Uneven Cylinder Head Temperatures Johnson 150HP 1987

[Will Bark]

My engine is a 1987 Johnson 150HP model J150tlcur; while running the engine on the muffs I noticed that the cylinder head temps felt different. After idling the engine at 950 RPMS for 15minutes I checked the temps with an infrared temp gun. The port heads were averaging 125 degrees and the starboard heads were at 156 degrees. Water pump is working well and is peeing good. Water temp exiting the lower exhaust ports was at 110 degrees. Air temp down here is 86 degrees. I'm suspecting thermostats need replacing as a first action. Any input would be appreciated.

 

[Will Bark]

Re: Uneven Cylinder Head Temperatures Johnson 150HP 1987

"Bump"

 

[emdsapmgr]

Re: Uneven Cylinder Head Temperatures Johnson 150HP 1987

The stats (one per head) open at 143 degrees. I'd expect that engine to idle between 143 and 155 in the lake. On the garden hose, maybe a little lower, as the city water supply could be cooler than the lake water now. Anyway, both readings indicate a big spread, so good idea to invest in two new stats. When you have the housing off, check the nylon pressure relief valves to be sure they are not heat-deformed, as they may not function properly when on plane and additional cooling is required. After the stats are replaced, I'd recheck the temps as you have done already. Hopefully they will be different. If you still have a similar temp spread, I'd pull the head covers (not heads) and visually check the water passages for any restrictions. (even including salt deposits.) What you see inside the head covers will also be in the water passages around the cylinders....

 

[Will Bark]

Re: Uneven Cylinder Head Temperatures Johnson 150HP 1987

Thanks for the reply, stats are here and I went ahead and ordered pressure relief valves just in case along with necessary grommets and gaskets. Will report back when installed.