Just picked up my 36 eliminator from the factory. It has some lightly used 500 EFI's in it. The starboard Engine is overheating to 190+. Eliminator changed the sending unit thinking that, that it would fix the problem. I guess it didn't. They did not check it again after installing the next sending unit and I pulled it home 1400 miles.

I have installed a manual guage and it still reads 190 degrees at idle and up. I have checked the impeller, it is good. I pulled the themostat and it seems to be opening ok and it is a 140 degree thermostat. There seems to be plenty of water coming out the back of the boat. The engines do not seem to be hot but all indications from the guages, it is getting hot.
I am at a loss as to what it may be. There is no indication of water in the oil. The engines were recently rebuilt by Teague I was told. Could there be a head gasket problem or could the oil cooler be stopped up. That is the next thing I am going to back flush out.
Any help will be appreciated.

The first thing I would look for is an obstruction somewhere in the cooling system. Follow the path of all the hoses. You also might want to hook up a water pressure guage and an oil temp guage to help narrow things down.

Back flush the Oil cooler and see if there is any debris. Does the boat have strainers? If you have "Plenty" of water something else must be wrong. Maybe check the timing too??.

More ideas for Robert to check??? He is tied up right now & his trip to LOTO is in jeopardy!!! I am monitoring this for him for now. Needs help ASAP!! Thanks

You may be getting steam pockets in the engine.....you can try and dril two 1/8" holes in the thermostat to help get rid of the air in the system....

If there is any trash in the cooling system it gets caught at the fuel cooler right below the harmonic balancer. Pull the hose off and you can feel inside the cooler if there is anything, it's kind of tricky because of the bend in the pipe. It's the hose that comes off the out going side of the pump. Good luck blairjeb

Thanks I will try that and let you know. Tomorrow, I have an appointment at the shop if I can't figure it out.

It may have the wrong thermostat bypass gasket. I have a lot of information on the Mercruiser thermostat housing on my new web sit. Check it out at www.mooreperformance.org
Sincerely
Dennis Moore

Based upon the description you gave in your first post you have a couple of contradictions that together could be the issue. "Lightly used" and "recently rebuilt" are rarely used in the same sentence in boating. Begs the question, why were they rebuilt in the first place?

If there are no obvious obstructions in the external systems (and if there were I would suspect the oil cooler before the cool fuel in a 500EFI) I would suggest that you remove the two block drain plugs and flush the block. If the blocks were tanked and not properly cleaned out, you could have all manner of crap in the block and heads.

Not to stir the s**t but are you sure that these are real 500's? Is it possible that the block or heads are older Mark IV pieces mixed with newer Gen VI parts? There are some coolant passage issues that could bite you there.

Finally, is your warning buzzer going off? The computer temperature warning buzzer is supposed to go off at 195°. It is driven by another sensor (separate from thr gauge sensor). If your are gauges are reading over 195° and your buzzer is not going off, you could have a sending unit issue (again).

Fountain 38,You said the thermostat "seems"to be opening OK? The holes are a great idea but for diagnostic reasons just remove the thermo.Sometimes we look for the worst things and it is the simple things get overlooked.Bob