Have an 1988 Merc 454, Bravo I combo. Replaced the raw water impeller, but now seems to be slightly overheating at cruise. Gets up to 160-180 at 3200-3400 rpm. Didn't let get any hotter. Runs nice at and cool at idle.

Always remember ran around 150 -160 max. Any thoughts? Maybe the thermostat? Checked the water flow and seems good.

Thanks

What shape are the manifolds in. If over 5-7 years, they might be getting plugged up. And not to be mean............was it running hot before the new impellers? If not you might have gotten the wrong impeller.

Engine is only 4 yrs old but RWC. Actually wasn't pumping any water before replaced impellor.

Were any of the blades missing on the old impeller? If so you will need to make sure they are not restricting the water flow. Try to back flush.

The old vanes may be stuck in your oil cooler.

I had same problem, it would only start to overheat at cruising and WOT. I replaced everything. Ended up cracking the head from the heat. After that is when I found the problem. It was the small hose that goes through the transom in the gimbal. Big time corrosion inside the hose. Many others had the same problem. It is the hose that attaches to the big hose that connects to your sea water pump at the transom. If you check everything else I bet you anything that is your problem.

I had the same problems last year. Turned out that when this one shop replaced the impeller for the previous owner, they never thoroughly flushed/cleaned the system and I had a handfull of impeller bits clogging the oil cooler. Check that 1st before you spend money on anything else.

I had this same situation a few years ago. Luckily, I found it before anything got ruined.

thermostat ............ quickest test ... take it out and run it ...

but after reading the post again ... does look like blockage ..the thermostat problem will generally let the motor run cool at idle or slow speed but the motor will overheat if you run it at normal speed ....

1) Pieces of old impeller in the oil cooler.
2) Collapsing water inlet hose, at that age the inside can separate from the wire and collapse.
3) Thermostat