Weird overheating problem on Merc 3.0L

[Pupek]

I have a problem with my 1984 Mercruiser 3.0L: It is getting very hot at the block and head - especially toward the rear of the engine. The temp guage reads 140 and the t-stat housing seems fairly cool. The water intake hose is cold, and the manifold is just a bit warm to the touch (not hot). The T-stat is brand new, but I checked it anyway - it opens at about 145. Checked the circulating pump - it seems OK - no leaks/drips, and the 'fins' look good. No issues with loose belt, etc. I'm about to replace the lower-unit impeller, but I'm not sure this will really help. It seems like water just isn't circulating well through the block. From what I can tell there are no obstructions in the block. Anything else to try? Do these symptoms sound like a bad impeller? It was replaced 3 years ago.

 

[180shabah]

Re: Weird overheating problem on Merc 3.0L

It is getting very hot at the block and head

How hot is "very hot"?

Do these symptoms sound like a bad impeller? It was replaced 3 years ago.

Replace it, it is overdue by a year or two.

 

[Pupek]

Re: Weird overheating problem on Merc 3.0L

Very hot is: too hot to touch without getting burned, the spark plug boot on #4 is smoking.

 

[Schmoe]

Re: Weird overheating problem on Merc 3.0L

that's too hot. I'd take off the water line coming off of the outdrive to see if you got a good flow. You may just have some blockage in the line or in the water intake on the drive itself. I wouldn't automatically assume the impellor is shot. I, and a lot of others, have had the same impellor in the outdrive for well over three years and no issues. It also depends on the water and way you drive.

 

[crownline182]

Re: Weird overheating problem on Merc 3.0L

Had a very similar problem earlier this summer with my 1993 3.0 XL. Problem was the exhaust manifold and riser cooling water passages were both obstructed. Replaced both, and cured the problem. Unfortunately the parts are expensive.

 

[Pupek]

Re: Weird overheating problem on Merc 3.0L

Just replaced the manifold with a brand new one - I suspect the heat may have cracked it. At the time I was thinking it rusted through, but now that I see this heat problem. it seems it was heat-related. The riser is still clean.
Is it possible to check the waterflow with muffs? Won't the hose 'push' more water up to the engine than the impellor would on the lake? What is considered 'good flow'?