Evinrude V4 115793S overheating

Hi, all.

I'm trying to help a friend with his boat, which has an Evinrude 115 V4 (115793S) supposedly a 1975 or 1976 engine. The symptom is it overheats at much above idle. Meaning, if you idle through canals and no-wake zones, it's fine, but past that if you open it past about half throttle, the overheat switch will trip within a couple of minutes necessitating an immediate shutdown. So far, this is what I know:

- the water pump is only a couple of years old
- the right side cylinder head seems new(er), it's bare aluminum where the left cylinder head is painted black like the rest of the engine
- the overheat switch has been removed and tested, it closes at about 214F and re-opens around 170F.
- the thermostat and Vernatherm components have been checked and are OK.
- on land, with muffs on the water inlets and the engine around idle, the engine seems to draw in water OK and the thermostat is obviously cycling open and closed.

one thing I noticed when running on the trailer is that when the hot water is being dumped, it seems "fizzy" not like soda but with fine bubbles in it. I don't have much experience with marine engines, but plenty with automotive engines and my first thought is that overheating under load combined with bubbles in coolant = bad head gasket. At this point the only thing I can think of other than a bad head gasket is an obstruction in the water jacket. either case, the heads will probably have to be removed.

am I on the right track?

Thank you.

I you think it,s the head gaskets, do a comp check and check your numbers. pull your heads and check the waterways, check your inlet ports on the lower unit, drop the lower unit and check the water pump impeller, see if it's burned or if it stays in the same shape that it was in when you pulled it out.

comp check and check your numbers

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can do.

check your inlet ports on the lower unit,

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ports are clear, the plastic screen on either side is intact aside from one missing bar on the right pickup.

drop the lower unit and check the water pump impeller,

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already done, no signs of burning and the impeller fans out with the housing pulled off.

welp, I think this mystery is solved. There's no evidence of burning on the head or cylinder wall, but we're definitely concerned about the flatness of the head.

coueytwo said:

I you think it,s the head gaskets, do a comp check and check your numbers. pull your heads and check the waterways, check your inlet ports on the lower unit, drop the lower unit and check the water pump impeller, see if it's burned or if it stays in the same shape that it was in when you pulled it out.

Click to expand...

Good job you nailed it..