1979 Mercruiser 5.7L 260 Overheating at 220?

jbark

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Jul 20, 2020
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Bought this project boat about a year ago and have worked through several issues, but now I'm stuck. One of the last test runs resulted in the boat over heating to a point temp gauge was maxed. The overheating seemed to cause a vapor lock and the motor would die after running for 15-20 minutes.

We have replaced the impeller (pre alpha), thermostat (140 degree), water pump, and fuel pump. We have removed the exhaust manifold and risers (a lot of rust came out) had them hot/acid bath and all the rust cleaned out as much as possible. One of the manifolds was cracked and we had crack welded. We have also tested temp gauge accuracy with temp gun at manifold and seems accurate.

It now will run without the vapor lock. However the temperature gauge now will reach 220 degrees and hold at that temp (that's running at about 3000rpms). We ran for another 15 minutes and it ran fine holding at 220 degrees. When we slowed to idle it ran for about 5 minutes (about 800 rpms) but then died at idle. When we tried to restart, starter would just click once (like battery was dead/but wasn't) and seemed locked. It stayed like that until engine cooled. After the engine cooled down to 175 range it started and ran back to dock. (Again jumped up to 220 as we ran back to dock but ran fine)

We noticed that the hoses coming out of the exhaust manifold, the water was crackling.

I'm not sure what to do next?
 
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Lou C

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Nov 10, 2002
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12,850
Sounds like you still might have bad manifolds, you can take them off and test for water leaks, but in any case I would not risk ruining an engine with a rusted and welded manifold! That might be why the engine locked up, hydrolock because there could be a crack internally where you can't see it.
 

jbark

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Jul 20, 2020
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I was concerned about that too but if water was getting in to engine block from a cracked exhaust manifold would it still be able to start/run once the engine cooled? Would water be in the engine oil?
 
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Lou C

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yes and yes it depends on how much water, when it gets really hot the crack you can't see might open up more and leak more water. The only way to tell for sure is pull the spark plugs ground the coil lead and crank it over. Does water shoot out of the cyls? Are the spark plugs showing rust on the electrodes? You can get water in a cyl and not hydrolock, each situation is different. If that is happening though you will wind up with milky oil and other problems.
 

jbark

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Jul 20, 2020
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yes and yes it depends on how much water, when it gets really hot the crack you can't see might open up more and leak more water. The only way to tell for sure is pull the spark plugs ground the coil lead and crank it over. Does water shoot out of the cyls? Are the spark plugs showing rust on the electrodes? You can get water in a cyl and not hydrolock, each situation is different. If that is happening though you will wind up with milky oil and other problems.
 

jbark

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Jul 20, 2020
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Thanks for the response. I think we will pull plugs and check for water. We do not have milky oil at this time though which sounds like something good for us for now....
 
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