1995 Johnson OceanRunner 175 Operating Temps via IR Gun

Group,

I'm having some intermittent overheating issues with my Johnson OceanRunner. Sometimes it runs for 3 hours and than overheats, other times I can't even idle at 1000 RPMs out of the river the boat is on before it overheats. It is the port side head and I'm hoping it is just the thermostat. Each time it does it if I remove the thermostat and clean/tap it, put it back it runs for the rest of the day without overheating, or at least the alarm doesn't go off.

Which leads me to my question. Now I don't trust it and don't want to hurt the motor. I've read stories here of guys claiming a single overheating cylinder has blown up motors, even though the alarm wasn't going off. I'm going to replace both thermostats, but I am going to bring an IR temp gun anyways just to check and make sure everything looks good.

What temperature should I get after 10 mins of 1000rpms? How about after 10 mins of 4000rpms?

If this fails to fix it, I guess impeller would be next. But I seem to have good pressure visually, and 5 to 8 pounds at 1000 to 1500 rpms, 18 to 20 pounds at 3500 to 4500. And its less than three years old. On the other hand the boat is used in shallow, muddy, bay water. So maybe the impeller is the issue.

Anyways, no matter, I'm wondering what temps I should get shooting the heads/cylinders with the IR gun.

Thanks.

My experience with port bank running hot.
The thermostat was warped and possibly binding in the grommet.( they unseat as pressure increases)
Both thermostats had sand in them.

I replaced both, but kept my old springs as they were stiffer and the engine idled 145 - 150 or so instead of 135 with new weaker springs.

You are on the right track.

I cant remember cruising temps but I think 165 or so.

Thank you!

I'm doing both the water pump and thermostats today.

First thing I did was pull the thermostats out and the housing cap off. Put a bucket under the housing and ran the motor for 20 or 30 seconds to flush anything floating around out. ( I had seen debris on the bottom of one of the stats when I pulled it a few days earlier ).

Than I ran the motor without both thermostats ( caps on ) for 10 or 20 mins or so at or just above idle and no area of either head ran above about 115.

I put the thermostats back in and it the starboard side ran about 155 to 165 and the port side ran about 165 to 175, all below 1500 RPMs. I scanned it multiple times and the temps were pretty consistent.

But ... I had to wait for someone at the ramp, so I figured just out of curiosity let me check them again. This time, the starboard side was running at 165 to 175 and the port side was 200 to 210. I don't know how long it had been that hot, but the horn never went off ( and it works since that is how I knew it was overheating in the days prior ). And again I never went above 1500 RMPs ( we we're crabbing in the bayside creeks ).

Went to pick up parts and told the guy about it scanning at 200 to 210 but no horn and he asked where I had scanned it. I told him the top of the cylinders and he said that is probably why it scanned a little hot but the horn wasn't going off. Since you are basically scanning the top of the combustion chamber. He said they scan them around the tstat housing and right next to the temp sender to check. So he didn't think I hurt anything and said that is why the horn wasn't sounding. I'll feel better when it has new thermostats and water pump for sure.

Just an update, maybe someone will come looking for similar info at some point.

As stated above, shooting the cylinder heads at the ramp got my temps of 165 to 175 on the starboard side and 200 to 210 on port side, with no alarm. Mechanic said alarm probably didn't go off because the cylinders were cooler and they cylinder heads are usually hotter because of combustion.

Well I did the water pump and thermostats late last week. Took the boat out and ran it for awhile.

Two noticeable differences: When checking the cylinder head temps I now got 145 to 160 on all 6 cylinders. Starboard side may have been a few degrees cooler.

I gained about 5 pounds of water pressure at all RPMS below 2000. Didn't gain much if anything from 2000 to 4200. Which just from some research indicates a worn out water pump impeller or bottom plate. Which also would make sense as to why the three or four times the engine actually overheated and sounded the alarm, it was running at idle or no wake speeds.

So IR temps on cylinder heads with no water pump and tstats were 145 to 160, pretty consistently whether at low RPMS or 4000.

Water pressure at:

800 RPMS - 10
1000 RPMS - 12
1500 RPMS - 15
4000 RPMS - 20

In case anyone runs into similar issues with a similar motor.