over heating with thermostat, cool without?

1990 4runner 3.0 v6 This is driving me crazy, i had the heads rebuilt 6 months ago. a machine shop did the valves and I put everything back together, when i put the thernostat in it overheats, but takes a while to get hot(maybe 5-10 minutes) . tried a new radiator today and that did not help. Without the thermostat it runs too cool and the tranny doesn`t shift properly of course. Tried flipping the thermostat out of desperation, that didn`t help either. the bottom hose stays cool when the car gets hot. Help please.????????????????????????????????????

It's possible that one or both of the head gaskets was put in wrong.

There is an easy way to check to see if the head gaskets are on the wrong head. It is easy to do if not watching they will physically fit but they ARE NOT symmetrical. :confused:
The gaskets are not interchangeable. :nono: They look almost identical. But if you look closely, you’ll notice a small hole in the driver’s side head gasket that feeds coolant to the thermostat. With the gaskets on the wrong sides, there is no coolant flow to the thermostat so it never opens. With the heads still on, look at the rear of the passenger side head. If you can feel the head gasket sticking out, it’s on the wrong side. I have a picture that shows this very well if your interested.
:2cents:

Thanks for the description. I couldn't remember how to tell the difference without taking the heads off.

Buy a new Thermostat. The old one could be stuck closed. You can test it by boiling it and checking to see when and if it opens at the right temp. They are pretty cheap and I always replace it whenever I do any timing belt/radiator repair.

Definitely. If the head gaskets are on correctly, replace the thermostat.