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Help! Need overheat advice.

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upwindskipper

New member
Engine is a 2002 50HP, 2 cycle, 3 cyl. I am overheating after about 5 minutes underway, will idle for 30 minutes with top cylinder hovering around 140 degrees but overheats with any load. If I block the signal stream, the temp will drop by about 5 degrees. I have replaced the thermostat (110 degree). I did the full water pump rebuild kit with impellor, gaskets, bottom plate and water tube. I have sent high pressure water up the tube and down from the thermostat hole. I have done a full Salt-Away treatment. I also did a flush with vinegar. Of note, I did not shut down immediately with the initial overheat. I thought it might be the faulty sensor in my oil tank so I tried to get to a dock, ran maybe 90 seconds with alarm. At that time, it was clearly a bad thermostat. I don't know if I did any internal damage but following that, compression numbers were consistent across all three cylinders. When idling or running (before overheat), engine is smooth.

Any advice most welcome. I have no more ideas. Summer is half over and I have yet to go boating.
 
Jeff: I have just had the Mercury shop replace the water pump, they tell me that the motor ran fine with muffs, idled for about 30 minutes with temp around 120, signal stream strong. Just took the boat for a spin, after 5 minutes underway at high idle, the overheat alarm goes off! I was running without the cover, quickly measured temps, top cylinder right around 178 degrees. It appears that the overheat alarm goes off at 180. Is that the the appropriate temp threshold? This is a 12 year old motor that ran fine for the last 11 years so it's not cavitation. Do I need to tear the whole powerhead down to check for a blockage?
 
With the professional water pump rebuild, I believe I can idle all day without overheating. At 2000 RPM, in gear, it's a steady climb to 180 degrees whereupon I shut her down. Signal stream temp is 90 - 100 during that time. If I idle down, in neutral, from 175 degrees, the engine will eventually cool down; interestingly, the signal stream is then too hot to touch, about 115 - 120.

If it is a blockage, how do I find it?
 
The usual method is to back flush the power head from the t-stat housing with the lower unit off. I use a combination of water followed by compressed air, then repeat a few times.

Jeff
 
Jeff:

I performed that flush in both directions, with water not air.

The temperature climbs very quickly when I advance the RPM's and the signal stream cools off. I don't understand the layout of the cooling loop but is there a chance that a head gasket leak is pressuring the cooling loop and keeping the water from reaching the top cylinder...just a thought by an amateur, will appreciate your thoughts.
 
upwindskipper said:
Jeff:

I performed that flush in both directions, with water not air.

The temperature climbs very quickly when I advance the RPM's and the signal stream cools off. I don't understand the layout of the cooling loop but is there a chance that a head gasket leak is pressuring the cooling loop and keeping the water from reaching the top cylinder...just a thought by an amateur, will appreciate your thoughts.
Click to expand...

Wouldn't a head gasket leak show up in the compression test?
 
I'm afraid water alone won't do it. You need the blast effect of compressed air AND water. That's what radiator shops use.

Jeff
 
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