Chrysler Repair: LHS coolant backflow problem, head gasket leak, coolant recovery

Question
I've got a '97 LHS and recently it started forcing coolant back through to the reservoir.  Enough that coolant is force out the overflow tube out onto the engine and ground.

What causes this?  How do I fix it.  The mechanic says there is air in the system and that it's tough to get out. They have changed the thermostat and a sensor they said was showing the engine temp to be cooler than it actually was.

Thanks,
Al True

Answer
Hi Al,
There are two possibilities: that you have a head gasket leak which is pressurizing the cooling system or that you have air trapped in the system. Of course this assumes that the thermostat and radiator are functioning properly.
On the way to get air out:
It would be good to start with only a partially filled system, so drain off three quarts from the radiator drain.
Then notice that the thermostat housing has a bleed valve and a nipple from which you can drain fluid. Put a 4' length of clear plastic hose on the nipple and direct the hose over the front of the car to a clean container to catch the coolant. Then open the bleed valve. Then slowly fill the coolant recovery bottle until fluid starts to flow out of the clear hose. The gently squeeze on the top radiator hose until all the air is exhausted from the system. Close the bleed valve and fill the system the rest of the way until it reaches the to of the overflow on the bottle. Remove the hose from the bleeder valve nipple.
If that doesn't correct the problem then it would be good to go to a radiator shop and have them attach an exhaust gas detector to the overflow bottle to see if they can identify exhaust gas in the effluent from the bottle. If so, then one of the head gaskets is blown. Another sign of such a leak is white smoke that has an odor of coolant that comes from the exhaust pipe when you first start the car. Of still another is a cloudy or foamy condition of the engine oil as seen on the dipstick. So check those out as well.
I would be interested to know how this is resolved.
Roland