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57 Mercruiser temperature issues

R

rand2474

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"Help! I have a 5.7 liter Mer

"Help! I have a 5.7 liter Mercruiser w/ Alpha one outdrive in a 1996 24' Maxum. Both were rebuilt in 2006 (or so I was told). They look new. Anyhow, I took the boat out this past weekend and after a short while at around 3000 rpms, the engine alarm sounded. The temp gauge was over 200 degrees. Once I reduced throttle, the temp went down but even at lower rmps, the temp gauge would rapidly fluctuate between 150 and 210 degrees. Any ideas where to start?"
 
"Impellor is a very likely sou

"Impellor is a very likely source, followed by thermostat. Both need checking out. Also if the impellor has disentegrated, small pieces could be blocking the water tube supply to the engine. The recirculating pump could also be involved, but clear the above items first.

If all above do not reveal problem it could be internal problem such as blown head gasket, or evn a freeze crack if improperly winterized.

On a less likely note, a loose/dirty connection at the temperature sensor or guage could cause irratic guage response, but the fact the temps went down (in general) as you reduced speed suggest the problem is real and not just an electrical malfunction."
 
"Check the belt tension on the

"Check the belt tension on the circ. pump. The temp. fluctuation can indicate that the T'stat is working, just not enough cooling water flowing. Is it raw or fresh water cooled?"
 
"Thanks for the quick response

"Thanks for the quick responses! The engine is raw water cooled and I may have put 20 hours on it since I bought it last July. Also, the boat sits in a wet slip in brackish water and the bottom and drive is cleaned monthly.

It doesn't get below freezing here (Charleston, SC) so I don't think anything freeze cracked."
 
"Did it cool OK last summer?

"Did it cool OK last summer?
If so, something is likely blocking your water intake on the drive leg.
Barnacles, crud, who knows what.

Rod"
 
"You may have a seafood buffet

"You may have a seafood buffet in the water passages. Mud and algae growth (slime gell) can interfere with heat transfer. Flush it with fresh water and a hose/muffs. Then run in in a tub of water with the hose in the tub but not connected to the OD and see how the temp. behaves.
uhoh.gif
"
 
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