TriStar 190 Running Hot
I picked up a '89 TriStar 190 with 351 Indmar last spring (2012) that had just under 600 hours on it. It always ran a little on the warmer side at around 170-180F. Towards the end of summer, it started to see 190-200+F. So I disassembled the water lines to make sure there was no blockage in the lines or the transmission cooler... only pulled out one piece of seaweed. With no blockage in the lines, I replaced the impeller and the thermostat (160F). I didn't see any improvement after the impeller and thermostat were changed, so I tried a 143F thermostat for the heck of it... still no luck. I put the 160F thermostat back in. Next, I replaced the circulation pump. The one I removed had some corrosion, but all the fins seemed to be intact. The new one was a bi-directional marine replacement from ski-dim. It didn't seem to bring the temperature down at all. That was at the end of last summer, so I winterized it and called it quits till spring.
This past spring I pulled the exhaust risers and intake manifold to check for blocked cooling channels. The cooling channels were corroded (not nearly as bad as I thought a 24 year old boat would be). None of the channels were blocked though. I replaced the gaskets and bolted everything back up. I took the boat out and took note of what the temp gauge was doing and when... After launching the boat and idling out of the no-wake zone (8 minutes), it was at about 170F. Brought the boat to plane and cruised for about 15-20 min and got to 190-200F. Took a couple ski runs (20 min) and was steady at 200F. Stopped the boat for about 15 minutes to rest up. When I went to start the boat back up, the temp gauge was at 200+F. I started it, and within 10 seconds the gauge started dropping till it stopped at 175F, hesitated for 30 seconds or so, then started to climb back to 200.
This got me thinking about the thermostat... maybe I have a nearly new bum thermostat. I pulled it and put in a pot of hot water on the stove. It started to open up at about 163F. OK, so maybe the thermostat is not seeing this temp in the housing. I hooked the boat up to the garden hose and fired it up. I let it warm up at idle for about 5-10 min till the gauge reached 170F and used an infrared thermometer to measure certain areas. The surface of the intake manifold ranged from 122-165F (165F right next to the temp gauge thermocouple). The exhaust manifolds/risers were between 108-122F. The thermostat housing was at 78-85F.
That has me very confused. I realize that the motor may need some more time and RPM to truly warm up, but should there be that much of a difference between the thermostat housing temp and the temp by the thermocouple? Particularly when the thermostat should be open due to the motor temp being over 160F.
Any thoughts/input/help would be greatly appreciated.... after all, summer is here and its time to ski.