I'm tying to diagnose an issue I'm having with my sister's '99 Astro. It has the 4.3L and is 2wd.

The van is running fine, holds at 210* max when in Houston traffic with the A/c on. After I shut the engine off the radiator overflow tank spills out a good gallon of coolant. I have flushed the system, replaced the radiator cap and tossed in a 170* thermostat. Seemed fine after that when the fluid was flushed...but now it's doing it again.

Any thing I may be missing? I have the coolant/water mixture at 50/50. Does this engine require special coolant? I've been using the general green stuff as I had it on hand and the "trusty" guy at the shop said to use it.

Thanks in advance Ladies and Gents.

 

Good job on changing out what you did!

What psi does your cap hold? A lot of times the symptoms you describe are from a faulty/weak cap.

Also, you might have air in your system. Park on an incline (can work on flat ground too but more efficient on an incline) and turn your car on (when COMPLETELY cold!) and take off your radiator cap, if you see bubbles, then that can be one or all of your problems. Let the car run a little bit without getting up to temperature (to be safe) and let all the bubbles get out of your system. This happened on a Supra that I owned but it was worse off...I heard air in the heater core. If the coolant level drops while bubbles get out, then just top it off.

Could be as simple as having too much water in your system too. Make sure your overflow tank is at its proper level. When cold, fill to the cold level. If you have a hot level, fill it to that when hot. A lot of times people load the overflow tank and radiator up with coolant thinking it will help but it really doesn't do anything for the car, the coolant will just spill out when the radiator dumps the water back into the tank.

 

As ifwecoose said, These vans are notorious for this. The heater core sits higher the the radiator cap and they get air stuck in them. Just get the rad cap above the heater core and let it run till up to temp and no more bubbles can be seen.
Another issue with them is the Dexcool antifreeze GM used in them. If the van overheats, this orange Dexcool coolant can gel, as in globs of jello.
From what I've read when this happens, it's flush flush flush and flush some more. I use the universal coolant in my 97 Astro and have no problems and a lot of guys just use the good ol' green stuff with no problems.