250L6 overheating over 3500 RPM

I just purchased a 1970 Sea Ray SRV190 with a 250 CID 165 HP. The previous owner complained of overheating "under load", he said it would cool down while idling. He said he had his mechanic replace the impellar in the outdrive. I ran the boat after buing it and yes, it overheats. My local Merc dealer suggested the exhaust elbow could be retricted. I removed it, surged water and blew it out with a compressor and found 4 small pieces of what appears to be impellar carcus. I estimate 25% blockage between the stainless sleeve and the elbow. I reinstalled it and ran the boat today. It overheats at WOT ~4300 RPM, but runs at normal temperature up to ~3200 RPM. Run it over 3300 and it instantly overheats. I've installed clear tubing in the water line going into the engine water pump, no bubbles, clear flow. Could there still be obstructions in the exhaust elbow? Could it be something mechanical with the block? Like head gasket, or cracked block?

Thanks

Re: 250L6 overheating over 3500 RPM

Howdy,

Well yes, there could still be obstructions.

Is this a fresh water engine or salt?

If salt (for it's whole life) you could have blockage in the block, head, manifold, and riser/elbow.

It's unlikely that it's a cracked block since you'd probably have other indications like water leaking in the bilge or water in the oil...(the oil level would rise too, since oil floats on water)

Also if you found impeller remnants it's possible that they're also causing blockage elsewhere.

If you can't flow enough water thru the engine it WILL over heat. The engine produces heat in direct relationship to the output power. Demand more power out of the engine and the cooling system must carry away more heat to maintain the T-stat temp.

If the cooling system cannot carry enough of the heat away, the whole engine will get hotter quickly....until something "gives"


Regards,


Rick

Re: 250L6 overheating over 3500 RPM

He said he had his mechanic replace the impellar in the outdrive.

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i think you need to go look yourself and see the facts. then you'll know. do the waterpump housing too.
i guess there's many possible reasons for overheats. start with the impeller. check the water hose for a kink. since you've found

impellar carcus

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(i love that, never heard that before) you need to be sure it's gone as best can be. remove t-stat, i removed the lower and the hose going up to the manifold end. then flushed water back and forth between the two ends. when flusing from fore to aft, observe to see if water is leaking out the gasket at the waterpocket cover. hope not. don't touch the bolts to that part. there's also a flapper in the exhust pipe. at the top of the exust pipe. it melts off under over heat. this may get lodge in the exhust pipe of anywhere up to the point of exit at the prop hub. i think manifolds and risers can be suspect too. a bad case scenerio for broken off impeller carcus D) get lodged inside the water jackets of the engine preventing flow. no idea what ya do or fig. if that's the propblem. bet others will let ya in on more than what i've said..

Re: 250L6 overheating over 3500 RPM

By the way,

Just to rule it out, you should remove the T-stat and stick it in a pan (of water) and heat it. See if it opens fully at around 140-150*F (this is a raw water cooler engine right?)


Cheers,


Rick

Re: 250L6 overheating over 3500 RPM

agree, boil it... the t-stat should start to open at whatevers stamped on it, like mine is 140. it should be full open at +20* of the stamp... for me that's 160*...

Re: 250L6 overheating over 3500 RPM

Check your intake ports on lower unit for the heck of it, and your circulating pump belt tightness. Could be engine circulating pump too, maybee. Maybe he put a new impellar but the housing or seals could be worn. May need a complete kit.

Re: 250L6 overheating over 3500 RPM

Fresh water boat, raw water cooling. I changed both the hoses off the circulating pump on the front of the block, peered in at the T-stat, didn't see any obstructions. I'll yard it out and boil it. I think I'll also take out the exhaust elbow, remove the stainless sleeve and inspect for more impellar parts. I have an outdrive kit I'll install also. Thanks

Re: 250L6 overheating over 3500 RPM

Thanks all for your help. I've solved the cooling problem. I replaced the stainless sleeve in the exhaust elbow, I also rebuilt the raw water pump in the outdrive. Upon removal, it looks like the first gasket in the stack may have not been the right gasket as well as others in the stack disintegrating. It also appears the pump housing was not torqued to spec. After the rebuild, normal cooling at all RPM ranges.

Kurt