Possible Overheating

P

Private Nemo

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Joined
Aug 8, 2012
RO Number
32985
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5
Hi , I have twin Mercruiser 1986, 260 motors. One motor runs at 145 degrees , never climbs above that. The other motor at idle climbs up to about 185 degrees. But if I increase rpm's , the temp comes down to about 150 degrees. I am not in the water yet, I am running these guys on muffs. New impeller and new thermostat. Do I have a problem or the beginning of a problem.

thanks

Drew
 
FWC or RWC? Do you have the correct T'Stats? If so, one may be defective. Are they mounted correctly? Are the impellers mounted correctly?
 
Both are brand new. Replaced both a month ago. Do not believe I made a mistake, but anything is possible.
 
Did you find all the pieces of the old impeller? Do you have an infrared heat gun to check to see if you are relly running hot? The fact that the temp comes down as you increase rpms. suggests decreased water flow that side.
 
Hi , Thanks for the reply. When I took the old impeller out , it was in good shape , not chew up. I did go around with a infrared heat gun and did not find any spot over 166 degrees. But gauge says about 185. The gauge and sender are new.
 
Sometimes you just can't get enough volume from a garden hose. If one engine is good and the other isn't start with the easiest things. Do you have the check ball thermostat housing? If so the springs could be weak. Another thing how old are the manifolds and risers? Your engines are old so be sure that all the block drains are clear, remove them and take a small screwdriver and clean out sand and rust also do the same to the manifolds.
Rob
 
If it does the same thing when floating, one possibility is a scored RW pump housing wear surface that is preventing normal suction level at low RPM when the impeller vanes need to have tight seal to that surface . Any slight scoring there can make a lot of difference in flow & temp. At higher RPMs the vanes doesn't even contact the housing and the pump just acts as a centrifugal pump.

On the other hand , from your checks with the IR gun either the new gauge or sender may be off if you checked right next to the sender near the thermostat. You can check the thermostat in a slowly warming pot of water with a cheap candy thermometer, but if it allows the motor to cool at higher RPM it's probably not the thermostat .

You can try swapping out the water pump and thermostat one at a time between engines to see if either causes the temp to follow. Gauges and sender too, one at a time.
 
Given the IR doesn't show the same readings as the guage is reporting, a quick check would be to swap the leads on the guages to see if the readings swap. May be much eaasier to verify at the dash than swaping things around in the engine room...
I have one temp guage that shows 5 or so degrees above the other. Found it by swapping the leads and the "cold" engine started reading higher..Hence the guage is off...

WRT the temp coming down when RPM's are added would lead me to suspect that there is a flow issue..
 
I can't add anything. These guys got the particulars.
 
" Stupid Human Tricks ". With that said I fix it; for those who are interested. When I replace the impeller there is a tube that goes from the lower unit to the upper unit of the outdrive. This supply's water to the engine. At the top of this tube there is a rubber bushing . I crushed it , so it was partially blocking coolant flow.

a big duh

thanks for the replies, appreciate it
 
Good save and it's always good to hear the solution.
As an inboard guy, I tend to expect that is what others are referring to if I/O drive is not mentioned in the post.
 
Drew,
Thanks for letting us know! The fact that you diagnosed and repaired it cancels out the duh. Listen I've even had to pay people to fix my screw ups so don't beat yourself up. Glad it's fixed enjoy the season!
Rob
 
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