Model 350 Port engine temp

I have closed cooling system.When cruising at 3500rpm my temp guage slowly starts rising, never overheats but you can tell it's getting hot. When I come off plane the temp skyrockets over 200 degrees.I've replaced the impellerer, changed the t-stat. I noticed a slight drips of coolant out of the exhaust manifold water inlet seals that go away when the rpm's go up, I know I need to replace those. Could this be my only problem or should I consider flushing the exchanger? The engine's has slightly less than 700 hours. Thanks for all your help.

What year is your engine. Unless you have reliable service records you may find your exhaust elbows or risers may be in need of change, U-coolers may be clogging, inadequate water flow thru the system to extract heat is likely a problem. Cleaning the cooling system may be only part of this issue. It was for me. It had been neglected so much it eventually required a top end rebuild and new manifolds, elbows and raw water pumps....Hours: 660

Thanks Knuckle, The engines are 1987,everything is original, well taken care of but also 23 years old.I'm going to start checking water flow this weekend.

I wish I could say what the flow should be but go online and find a few PDF's for the pump and hopefully you have access to an engine parts manual and service manual. They have been a real benefit at times...........AL

You may wanna stop by the hardware store and buy a 5' length of clear PVC hose, with a 1.25" ID. it can come in handy when checking for adequate water flow.

Sounds like inadequate water flow somewhere. Reminds me of what you see now and then at a gas station on a hot day: Car pulls up with the AC running and switches off to put gas in. A minute later the radiator blows out some steam. Why? Heat soak. The motor is still plenty hot, but the cooling system that kept it under control is suddenly shut off.

In this case the cooling system--though not shut off--has been throttled down along with the engine and is no longer adequate for the heat load.

I'd start looking for an air leak before the raw water pump, a heat exchanger that needs cleaning out, and/ or a worn impeller, for something is not delivering the cooling the motor needs.

Jeff

Thanks Jeff, I'll look for air leaks. but how would you recommend flushing the exchanger? Would you use some kind of chemical flush kit?
P.S. Thanks all you guys your big helps when it comes to this s@#$.

Sounds like maybe the hose from your raw water intake could be collapsing on you. When at speed there's enough pressure from your clam shell forcing water in to keep it open, but when you slow down the suction from the water pump causes it to close up. You can't tell by looking at the hose on the outside because these things de-laminate sometimes so the outer shell looks normal but the inside is actually blocked.

Just a thought. Hope you get it figured out.