99 Alero:Overheating/Eatting Coolant

Good morning. I have a 1999 Oldsmoblie Alero, 6 Cylinders 3.4L SFI, with 140,000 miles.

After sitting for a while (overnight) there is no coolant in the reservoir.
You can then fill the tank and drive 2-3 miles before the thermostat starts to read as overheating. This inturn causes the motor to stall or run slower.
At this point there is still coolant in the reservoir.
If you then turn the engine off then restart the car, the thermostat will not get over half on the gauge.
The car then runs fine except for fluctuation in the temperature of air coming out of the heater.
There is no coolant in the oil or going through the engine that I can tell.

My questions are: where is the coolant going? and what is causing it?

Thanks

Sounds like an intake manifold gasket to me. Very common problem with this engine.

Just fixed the same symptoms on my car...I don't think it is a LIM gasket. If it was you'd either have coolant in the crank case - check oil cap and level. If white gunk around valves then its leaking in, or if oil level is way up also tells you its leaking in, or the coolant is leaking out and you'd see puddles in yoru driveway etc.

It could be a bad head gasket, that's what my problems were. High pressure gas was getting into collant system and was pressurizing the coolant, it would then vent our the cap and drain the system. Then resultant air in line prior to thermostat would heat up driving temperature up. I'd stop car and wait, on restart thermostat would open as some coolant came in contact then air in system was vented to tank. That also explains why your heater is intermittant, you have air in your line.

Apparently there is some test to check for combustion gases in the coolant line, that would be the sure way to confirm. Alternatively, run car and see if coolant flows out of resevour. YOu can do this with cap on or off.

Did you do the basics and check to see if it is leaking onto the ground? I had a couple o-rings go out on me after about 140,000 that caused it to leak coolant pretty bad. They were the orings at the top drivers side of the engine where the coolant returns to the rad. $4.00 fix.