7.4 Mercruiser overheat

PlayDate said:
I agree. This engine is done. Save your money for a replacement.
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Thank you for all of the advice! It is much appreciated. I think I will just replace the head gasket and put it back together. Hopefully it will run well enough (and not overheat), to get a few more runs in before it gets pulled out for the winter. I guess it’s time to go motor shopping.
 
Punt, Michigan Motor, Jaspar..both have great long block prices.
And for me I’d go long block, plug and play. It’s not cheap but if it was mine that would be the way I’d go
 
Not to hijack, PD and Scoflo, the tops of the pistons are they supposed to have those marks?
 
Yes; valve relief pockets. They only are there for the intake, but they cut those eyebrows on both sides of the piston so that they'll work in either side of the engine.
 
blaster said:
The pitting did not look good.
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FWIW, that cylinder does have good compression. The mechanic at the marina said he thinks it is corrosion from coolant sitting in the cylinder over the winter.
 
PlayDate said:
I agree. This engine is done. Save your money for a replacement.
Click to expand...
Well, I got it all back together and took it for a long cruise. The overheat issue is fixed, and I can now confirm the head gasket was the cause. It ran great at cruise RPM, good oil pressure and temps. However, it still sounds like marbles in the transmission at idle. Others have related that to a miss-fire, and cylinder 6 still only has about 70 psi., all others over 100. That being said, I decided to order a new long block from Michigan Motorz.

In retrospect, I am still glad I replaced the head gasket as I had never done one before. It was a great learning experience for me, and I learned a ton about my boat! Thank you to all who offered advice along the way.

All was going well on my ride until the starboard motor (the good one) overheated on my way back to the marina. Unbelievable luck! The coolant escaped from the cap! I did a quick compression test and all looks good, so I replaced the coolant and ran it for a while and could not duplicate the issue.

The boat is now out of the water and shrink wrapped until spring. I guess I will start the season with one new motor and another ghost to chase!
 
94SD370 said:
Well, I got it all back together and took it for a long cruise. The overheat issue is fixed, and I can now confirm the head gasket was the cause. It ran great at cruise RPM, good oil pressure and temps. However, it still sounds like marbles in the transmission at idle. Others have related that to a miss-fire, and cylinder 6 still only has about 70 psi., all others over 100. That being said, I decided to order a new long block from Michigan Motorz.

In retrospect, I am still glad I replaced the head gasket as I had never done one before. It was a great learning experience for me, and I learned a ton about my boat! Thank you to all who offered advice along the way.

All was going well on my ride until the starboard motor (the good one) overheated on my way back to the marina. Unbelievable luck! The coolant escaped from the cap! I did a quick compression test and all looks good, so I replaced the coolant and ran it for a while and could not duplicate the issue.

The boat is now out of the water and shrink wrapped until spring. I guess I will start the season with one new motor and another ghost to chase!
Click to expand...
Might be the impeller on the Starboard motor.
 
PlayDate said:
Might be the impeller on the Starboard motor.
Click to expand...
Water flow was normal. What would cause the cap to release the pressure? Or better put, cause the pressure to exceed 16PSI (assuming the cap held to that value). A head gasket would certainly pressurize the coolant side, but I would see that leak on the compression test. Maybe the cap is defective? Or maybe air is being introduced somewhere causing the coolant to boil over. Lots of possibilities.
 
94SD370 said:
Water flow was normal. What would cause the cap to release the pressure? Or better put, cause the pressure to exceed 16PSI (assuming the cap held to that value). A head gasket would certainly pressurize the coolant side, but I would see that leak on the compression test. Maybe the cap is defective? Or maybe air is being introduced somewhere causing the coolant to boil over. Lots of possibilities.
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Assuming this has not happened before......I would not jump to a head gasket.....although I can understand your focus on that.

The engine really overheated which in the scheme of things is means 230 degrees (engine alarm blaring) or so to create that internal pressure to pop the pressure cap. Normally two items create this problem: 1) the thermostat stuck closed 2) the impeller did not provide enough water flow to cool the heat exchangers.

Yes....it can be a dozen more things but 90% of the time it is one of those two things.
 
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