Strange Cooling Problem

[Lucky Penny]

I do not know if anyone has ever run into this one but it is new to me. we were out on the bay and set the hook, enjoyed the day. as I was getting ready to pull the hook i started the engine (started right up no problems) set the throttle to idle 725 rpm and let it run will recovering the anchor. chatted and joked for about 5 minutes then went to the helm station engine temp was nearly 200F. I shut her down raised the I/O to make sure that the water intake was clear (it was) lowered the leg and restarted the engine. this time I put her in high idle (1200rpm) and the temp returned to normal 150F dropped it back to normal idle (725) and the engine started to get hot again. put it in gear and started back to the dock. everything went fine until I edged the throttle to 3200rpm then it started to overheat again. dropped rpm and at 2700-2800 rpm the temp stayed normal, above 2800 or below 1200 it would start to over heat.
Question: which pump is it could it be the pickup pump in the lower unit or the circulating pump. one is an easy fix the other a real bear
the engine is a 1977 small block chevy (mercruiser) 5.7L 250HP

 

[Bondo]

Re: Strange Cooling Problem

Ayuh,... The Impeller in the raw water pump... In the drive...

 

[Lucky Penny]

Re: Strange Cooling Problem

I was hoping that would be the problem. although i do not have to pull the engine to get at the circ pump i really do not enjoy standing on my head. I have never had a pump failure before, I alway replace the pump every 100 hours or so, and the current pump has less than 50 on it. I would never have guessed it to fail with so few hours on it. what is the primary cause?
by the way, thank you

 

[Don S]

Re: Strange Cooling Problem

All it takes is one plastic bag over the intake, and instant overheat.
When you shut the engine off, the suction was gone and the bag fell off.
Just an observation, have no idea if that is what happened or not, but it happens all the time.