run pretty hot at idle. I would doubt if the thermostat is the culprit being as it goes right down to 165 with some RPM. It is obvious you have enough water with cool risers that is what gets me leaning toward the nylon balls, the may be worn or or the spring broke. Worth checking out, if badly worn you can turn them around on the shaft putting the good side toward the housing. The spring tension should not be set to high or to light, if the springs look real nice and the balls are very round not tappered adjust with about 1/8 to 3/16 of thread showing outside the lock nuts.

Phil

[vomv]

Thanks for the responses. Yes I did replace the thermostat with factory specs. I also have a new aftermarket tstat housing I recently installed. Do these parts still have the nylon balls in them since it is not a Mercruiser part?

thanks

[MOP]

I would think they would still use the check balls, there was another post back awhile ago that was fixed with an install of the ball kit.

read post #41
http://www.donzi.net/forums/showthread.php?t=33905&highlight=overheating

[Murphy]

Questions:

What year is the engine?
What drive?
Which Thermostat housing gasket did you use, full open flange, full closed flange, small hole in flange?
Also, check balls do not come in all thermostats. I think the design changed in the late 80s.

All this might help in diagnosing the problem.

Murph

[Murphy]

Check balls are in the thermostat housing, not the thermostat...just a clarification.

Murph

[vomv]

MOP thank you very much for your advice. I checked the nylon balls in the thermostat housing and found that there was no spring to them. I replaced the springs and ran the engine at idle and did not notice any overheating. That was the trick. It is amazing that something so small as a spring can cause such havoc to your raw water cooling system. Thanks again MOP

[MOP]

Thanks! But the real thanks should go to Scot the HM for getting the board going and keeping it running. There are a lot of far smarter guys up here then me. I have learned quite a bit up here as do many others, and I also thank the Scot and the many others who selflessly pitch in to make this the best damn boating site on the net.

Phil / MOP

[rayjay]

...But the real thanks should go to Scot the HM for getting the board going and keeping it running. There are a lot of far smarter guys up here then me. I have learned quite a bit up here as do many others, and I also thank the Scot and the many others who selflessly pitch in to make this the best damn boating site on the net. - Phil / MOP

Here, here!! (CLAP, CLAP, CLAP)

I wholeheartedly agree with Phil! I know a bit about engines, but have learned more about the rest of a modern boat than I would ever have on my own, plus what is unique about Donzi's.

I have an RG500 and an XJ-S, both of which have very active net groups just like this group. And I am sure most of the members here also have some other extremely interesting toys! Guess we all like interesting things with a touch of soul.

Thanks Scot!!

rayjay :wavey: