Chrysler Repair: over heating, head gasket leak, chrysler concord

Question
I have a 1997 Chrysler Concord,3.5,24 valve.The motor is over heating.I've installed a new thermostate but it still over heats.The heater blows cold at first then it gets warm,but it cools off again after a few minutes.Do you think it is the water pump?Can you lead me in the right direction.Thank You

Answer
Hi Bruce,
I am limited in specific knowledge about the 3.5L engine; you may recall my claim to expertise is limited to 1995 and earlier models.
But, are you getting coolant blow off into the overflow bottle, which would confirm the overheat situation?(for example it could be a faulty temp sender unit giving you a false positive on your temp gauge). If so, then the possibilities are; compromised flow thru radiator due to buildup of crud in the small tubing (try flushing with a heavy duty product), or there could be air trapped in the system which is impeding the flow of fluid (you might try draining the system: after starting the engine and turning on the heat control so that the radiator core will drain out and you also know the heater valve is open, then turn off the engine and drain off the fluid, but collect it for reuse. Then refill it and try the heater again, with the hope that trapped air is out of the system), or as you suggested the impeller of the pump may not be doing its job due to damage or looseness on the shaft. (Can you observe the fluid in the radiator thru the filler cap opening, and if so is the fluid giving the appearance of flowing when it it warm, but not so hot as to be under pressure which would be dangerous to remove the radiator cap to see?). Finally, are you loosing coolant slowly such that you have to add to it peridically? It could be that you have a head gasket leak (which is often manifesting itself by engine overheating). You might confirm this if you see white exhaust smoke (steam, which has a slight odor of antifreeze if you let it condense on you hand and smell it) or if the oil in the crankcase is cloudy or frothy (if the leak is not going out the exhaust but rather into the crankcase). You could also confirm this by having a radiator shop put an exhaust gas detector on the radiator filler neck which if positive result proves a headgasket leak.
Those are the typical possibilites to run through, now that you have replaced the thermostat.
Let me know what you find out.
Roland