Mid Range rpm overheat - 1992 200 HP Mercury

[jdupree]

I have had this problem for some time and still can't figure it out. This motor will overheat between 1500 to 2000 rpms after several minutes if you run it in that range. At WOT she runs cool and at idle she will be around 140 to 170deg.

The motor was re-built 2 seasons ago right before I bought the boat. Within the last 6 months it has had water pump, t-stats, poppit. I also have a water pressure gauge installed. At idle showing around 2 to 3 psi which I think is normal. Between the 1500 to 2000 rpm's the gauge is showing 5 to 10 psi.

The motor runs great but will overheat in that range. The motor is a 1992 200 HP Mercury Offshore. Any input would be greatly appreciated.

 

[j_martin]

Re: Mid Range rpm overheat - 1992 200 HP Mercury

Go back to the water pump. It's a flow problem At that speed and pressure, it should be cool. perhaps there should be a bit more pressure.

Look for suction side air leak (base gasket), restrictions (junk in the pickup passages) baseplate wear, housing distortion, and impeller condition.

An impeller that looks pretty good, but keeps a definite set and has hairline fatigue checking in it will not pump efficiently.

The last folk I helped with this was looking at all the techno quickies with no help and finally went back to the water pump he was sure was perfect and found a piece of wood lodged in it.

Also check for free flow through the riser tube into the engine.

As you can see, there's a lot more to the water pump than just the impeller.

hope it helps.
John

 

[jdupree]

Re: Mid Range rpm overheat - 1992 200 HP Mercury

Thanks for the advice. I just replaced the water pump in March of this year, so in my mind I was ruling that out. I guess it won't hurt to drop the leg and take a look.

As far as water flow goes, when I have the leg off, can I shoot some water up the tube to the motor and see what kind of flow that I have coming out of the t-stat housings? I really think that I have good flow up in that area but was just looking for ways to check. Thanks.

 

[jdupree]

Re: Mid Range rpm overheat - 1992 200 HP Mercury

What is a good way to check the riser tube that goes into the powerhead. I have the leg off, but have not looked at the water pump yet.