Merc 350 overheating. stumped...

[mattg1024]

1987 Mercruiser 350 Alpha 1.

Starting it up this spring and overheated after a few mins. Temp sensor tested good, Gauge is good, Thermostat is new. I have great water flow from the inlet. I found all 4 manifold plugs packed with debris. Cleaned out all the debris, ran engine for a few mins with the plugs out, great flow from all plugs. Did the same for the riser plugs. Put all the plugs back in, ran it and still overheats. All the hoses have water flow, all stay fairly cool except the hose from the t-stat housing to the circulating pump. If I take the temp sensor out and backfill the intake with water, it comes out of the pump and t-stat housing, so I can assume no blockages.

The circulating pump "looks ok" but I've seen many Ford water pumps look ok but when the heat up the fins slip on the shaft....

Might I have that issue here or am I looking at a cracked block or head gaskets?!?!

Thanks

 

[Bt Doctur]

Re: Merc 350 overheating. stumped...

Found the impeller broken off the shaft twice. manifolds and risers were cold, and the motor was overheating..Take it off and shake it or remove the backplate.

 

[mattg1024]

Re: Merc 350 overheating. stumped...

Did that. All seems fine. Which may be my problem. Like I said I'm a Ford tech and I've seen pumps that look brand new when you take them out be junk because when they heat up the fins slip on the shaft, But can never tell unless you replace the pump. Just trying to avoid replacing a good pump

BTW, 130-142 psi compression across the board

 

[LIsound]

Re: Merc 350 overheating. stumped...

Had same condition recently. I thought it was the impeller, checked and found impeller and water flow to t-stat housing fine. Risers and manifolds seemed fine. I took a shot and pulled off the circulation pump (ie: engine water pump) and found the pump faulty, the shaft which connects the pulley side of the pump to the bronze impeller inside the pump had completely sheared possibly due to electrolysis.
There was suprisingly no play at water pump pulley because the bearing within the pump was fine. Hope this helps.