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3 litre Mercruiser running COLD A mystery

B

be080

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"Hi,I've a Bayliner 3 litr

"Hi,I've a Bayliner 3 litre with Alpha 1 leg, done 150 hours. Two trips ago I noticed the water temp guage didnt move off cold. Got back and saw water circ pump had rusted out and leaking water from seals so I replaced it. Whilst doing the pump I checked water temp guage by earthing wire and got full guage movement ok. Checked coolant lines ok, eng oil ok, put it all back together and went for a two hour run. To my surprise the water temp guage still not registering any temparature. Laid hands on the engine after 30 minutes running and it wasnt even warm. All hoses failed to register any temparature, exhaust manifold was cool, engine block cool, the oil filter was just warm to the touch. Oil pressure guage reading normally at 40psi. After 2 hours of alternating between slow and WOT the hottest I could get it was about a 1/4 temp guage reading which soon fell down to zero reading again after slowing the pace. Engine block/manifolds never more than slightly warm / all hoses the same. The oil filter did appear to get up to temparature after the 2 hours though never hot. INVESTIGATION....> today took the thermostat cap off expecting it to be jammed open. Nope, the thermo was shut and although I didnt remove it I pressed it open a few times and it shut ok without sticking. Again I tested the temp guage by earthing out lead and guage registered fsd ok. Even if the thermostat did lock open whilst running would the engine run this cold. Before this happended the boat always ran at half temp on the guage and there was never a problem. The water circ pump I replaced was rusted rotten, maybe some gunge came off the pump and went through the system but how would that cause the engine now to run so cold. I'm beat. Any ideas would be appreciated....the boat is running great but running this cold is just wrong. Thanks Brian"
 
"I would replace the thermosta

"I would replace the thermostat with a new 143 degree, Old should also be 143. Just a precautionary measure. It may be opening all the way vs controlling temp.

The sending unit is a variable resistor.
It has a resistance value of ~ 60-80 ohms to ~240 ohms.

I would remove the sending unit and clean the threads of both the sending unit and the motor threads. The body of the sender is its ground.

Typically when the temp never goes to a "normal" temp reading it can be traced back to a weak ground connection. Also check to make sure the ground at the back of the guage is also a good ground.

If that does not work than I would just replace the sender as a precautionary measure."
 
"i would bet on the thermo sta

"i would bet on the thermo stat, as you "hand tested" the thing and still have skin left
Were it the guage or sender, the engine would be hot and would read cold. Yours reads cold and is cold. If your lake water is in the 70(or even 80) degree range and your thermo opens full and stays there, i can easily see how this could happen(it is Freshwater cooled, yes?) I can't recall exactly, but my v6 runs in the neighborhood of the 120-130's and I don't *dare* touch the oil filter once she's warmed up

--chris"
 
"Chris,

Mine does the same

"Chris,

Mine does the same and it is probably incorrect.

It did read correctly for many years but I suspect the ground is a bit oxidized or the sending unit is off a bit. For me I do not care as I know the motor is running properly and at a normal temp. If it gets to warm it will show it. I have tested it with the outdrive off.

The reality is when all is right a guage should read 140-150 degrees when warmed up at idle and should read 150 -160 when running hard. On a very warm day or in warm water it may even reach 170ish."
 
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