Thermostat Problem

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Hey guys.

My friend bought a 89 ea spack, manual, 3.9

The problem he is having is when the car gets to operating temperature and the thermostat opens up it runs like crap, coughs and splutters,

But if he revs it over 2 grand the problem dissapears

He has replaced alot of things. map sensor, thermo sensor,all of the vaccum lines etc

Put a brand new thermostat in and it still does the same thing

Even had the ecu looked out and theres nothing wrong with it.

If any one would have a idea of what might be cuasing it, would be good

I'm not a mechanic but it could be a headgasket? Leaking slightly?
Id suggest a compression test.
Cheers Jason