I had my engine taken out of my 1991 gmc sierra to get the rear main seal resealed, and the intake manifold resealed. After Putting the engine back in, it overheated. So I replaced the thermostat, radiator, and water pump, but it still overheat. If I take out the thermostat it does not overheat, what could be the problem? BY THE WAY THE TIMING WAS ALSO ADJUSTED BEFORE THE TRUCK STARTED OVERHEATING.

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Originally Posted by strategy28

I had my engine taken out of my 1991 gmc sierra to get the rear main seal resealed, and the intake manifold resealed. After Putting the engine back in, it overheated. So i replaced the thermostat, radiator, and water pump, but it still overheat. If i take out the thermostat it does not overheat, what could be the problem? BY THE WAY THE TIMING WAS ALSO ADJUSTED BEFORE THE TRUCK STARTED OVERHEATING.

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You may have inadvertently installed a faulty t-stat?
Is it possible the intake manifold gasket was installed incorrectly, or possibly the wrong application?
Subtle variations from year to year, and from engine to engine make visual ID difficult.