Mercruiser 5.7L Engine UNDER-heating..

I have a 2000 5.7 Merc w/B3 outdrive in a 2001 Bayliner 2655. Starting last year I noticed little to no activity on my temp gauge. It stayed at, or near bottom.

This year, replaced sending unit (even though it checked out) raw water pump (impeller was only slightly damaged), new 140 degree Tstat, new Temp gauge (not the problem, but wanted new dash gauges). I grounded the temp gauge wires at the sending unit, and it registers as expected, so it's not a wiring problem to the bridge.

I flushed entire system from outdrive to pump, from pump to thermostat housing, from housing through risers and back out the exhaust system. Ran with the muffs, water spews from outdrive as always. All cooling systems check out however, when running it reaches 140 and you can place hand on t'stat housing feel it get warm, then cools again within 30 seconds. Temp gauge shows almost no activity and sits at, or near bottom. Stays this way for 5 minutes, and then repeats. It seems my engine is running way too cool. Risers are warm somewhat evenly.

This is a real head-scratcher..
Eternal thanks will go to the one knowing this secret.

Re: Mercruiser 5.7L Engine UNDER-heating..

You should be running a 160? thermostat, not the 140?. Replace it with the correct Merc stat and see how it works. You should also get one of those temp guns, lot more accruate than "Feels like"?

Re: Mercruiser 5.7L Engine UNDER-heating..

I hate to jump in on you'r problem But do You even need to run a thermostat on the merc 5.7 If so should it be a 160 even on a 1985. Thanks and good lock with you'r problem I have the opposite overheating.

Re: Mercruiser 5.7L Engine UNDER-heating..

badbowtie said:

I hate to jump in on you'r problem But do You even need to run a thermostat on the merc 5.7 If so should it be a 160 even on a 1985. Thanks and good lock with you'r problem I have the opposite overheating.

Click to expand...

Do you need a thermostat? Yes.

If you are running a raw water cooled engine, then from my understanding, you need to go with the 140.

Re: Mercruiser 5.7L Engine UNDER-heating..

Don is correct, the latest are 160s. Remember, they control minimum temp not maximum . . . so running without is never a fix for overheating unless the old one was stuck closed, and of course then you need a new thermostat, not a removal . . .

Re: Mercruiser 5.7L Engine UNDER-heating..

Know I went to my local marine today and they looked it all up and it called for a 140. But you are saying 160. It is a reputable marine shop.

Re: Mercruiser 5.7L Engine UNDER-heating..

badbowtie,

You need to start your own thread . . . thom's is a 2000 and is indeed a 160 degree thermostat Eitherway, overheating has nothing to do with the thermostat's opening temp (140 or 160). Again, thermostats only control minimum temp, not max . . .

Re: Mercruiser 5.7L Engine UNDER-heating..

Okay thanks and good luck thomgallagher I am sorry to jump in on your thread.