Overheating starboard side of engine

[Carl]

What would cause only one side of an engine to run hot? I have mercury 150hp and the port side runs normal (this side has the temp sensor which sends the signal to the guage). The starboard side runs HOT !! This side houses the sending unit which activates the high temp alarm. I replaced the sensor and sending unit. The temp guage reads normal, but the alarm sounds. I found the the port side felt normally warm to the touch, but the stbd side was so hot that I could barely keep my hand on the head. The water runs well out the telltale.

Any thoughts?

Thanks

[THEFERMANATOR]

Thermostat would be my guess. There is a thermostat in each cylinder head in the top of it where the hose connects up at. Also the poppet valve is a big concern with overheating on MERCS as it controls when the t-stats get bypassed to allow for full cooling. The poppet valve is located on the lower stbd side of the engine and is very common for it to get salted up over the years and stick open causing overheating. Normally it will cause it on both sides though. I would start by putting in a set t-stats first and see if that does it. And also make sure the hoses coming off the housings are in good shape with no kinks in them and are free flowing.

[Carl]

Thefermanator,

I replaced the soft parts of the poppit valve and ran the engine without t-stats just to see if they were creating the problem. It seems to be happening regardless.

Another crazy thought - could this be an oil issue? The previous merc I had did not have direct oil injection I just premixed. This engine had the oil pump and tank all connected so I left if assembled. Does the oil pump feed the engine between port and starboard or top middle and bottom through the carbs?

Thanks

[THEFERMANATOR]

The oil injection is fed through the fuel line before the carbs so all cylinders get the same fuel oil supply. Take the hoses off teh t-stat housings and see if water comes out of them with it running. You may have a plugged hose or a blockage somewhere inside the block.

[Carl]

Thanks for the info on the oil injection. I'll check the hoses too. Another question - If there is a blockage in the engine block, would I gain enough access by pulling the cylinder head, or would it require a more invasive disassebly?

Thanks again

[THEFERMANATOR]

Most problems with low flow can be traced back to a stuck open poppit valve, but if there is a blockage then yes the heads will have to come off. i've never heard of this kind of problem on the MERCS though.