Over heating under load please help

sebas54

Petty Officer 1st Class
Joined
Jun 17, 2007
Messages
334
Hi, I have a 1988 4.3l merc. I/o that over heats only when underload at high rpms. I was told that I may have a blockage of some sort but don't know.
I thought that it was the T-stat but it wasn't. When I take the T-stat out the boat runs cool.

I checked the inlet hose by disconnecting it and water was flowing steady, but it wasn't flying out.

When docked the boat cooles itself like it should but once underway it over heats.

I recently changed the impeller and the old one looked fine. I also changed the risers/manifolds and water cirrculating pump.

Could I have put a gasket in wrong when installing the impeller that would cause these symptons.

Should I take the lower unit apart again and install another impeller with new gaskets?


Also I was not having any of these problems untill I changed the impeller.



Thanks
 

Bry21317

Chief Petty Officer
Joined
Apr 9, 2002
Messages
552
Re: Over heating under load please help

I had the same types of issues but mine would cool at Higher RPMS, but get hot at lower ones. I had a mechanic do the impeller, he had to redo it, and then it still did the same thing. So I redid it again, making sure to replace every gasket and the housing and then it started to work perfect.

So I had to do it again myself, to make sure that I KNEW what was done.

Bryan
 

sebas54

Petty Officer 1st Class
Joined
Jun 17, 2007
Messages
334
Re: Over heating under load please help

If I put the face plate in updside down could this cause the problems. I think I may have face the lip up instead of down. I am not sure if I did.


So, Is my next step is to buy a new impeller, gasket and housing?
I really don't have any extra money.

Thanks
 

freddyray21

Commander
Joined
Jun 10, 2006
Messages
2,460
Re: Over heating under load please help

if the water pump housing it worn it can cause it to suck air at high rpms and not deliver a good flow of water. Did you change the housing or just the impeller?
 

sebas54

Petty Officer 1st Class
Joined
Jun 17, 2007
Messages
334
Re: Over heating under load please help

I just changed the impeller, 3 gaskets and some o rings.



So, should I change the lower housing also or just the upper.


thanks
 
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