87 comp temp / water pump issue

[steelers]

87 supra comp with pcm 350 chevy

tstat usually kicks about 170, but it started getting hot (north of 200), so I got towed back in. before I shut it down, I noticed that getting on it a little or really just changing rpms would quickly drop me back to normal running temp or cooler. then it'd climb back up.

changed impeller, tried again. same results. got hot, some throttle would bring it back down, hot again, change rpm, back down. I babied it back in that day making sure it never got much past 200.

so I ordered a new thermostat next. I had an auto one in, found out this one is special, so figured that had to be it.

I get the new tstat in and fire it in my driveway with a hose in a bucket. after watching the temp gauge for a minute or 2 at idle, I notice the bucket starts to overflow. I give it some throttle, then bring it back to idle, bucket gradually drains. for maybe 20 seconds, the bucket stays empty, boat is pulling water as fast or faster than the hose (which is usually the case). then, bucket starts filling back up and overflows. throttle again, back to idle, empties. repeat.

so its like the engine stops pumping water after some time, then a rev or change in rpm gets it pumping again.

any idea what is malfunctioning here?

[h2o-ski]

On my last boat, at idle the water pump sucks less water than what the hose flows. 1800 rpms would be close to the flow from the hose.

If water is coming out the exhaust, your raw water pump is pulling water.

I also had an issue where one of the exhaust manifolds would get vapor locked. I would just pull the drain plug until water flowed. It would do similar to your issue and temps would spike until the vapor lock was removed.

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[Kevin1522]

Thermostat closes after cold water enters engine and then higher rpm heats it up and stat opens and more water gets drawn from the bucket?