Johnson Head Temp

[wreckless]

I recently replaced the water impellor and thermostats on my 1988 175 hp Johnson. I checked the head temp with a laser thermometer. All three heads on the left bank were running 135 degrees; the right cylinders were reading, from top to bottom, 145, 165, and 178 degrees. Can anyone tell me what the normal head temp should be for that engine and what would make the right bank run hotter than the left or is that normal due to the water flow. I would rather not pull the heads to check for blockage if not necessary.

 

[wilde1j]

Re: Johnson Head Temp

Around 140?F is a good target. Hot could be sticking thermostat, debris or other blockage (sand, etc.). The right side is the one to be concerned about.

 

[tcindie]

Re: Johnson Head Temp

I think in the "adding a tell-tail" thread it mentions that the starboard side tends to develop air pockets and run hotter than the port side, if you were to add a tell-tail tapped into the top cylinder on that side it may correct the imbalance. But I'm pretty noobish, so don't take my advice as gospel.

 

[wilde1j]

Re: Johnson Head Temp

Not that much.

 

[iwombat]

Re: Johnson Head Temp

If it were air-pockets the top would be the hot one. This is probably either blockage, t-stat, or some bad deflectors.

 

[emdsapmgr]

Re: Johnson Head Temp

The thermostats open at 143, so a warmed up engine should idle in the 145-150 range in the summer in warm water. Your engine will sound a hot horn at 212 and it will silence at 175, so your bottom cylinder is running hot at 178. The water flow in the block on the starboard side is from bottom to top in the block. (around the water deflectors.) Then through the thermostat housing at the top, then down the head cover from top to bottom. It would make sense that the head cover on the right side would be hotter at the bottom, just before it dumps back into the adapter. I'd probably pull the right thermostat cover and check the new thermostat to see if it operates to spec. I'd pull the starboard head cover and have a look. Does the starboard side cool off when on plane? Double check the telltale hose. It should run up the back of the block, to the top of the engine, then curve back down to the lower cowling nipple.

 

[JoLin]

Re: Johnson Head Temp

According to "the book" for my '95 175 hp, when using an IR thermometer, you shoot the port and starboard heads right above the thermostat housing. That's where you should take your measurement. For my motor, correct reading is 140 +/- 15 degrees.

It makes sense to me that your lower cylinders would read hotter as they're farther away from the cooling water coming into the top of the head, but unless you have reason to believe that one or more of the lower cylinders is running abnormally hot, I don't know if those readings really mean anything. The Johnson and Seloc manuals don't mention them.