23 Cobra Surging temperature

"I have an odd overheating iss

"I have an odd overheating issue--and some ideas as to what MIGHT be the cause. I'm interested in what you might think is causing this.

The temperature on my engine after warm up creeps up to 190 degrees then shoots back down to 140 very rapidly. At 140 degrees, the cycle starts again creeping back up to 190 then shoots back down. It doesn't matter if I'm running at idle or at half throttle--the end result is the same. However, If I run the engine wide open, the engine will actually get so hot you can hear it clatter and I immediately shut it down.

I think it's the thermostat but the dealer I just purchased this boat from swears he just changed it. I also read somewhere that it could possibly be the impeller on the water pump.

What makes me think it's the thermostat is how the engine cools down so rapidly after it reaches 190 degrees then slowly climbs back up.

Would anyone else like to share their wisdom with me on this matter?

Thank you."

"Providing cooling works (

"Providing cooling works (manifold stays cold?), I would suspect a leaking head gasket or cracked head."

"Perhaps, but there are no cla

"Perhaps, but there are no classic head gasket or cracked head symptoms like water in oil, oil in water, and no oil or water leaks. No weak compression or power loss."

"What about the exhaust manifo

"What about the exhaust manifold temp? That is the best indication if Your cooling/pump works in basic. If it stays cold, Your impeller pump works, if getting as hot as engine, You are loosing cooling water."

For the sake of $10 bucks or s

For the sake of $10 bucks or so go ahead and change the T-stat (again?)at least then you'll know it's been changed. Possibly you could be sucking air along with the water up from the lower unit. Some guys have installed a clear hose from the transom to the t-stat to watch for air bubbles.

"Thanks for your input guys.

"Thanks for your input guys. Before I broke out the wrenches, I contacted the dealer that sold me the boat and advised them of the issue. They are insisting that I bring it back to them for repair. Appearantly, they pride themselves on making sure their used boats run well when sold and that the customer is satisfied.

This is a bonus I was not expecting--especially when I purchased this boat "As Is".

I'll keep you posted....Thanks again!"

"Just a follow up for you guys

"Just a follow up for you guys who have been very helpful.

I took the t-stat out and dropped it in a beaker of 200 degree water. It never opened. Replaced it with a new 160 t-stat. Temp rises to 162 and drops down to 140. Temp surges between 140 and 160 at various speeds while running with muffs--which I believe is normal.

Sunday, I'm going to try it under load.

Thanks again for all your help!"

"Pulling my hair out again wit

"Pulling my hair out again with this issue.

Under load at full throttle on the lake, boat ran fine for about 90 seconds then the engine began to clatter and lose power. Overheating--again. Also, same problem I had before with the surging temparature gauge.

So far, new fresh water impeller and a new t-stat. Only thing left I guess is the circulating pump. That or a shotgun to the engine block...very frustrating."

"Sorry to hear that, thought y

"Sorry to hear that, thought you had it with the t-stat. Wonder if your pulling air up with the water from the lower unit? If you can get a piece of clear tubing put it in between the input hose and the t-stat and watch for air bubbles. Not so sure about the circulating pump, those things seems very solid."

"Changed the circulating pump

"Changed the circulating pump that solved the heating problem. Now, I have a new problem. When the circulating pump was being changed, it was noticed that the timing belt was getting ready to go so it was changed. Now, the boat will not pop up and plane out like it did prior to the change of the timing belt. If it planes out at all, it takes several minutes instead of a few seconds, and the top speed is down about 5 mph.

I'm getting ready to shoot this engine. Any ideas?"