2004 24v 5.7l Mercruiser overheating

[jbrandonlat]

1st please excuse my ignorance when it comes to marine mechanics. I have read through every overheating post I could find and I have done all of the suggestions.

Problem: motor overheating at idle
Steps already taken:
1. Impeller changed. Correctly
2. All hoses flushed including trans cooler
3. Circulation pump changed
4. CLR flushed
5. Compression tested all within spec w no leak down.
6. Belt changed
7. Thermostat changed
8. Crossover that from the sides of the block back into the cooling system replaced.
9. Temperature sending units changed

While running 5 hoses stay hot. The two that run from the thermostat housing to the valve on the bottom of the exhaust manifold. The hose coming from the circulation pump. And the two hoses front the side of the block and run back into the cooling system.

I am at a complete loss and would appreciate any help possible.


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[TeamAllen]

Was it used in salt water? Have you run it on the trailer with a fake-a-lake to see if water comes out of the exhaust ports?
Do you know if it has a closed loop cooling system?

[jbrandonlat]

To my knowledge, it has never been used in salt water. But please explain your thoughts if it were. It heats up instantly on a fake a lake. My fake a lake won't keep up with how much water the impeller pulls in. It is a raw water system.


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[JCP]

Does it only over heat at idle. Is it normal temp when cruising on the lake?

[jbrandonlat]

It runs just north of 175 when underway. But when it idles it shoots up to 250 quickly.


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[jbrandonlat]

Just checked the thermostat as well, it opens appropriately.


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[JCP]

Running cooler at rpm would point more towards a circulation problem versus a mechanical issue (head gasket). More rpm = more circulation. Have you tried running it without the thermostat just to eliminate any possibility of that being the issue?

[jbrandonlat]

Oh yeah... This all started with a head gasket issue. I have ran it without the thermostat.


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[ChrisSnow]

Is the gasket in the right way around? If not you could have blocked passage ways. May not be possible to do this though......

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[jbrandonlat]

Head gasket is on correctly. I triple checked before I put the head on.


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[NICKYPOO]

If it was run is salt water or in a lake with high alkali (salt), that salt can build up in the water jackets over time (around 4-5yrs) causing a blockage.

What was the reason for the new head gasket? Did you notice any white build up in the water jackets?

In our younger days, we ran a buddy's boat at lake with high alkali content. After 4yrs of running it out there every weekend, it ate through the head gasket causing water to leak into the oil. We always flushed it thoroughly after every trip but, just on the hose. Alkali solidifies at around 170 degrees. Once it solidifies, it starts eating whatever it is stuck to.

I'm not sure is SaltAway would clear it but at this point it would be worth a shot.