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1990 90hp stubborn overheat problem

R

riopga

Regular Contributor
Here's what I have tried:
Replaced entire water pump kit w/ new impellor
Replaced thermostats and poppit valves
Removed cylinder heads and replaced water diverters
Forced high pressure water through copper tube
Cleared small hole in thermostat housing
Notched the poppit valves

It will run at 4200 rpms for over an hour with no issues, but at 4500 rpms it overheats in about 10 minutes. The motor was trimmed down.
 
It will be a minute before I have time to remove the cylinder heads.
If it makes any difference, I use Johnson engine tuner once a year. Compression is between 112 and 118 on all cylinders.
 
Do you have a water pressure gauge installed? You might try that and watch the pressure as you approach 4500RPM and see if it starts to drop.

With everything you've done it sounds like the engine side of the equation is in good shape. I'm curious if you may have an issue with motor height relative to the keel or some device installed on the transom too close to the LU that is causing cavitation around the water intakes. It may not be enough cavitation to cause prop slip but if you're getting air bubbles in the water stream it could cause problems with cooling.
 
I posted a similar problem on this website and several others. On one site, a guy had the exact same boat and motor as mine and he had figured out the problem. I have a live well in the back of the boat and I use it to store my anchor. SInce the rope is tied to a cleat on the transom of the boat, the lid on the live well can't close completely because the anchor rope comes out from under the hinged lid. I also have the drain plug out of the live well because I lost the one that I use in a a built in cooler in the bow. My motor was constantly overheating at around 4,000 RPM's. The guy with the same boat and motor said he realized that his motor did this during the summer when island hopping and pulling kids on tubes, but once fall fishing started, he didn't have any problems. He realized that when he reached a certain speed/RPM range that the transom was out of the water enough for air to be sucked from the open lid of the live well and through the through hull overflow from the baitwell and causing cavitation around the water pickup on the motor! He said when he replaced his impeller it was all burned up from running dry. I had removed and cleaned my exhaust cover, cylinder head covers, cyinder heads and had removed thermostats and replaced poppit valve. I'd replaced the impeller a couple of times during that summer too, but still had the problem. All I had to do to fix the problem was to put the drain plug back in the live well! So, there's lots of ways to cause an overheat situation!
 
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