mercuiser overheat

[Thumbtack]

Mercruiser gen II. 5.7L carbed.

The engine starts to get hot after running for a few minutes at high rpm. As soon as you back off the throttle it instantly cools off. I did the pump impeller in the winter so I didn't think that was the problem.
So far we did the following to it but it would still overheat after each thing.

1) replace thermostat
2) removed thermostat
3) replaced manifolds and risers as they were clogged
4) replaced pump impeller again along with housing
5) removed the ball and spring things inside the thermostat housing

We checked the waterflow coming in at the thermostat housing and saw plenty of water. We also checked to make sure the power steering oil cooler was not clogged.
I'm lost....

 

[Bt Doctur]

Re: mercuiser overheat

replace the balls and springs inside the therm housing first.
the purpose is to allow water thru the exaust system to keep the rubber from burning, when the therm opens there supposed to close and allow flow thru the manifold and riser.Did you replace the lower water pump housing also.
Timing correct?.

 

[wvit1001]

Re: mercuiser overheat

sounds like the impellar has something wrong with it. Take the outdrive back apart and check it before you go any further.

 

[Thumbtack]

Re: mercuiser overheat

The impellar was done twice so far. There is plenty of water coming in the intake hose while running for a second disconnected. And no I did not replace the lower water pump housing. With good water coming in do you think that can still be a problem? Thanks

 

[Thumbtack]

Re: mercuiser overheat

111

 

[Bt Doctur]

Re: mercuiser overheat

The water pump assembly in most cases is above the water line at hi rpms, the only water that is there comes from the exaust discharge along with hot exaust gasses, you could be sucking hot air at high rpm.

 

[JustJason]

Re: mercuiser overheat

A high rpm overheat can indicate its pumping air. I'd replace the entire housing. Also... are you sure you twisted the driveshaft the right way (clockwise) as you pushed the housing down?

 

[Thumbtack]

Re: mercuiser overheat

Yes I did twist the shaft in the correct direction. I am going to try the lower pump assembly now. I will respond in a few hours when I am done and have it test ran. Thanks for the info so far. Hope this is it.

 

[Thumbtack]

Re: mercuiser overheat

Problem fixed. I just replaced the lower pump assembly and test ran the boat. The overheat problem is gone. Thanks for all your help and suggestions.

 

[JustJason]

Re: mercuiser overheat

Congrats!!!

It was probably getting airbound.

 

[nazz2005]

Re: mercuiser overheat

hey just wanted to ask, is it normal for one riser to get hotter than the other? inbord merc 5.7. One is bearable to touch at operating temp while the other is not..? does not overheat..complete cooling system just redone