Please Help - Going crazy trying to figure this out:
I have a 1988 Pursuit Tiarra with 2 - 350 Chevy Merc Motors, straight drive. EACH motor has a heat exchanger and raw water pickup system. The boat is kept out of the water on a lift when not in use.
At the end of 2011 Fishing season, I noticed a leak had developed on one of the starboard manifolds. I replaced the manifolds and risers, and all water supply hoses. After this repair, the motor continued to overheat after reaching approximately 2400 rpm's. (We noticed this overheating problem on the temperature guage and steaming out of the starboard side exhaust at rear of boat). We then replaced the raw water impeller, and new 160 degree automotive thermostat. Test run showed overheating problem same as before.
A mechanic then did a flow test comparison with the port side motor, and found that the starboard engine had better flow/volume than the port side. We inspected the heat exchanger and it appeared clear and clean of any debris. But we still ran a wire through each tube to assure there was no corrosion buildup.
With all of the confusion, we then decided to do a compression check, and found that 2 cylinders had low compression so we decided to change/rework the heads. We inspected the water jackets in the block, (keeping in mind that we have a heat exchanger), and there was no corrosive areas in the block. After removing the heads, we discovered that the head gasket was blown, dumping exhaust water back into the water jacket. We had a head cylinder shop rework the heads, and we re-installed with all new gaskets, bolts, etc.
Did test run, same problem! Temperature at low rpm's would read 160, and upon increase of rpm's the temp. increases above 210, close to 220. At which time we limped back in.
It has been our practice to flush the engines with fresh water after each use. We also just recently inspected the water inlet on the bottom of the boat for the raw water pickup supply. All was clear, free of any barnacles, sand, debris, etc. At this time, we inspected and cleaned the sea water/raw water inlet strainer. The strainer/filter did have some small sea shells and sand, but the flow was very free and easy, both before and after cleaning. We tested the water circulator pump and all fins inside pump are brass/bronze with no sign of wear.
We need any suggestions or ideas on what could be the source of this continuing problem. Our only thinking now is to start by replacing the replaced thermostat, again.
If someone could offer ideas, it would be a life-saver!