I have a 71 Marinette Seacrest with Chrysler 318's. Raw water
cooled, twin impellers.

The drive belt on starboard engine would slip at low rpm. The RWP
pulley was not turning sometimes at low rpm. Pump finally
completely locked up. Engine NEVER overheated or ran hot.

I rebuilt RWP. New impellers, seat, seal, and bearings. Old impellers
were a little rough at tips of vanes but still intact.

After rebuilt RWP installation, engine will run between 210 and 220
degrees below 1000 rpm. Any higher and temp guage pegs. Plenty
of water from exhausts.

Removed thermostat, no change.

Checked all hoses and blew air thru stainer and tranny cooler. No
change. Checked new pump impellers thru pump outputs and all
seemed ok.

Again, this engine never overheated prior to pump rebuild and I don't
think any old or new pump parts were lost into the system. I'm
stumped and ready to call my mechanic. Any thoughts?

June Bug

Thermostat has to be in place on these engines. Also check the skirt seal in the bottom of the thermostat housing. Make sure you get the right stat.

I "boiled" the stat on my cooktop and it worked fine. The engine
overheated with and without the stat in place and worked fine before
the pump failure.

Do you really think the stat is the problem?

Thanks
June Bug

Are you sure it is really hot. Is vapor coming out exhausts? Could be the temp sending unit. Are the manifolds to hotto lay your hand on, if not that may be the problem.pirate38637.7041087963

I was thinking that maybe your pump housing was rotated and as a result the suction and pressure side of the pump would be reversed - starving the engine of water, but I see you say you have water flow out of the exhousts. OHB is right on the stat has to be in place - it would seem that something is keeping the stat from opening - is the gasket on the right side? you say you boiled the stat and it worked - if the new stat is automotive it could be a 190-195 degree one which would give you a high reading - I think your engine would use a 145 degree. Do you have a thermometer to check for stat temp for opening at 145 degree in a pot of water that is heating up. Or swap stats in the engines

The stat worked fine before the raw water pump was replaced. I
only removed and tested it to eliminate it as a cause for overheating
after replacing the pump.

The only variable that changed since the engine began overheating
was the rebuilt pump. I'm confident I rebuilt and installed it properly.
Wondering if there is possibly some kind of air lock in the plumbing?

I don't think you'd get and air lock - As long as you are sure you're getting water flow from the rebuilt pump it should work- you should be getting at least 5 gal/min out of the pump. I have removed the hose and put a hose extension on and checked to see how much I fill a 5 gal bucket in 30 sec. - Possibly you have something wrong with the block circulating pump - Make sure the belts are routed correctly

I know it's been a long time since your post but I just saw it tonight.  I had the same problem; plenty of exhaust water but engine was heating up.  What makes it difficult to diagnose is the dual impeller water pump.  One impeller supplies water to the engine while the other supplies water to the manifolds so...you can have lots of water coming out of the exhaust but none of it is going through the engine.  Could be a bad impller or missing woodruff key for that impeller.  My problem was a bad impeller.