This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
The clamp on the hose to the overflow bottle wasn't tight enough; it had
loosened during recent pressure-checking of the cooling system.
Ross Gunn heard air (exhaust) bubbling back through the coolant overflow
bottle and had to replace the head gasket was the solution. The dealer
estimated $500; Ross did it himself for under $100.
I managed to trace a slow coolant leak to the point near the firewall
where rubber coolant hoses are clamped to the metal ends of the heater
core tubes. Tightening the clamps a few turns ended the problem. I
probably never would've discovered the culprit had I not noticed that
a nearby cable had an odd green-ish tint. Roger Fradenburgh
The 2.2/2.5 liter cooling systems *MUST* be purged of air before
operation; otherwise, coolant flow blockage will result (i.e., hot,
possibly REALLY hot, engine). Partial purging will
cause the engine to run hotter than normal; the temperature
will gradually drop to normal as the system purges itself over
several days/weeks. These engines' cooling systems can be purged
easily by parking the car on an incline (front much higher than
rear) and cycling the engine on/off until the thermostat opens and the
air is expelled into the radiator. The proper way is to use the bleed
screw in the thermostat's housing...on some engines (like my 2.5) this
screw is frozen in place because of lack of use; hence, the heat
soaking of the thermostat's housing. Bohdan Bodnar (note:
letting it "purge itself" may lead to negative consequences).
Sometimes you can fry your brand new thermostat if you don't
burp it properly. I would always just crank the heat,
leave the radiator cap off, and start the car to purge the
cooling system. Terry L. Howe
We just went through this with my neighbors 3.0L Voyager. He complained
of white smoke in the exhaust, we found oil in the water and vice versa.
The dealer told him $1,000 to replace head gasket; we spent under $300.
We also replaced the timing belt, water pump, plugs & wires, etc while
we were therethe parts weren't expensive. We also replaced the
speedo cable as the best way to get at it was when the heads were off.
The new gaskets solved his problem, and he has more power.
If the cooling system is low on water, the highest parts of the engine
tend to overheat, causing the head to warp and the head gasket to blow
out through the gaps left by the warping. Plymouth Reliants have
temperature gauges and show a high reading within a few minutes of
starting the engine IF it is low on coolant.
If the heater/defroster fan doesn't blow toasty warm when the engine is
hot, you are probably low on coolant. or have to bleed the system more.
It helps to park the car on a grade (front end high), turn the heater
temperature control up all the way, and idle the engine with the
radiator cap off. Then, fill the cooling system. [Use distilled water
about fiftey cents a gallon from the supermarket] (Tom Johnson)