I have a 1998 3.8L Grand Prix in absolutely perfect condition with 230,000 miles on it. I live in Phoenix so there isn't a spec of rust on it and it runs flawlessly. The problem I'm having is that the coolant in the overflow tank does not seem to be being sucked back into the radiator. I've changed the radiator cap, the hose between the overflow tank and the radiator, the overflow tank cap and hose (which comes from GM as an assembly by the way) and there appears to be NO leaks anywhere in the cooling system. I've pressure tested it with a Stant pressure tester and it holds pressure perfectly. The car has had the lower intake manifold gasket changed to the FelPro premium aluminum one as the original with the plastic carrier had been eaten away by the OEM Dex Cool. The upper intake has been switched out for an ATP version and was installed as well with a FelPro plenum gasket. All of this was done over a year ago. I'm certain that the notorious B.I.G. intake problems are not haunting me. The new lower and upper intake manifold setup is tight.
When I open the radiator cap the level is about an inch below the cap and the volume of antifreeze needed to get the level even with the cap is about 90ml (3 oz.). The car does not even come close to overheating and I'm certain that there is no thermostat issue as it has been changed as well. Why isn't the antifreeze up to the level of the cap? That's my question. I've used the bleed screw on the top of the thermostat housing to remove the air from the system. Any suggestions?
