Mercruiser 3.0 overheated, Is my motor shot?

nola mike said:

His overheat occurred after the transom

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Correct, I would assume the impeller was doing what it should.

ScottinAZ said:

its not the manifold EXTERIOR that is the problem. The inner wall between the exhaust passages and the water jacket is where the issues will be... the exterior of the manifold is between air and water, and wont typically get that hot regardless. If its cracked internally, you WILL get water in the engine where you dont want it......

I had an issue with a worn out manifold leaking water on the hose back into the engine, and nearly hydrolocked it.... not a fun time... a cracked manifold would do the same, let water get where it doesnt belong

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Any way to test that with it on the motor or is it a have to pull to inspect deal?

Hatch19 said:

Any way to test that with it on the motor or is it a have to pull to inspect deal?

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easiest to pull it. check or signs of water intrusion as you go.

once off, fill the water jacket with acetone, and see if it holds.... you WILL see it if the acetone starts leaking out, it will go through smaller cracks than water.

Hatch19 said:

Any way to test that with it on the motor or is it a have to pull to inspect deal?

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Assuming you have a raw water cooled system (or even a half system), you would need to take the riser off and block off the water passage between the main manifold and the riser. You can do so by putting a solid cork gasket between the riser and manifold, then put the riser back on and tighten it down. Then you would put the pressure tester (can be rented free at AutoZone) in the water hose going into the manifold. Pressurize it to 13ish PSI and see if it holds that pressure for 15 minutes or so. It will only cost you a cork gasket and a riser gasket.

Or if everything else checks out with the engine, you can run it for a couple minutes. After a couple minutes, shut it down and pull all the spark plugs to inspect for water. Then do the same thing after 15 minutes if there was no water after the first couple minutes. If there is water on the plugs, you have water in the cylinders so you will want to deal with that, which is why the pressure test is better.

How about the water tube grommets?

Did the hot dry gases pass over the prop hub ? Around the Lower Unit oil and bearing seals ?

Lou C said:

How about the water tube grommets?

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Again, he shouldn't have any issues with the lower unit

Hot exhaust passes thru the lower unit and I do think that the water tube gaskets can soften or melt if you lost all cooling water. That's how the thru the prop exhaust works...thru the lower unit...

Lou C said:

Hot exhaust passes thru the lower unit and I do think that the water tube gaskets can soften or melt if you lost all cooling water. That's how the thru the prop exhaust works...thru the lower unit...

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The grommets and water pocket cover are still getting water if the impeller is working though.

nola mike said:

The grommets and water pocket cover are still getting water if the impeller is working though.

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not to mention this happened on the water... there is some amount of "backflow" in the area naturally, especially at slower engine speeds. if it was on the hose...id be more worried

Alright guys need some help…

Went to do a compression test and did the following steps, I obviously did not warm it up since I’d rather not attempt to run right out the gate..

Disconnected fuel line from carb
Removed all spark plugs
Placed in WOT
Tested all cylinders but turning over for about 10 seconds and all showed “0”.

The gauge did jump as it was turning but never built up

Do I really have ZERO compression or did I get a bum gauge??

When I put the plugs back in and turned it over there is an audible difference in how the motor sounds like it obviously has some compression…

Bad gauge. The gauge shouldn't jump up then go to zero, it should stay at whatever the pressure was. Sounds like you did everything correct. If you want to confirm there is at least some compression, put a piece of masking tape over a spark plug hole and crank the engine over. The compression should blow the tape off. Or even dangle a string of yarn over the spark plug hole and watch for the string to get blown by the compression stroke when turning the engine over.

Kola16 said:

Bad gauge. The gauge shouldn't jump up then go to zero, it should stay at whatever the pressure was. Sounds like you did everything correct. If you want to confirm there is at least some compression, put a piece of masking tape over a spark plug hole and crank the engine over. The compression should blow the tape off. Or even dangle a string of yarn over the spark plug hole and watch for the string to get blown by the compression stroke when turning the engine over.

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Or you could just man up and get the wife to put her finger over the plug hole while you turn the key .
Sounds like the valve in your compression tester is jammed open ,they look like a car tire valve ,maybe the centre core is bent .