Not your basic overheating problem

whodunnit

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May 18, 2011
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Mercruiser 140 hp. used regularly, maintained VERY well. During an outing, temp gauge rose from its normal 150 degree position to 220 before it was noticed and shut down. The following remedies were preformed one at a time. Both water pumps replaced. Temp. sender and gauge have been replaced. The use of a hand held infa-red thermometer shows the entire block, manifold, and all but one spot on the head (@ the #1 exhaust port) at 150 degrees. The hot spot rises to over 200 if you let it. So, then the head was inspected, resurfaced, and has new stainless valves. If one didn't know better you'd say it was in perfect condition. BUT, after running approx. 5 minutes the hot spot reappears on the head, this time between the #2 and 3 exhaust ports. The rest of the engine remains at 150; the gauge reads 200+. WTF?
 

Don S

Honorary Moderator Emeritus
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Aug 31, 2004
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62,321
Re: Not your basic overheating problem

It's going to show hot at the exhaust ports on the head right where the manifold hooks up, there isn't any cooling water right there.
What does you temp gun say when you point it at the base of the temp sending unit right where it goes into the engine. Does it match the gauge?
 

whodunnit

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May 18, 2011
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Re: Not your basic overheating problem

150 degrees everywhere except that one spot. Old and new gauges read 150 at first then rise steadily to 200+. Remember that the "hot spot"
moved when the head was replaced and again when the valves were RE-set. I say "re-set" because we did them again thinking it wasn't correct because of the transient hot spot.
 

KCKracker

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Jul 28, 2008
Messages
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Re: Not your basic overheating problem

so is the gauge grounding out? bad?
 

KCKracker

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Re: Not your basic overheating problem

SO LET ME SEE IF I GOT THIS RIGHT.

1-gauge says its overheating
2-but laser says engine really not that hot
?

try a new battery in the laser. then: at he point your gauge says its overheating

get us a reading right in front and behind the T stat,
and also on the side of the block / cylinder passages
and also the hoses.

-- did you do complete pump kit replacements or just impellers?

--have you checked for any leaks? even a tiny leak lowers your boiling point and will spike temp.

--check manifolds for blockage. flow doesnt mean great flow.
 

whodunnit

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Joined
May 18, 2011
Messages
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Re: Not your basic overheating problem

1- Yep
2- Yep

Already thought of it and replaced the battery. Same readings.

150 degrees max. on every inch of the engine block, manifold, and head except for the previously mentioned "hot spot"
(I've been working on this for a few months and am confident in the info )

--over the years I've done the complete kit only when there was wear on the plate. There was a new (4 month old) impeller installed
when the problem arose. Another new impellor was installed as the first thing on the previously mentioned list.

--No leaks detected

-- Manifold was inspected and magnifluxed when valve job was done.

Also, only OEM parts are used, no auto or Sierra. I'm led to think an exhaust valve is not set properly, allowing the hot air to create the hot spot.
IF thats the case, would one small area heat the water and circulate enough to register it on the gauge?
 

kd7isf

Petty Officer 2nd Class
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Aug 20, 2004
Messages
162
Re: Not your basic overheating problem

sounds like the wire between the sending unit and the gauge is damaged somewhere. Or wire supplying power to the gauge.

Are any of the other gauges reading differently than they should?
 

emoney

Commander
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Jul 19, 2010
Messages
2,551
Re: Not your basic overheating problem

Have you verified that the guage, since it's been replaced, will still read 200? Maybe you solved the problem the first day.
 

dingdongs

Chief Petty Officer
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May 29, 2009
Messages
649
Re: Not your basic overheating problem

is the thermostat opening and test in a boiling saucepan.is there a blockage in thermostat housing?
 

whodunnit

Cadet
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May 18, 2011
Messages
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Re: Not your basic overheating problem

I'm sorry, but I don't understand your question. Please re-phrase or elaborate.
The guage question.
 

whodunnit

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Joined
May 18, 2011
Messages
18
Re: Not your basic overheating problem

Thermostat (s) working fine. Tested the existing - OK. And The new before installation. All hoses, fittings, housings and manifolds are clean.
 
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