WOT Overheat - Out of Ideas

[JoLin]

1995 175 hp Oceanrunner (carbed)

A week ago I sucked a load of sand into the motor. The overheat alarm went off and the motor dropped into SLOW mode and I limped home at 2000 rpm.

I hauled the boat out, pulled the lower unit and cleaned out some sand. Impeller was fine and water pump housing was not scored. Found some grass up in one of the thermostats, so replaced them, too. The motor still overheated in the "tank."

At that point I pulled the heads, flushed 'em, cleaned sand out of one of the cooling passages in the block (it was the only one that had any junk in it), and carefully put it all together with new new o-rings, RTV sealant and a torque wrench. Started it up and the alarm went off again. i swapped out the temp sensor with an old one I had around and bingo- no more overheat alarm. (I do believe that the motor overheated when I pulled in the sand, but I also think the first temp sensor fried itself when it happened). Temp at idle (measured with IR thermometer) was bang on spec for both heads, and it had never been so close from side to side. Damn near perfect.

Everything was now fine in the driveway, so wife and I relaunched a couple days ago and took a long ride at our normal cruise speed (4000-4500 rpm). The motor never ran better, and even my wife commented that it was smoother at idle than it used to be. The motor has always run well, but it looked like the head job had improved things all around.

Today I went out by myself and ran non-stop for an hour at cruise. Motor ran perfectly. The bay was calm so I decided to open it up on the way back. After 5 minues at 5000+ rpm the overheat alarm went off again and the motor dropped into SLOW mode. I cut it back to idle speed, and after a few seconds the temp light went out. Back to 4500 rpm for awhile with no problem at all. Pushed it to 5000 rpm again and after about 10 minutes the same thing happened. Rode back home again (30 minutes or so) at 4500 rpm without a peep from the alarm.

Now, I seldom if ever run the motor beyond 4500, but it frustrates me that something is still not right. Any ideas, folks?

 

[CharlieB]

Re: WOT Overheat - Out of Ideas

Is 5K your max WOT RPM?

You may be slightly over-propped and the overheat/overload does not appear until the upper RPM is reached.

Also check timing, a couple of degrees off can cause WOT overheat.

 

[JoLin]

Re: WOT Overheat - Out of Ideas

Thanks, Charlie-

My max WOT rpm is 5300-5400. Johnson spec calls for 4500-5500, so the motor isn't overpropped.

A timing issue had occurred to me (and I'll recheck it when I pull the boat for good in a few weeks), but I checked timing earlier in the year using the "Joe Reeves WOT Timing Procedure" and it looked right.

Also earlier this season I did extensive WOT testing when I was adjusting my new Smart Tabs and never had this issue

 

[Chris1956]

Re: WOT Overheat - Out of Ideas

Do you have a water pressure gauge? My '98 Johnny 150 runs 30+ PSI at WOT. You might check yours. At cruise it should have 20+ PSI. Any less and your water pump ain't doing it's job.

You might replace the thermostats. Maybe they do not open enough for WOT usage.

 

[CharlieB]

Re: WOT Overheat - Out of Ideas

Take a close look at the piston domes thru the spark plug holes.

Do any of them look noticably cleaner than the others? Steam Cleaned?

A slightly leaking head gasket will allow combustion gases into the cooling system, displacing water and allowing the motor to heat up. If the gasket holds at low throttle and only begins to leak at WOT, the engine would remain cool until the point that the gasket begins leaking.

 

[freddyray21]

Re: WOT Overheat - Out of Ideas

Even though the impeller and housing looked good did you replace them? Those pumps run differntly at wot then they do at idle. It is common for a worn pump or impeller to suck air at wot. Sounds like that might be the case here.

 

[JoLin]

Re: WOT Overheat - Out of Ideas

Freddy and Chris-

I've pretty well ruled out any water pressure or water pump issues. As previously noted, there was no overheat problem up until the "accident." I had rebuilt the water pump at the beginning of the season and when I pulled it apart again, the impeller still looked like new and so did the pump housing. I inspected it all carefully. There was no damage from the sand.

The t-stats are brand new, they're holding temp right on spec at idle, and when I held the IR thermometer steady on them for a couple of minutes, I could clearly see the temp rise and fall as the stats opened and closed. No issue there, either.

Charlie B-

It's always possible that I messed up when I pulled and replaced the heads, but I was awfully careful placing the RTV sealant around the cooling passages. My 175 doesn't use gaskets. Each cylinder is o-ringed. They're all brand new and were VERY carefully placed and torqued in stages. I'll of course check it all again (probably pull and reseat them) once I pull the boat for the season. Can't check anything with it in the water.

Could it even be that the heads simply need to be retorqued now that they've gone through a few heating/cooling cycles? I wonder...

Thanks for all the responses, guys. I appreciate them.