1978 Mercruiser 140 overheats

[rishar1]

I have a 1978 mercruiser 140 in my 4 winns deck boat. The motor overheats after running for a bout 10-15 min. It's ok when it idles but at high running speed it over heats. I have replaced the outboard impeller twice, replaced the inboared water pump, had the heads shaved and checked with new headgasket, have had the engine torn down pistons and rings replaced pretty much everything and it still over heats. I have run out of options and am in desperate need for advice.

 

[Showtime22]

Re: 1978 Mercruiser 140 overheats

Welcome to Iboats.

Could the thermostat not be opening? I have a 1981 Merc V-8 and mine is supposed to open at 140 degrees. You also could try disconnecting a water line or two and checking for blockage.

 

[SeanMcl]

Re: 1978 Mercruiser 140 overheats

I'm sure this is the dumbest thing you'll hear all day, but is the water intake clear? If the screen is mashed up, or if something got sucked up inside it will restrict water flow.

 

[rishar1]

Re: 1978 Mercruiser 140 overheats

First of all I took the thermostat out and let it run without it and it still over heats. Secondly im not sure if there is any blockage The boat was from northern ohio and had alot of lime scale on the hull and out drive casing. We cleaned the out drive and then pumped CLR through "which did help a little but then it went back to the same old problem". could be a blockage but im not sure when I tore the engine down I had the block and heads boiled in acid to clean it out so there shouldnt be anything in there. Also I took the lead in hose from the outdrive lose yesterday and ran it in the water to see if it was pumping and it would only give little spurts when I reved the engine. Took the outdrive apart and checked the impeller and friction plate both are in perfect condition which leads me to believe there is a major leak somewhere.

 

[Showtime22]

Re: 1978 Mercruiser 140 overheats

Sounds like you'll have to get into the drive find out whats blocking the output.
There isn't much distance between the impeller and the motor, but im sure there is enough bends to be holding something.

Luckily it sound like you are hands on. If you get it apart you could try attaching a hose to the output to reverse the flow and seeing what comes out. Maybe with the lower unit off.