Another Mercruiser overheating problem

[Tin Cup Chalice]

I have a 1985 Mercruiser 5.7, raw water cooled, no heat exchanger. It is in a 25 foot Formula Performance Cruiser. Used exclusively in Salt water. The problem I am having is unlike the others I've read. My engine is overheating at IDLE. The temp will increase to 200plus when at idle or headway speed. Once I throttle up to 1200rpm or more, the temp begins to drop slowly. I changed the circulating pump in the spring, and recently replaced the thermostat (140 degree). I also removed the thermostat housing and cleaned any rust and scale from the inside. I have hot water in the return lines, so I know the thermostat is opening. Can anybody give me some direction as to where I should be looking next?

 

[captmello]

Re: Another Mercruiser overheating problem

assuming you have an alpha one, sounds like the impeller in the drive should be changed or your risers are getting old and corroded from the salt. When were these parts last replaced/checked?

 

[Bondo]

Re: Another Mercruiser overheating problem

Ayuh,... Replace the Raw water Impeller,+/ or the whole pump...

 

[Tin Cup Chalice]

Re: Another Mercruiser overheating problem

Thanks guys... I was afraid you were gonna say that! I think the risers were replaced 3 years ago by the previous owner, I will have to check the records. I believe I can test the impeller theory by using muffs and forcing water correct?

 

[Bondo]

Re: Another Mercruiser overheating problem

I believe I can test the impeller theory by using muffs and forcing water correct?

Ayuh,... If it hasn't been Replaced in the last Year or so,.... Replace it...

 

[Tin Cup Chalice]

Re: Another Mercruiser overheating problem

OK.. Thanks for your hep Bond-O.. Ill check it today.

 

[Bondo]

Re: Another Mercruiser overheating problem

Ayuh,... Don't forget to come back,+ Up-date on what you find...

Btw,...
When you find it's shredded to pieces,...
Be sure to Backflush from the T-Stat housing to flush out the missing Pieces....
Otherwise, the bits will come back to bite you in the butt...

 

[Tin Cup Chalice]

Re: Another Mercruiser overheating problem

Latest update.... I put the fresh water muffs on AND lowered the drive into the water. Same result. pegged the needle at idle.Increased the RPM to @1200 and the temp came down to @ 160-180. guess its time for that impeller. I will update when I get to that. If anyone has other suggestions.... feel free to throw them out.

 

[Tin Cup Chalice]

Re: Another Mercruiser overheating problem

Latest update... I removed the hoses leading to the risers and flushed them. Some debris (rust,black water) was evident. Pulled the drains on the manifolds and ran water through them. (same scenario) reinstalled everything, and took her for a ride. Temp continued to rise above normal at headway speed, but not quite as high. (190-200 this time). Temp declined to 160-175 at 1200 rpm. Upon inspection, right riserwas Hot to touch, but left was noticeably cooler. I think Im making some progress. Would it benefit me to pull the risers off and inspect/clean then thoroughly?

 

[deves3000gt]

Re: Another Mercruiser overheating problem

It might but like stated above it;s time for an inpeler.