Chrysler Repair: Coolant system leak question, dodge grand caravan, coolant levels

Question
QUESTION: I have a 1997 Dodge Grand Caravan LE, and have noticed the coolant level remains steady unless I turn on the heat, at which point the level drops below the min level in the coolant jug.  Temp gauge never goes up, but I am now checking coolant levels on a daily basis before driving the vehicle. Any ideas or suggestions would be greatly appreciated. I was told by a Chrysler dealer that I had a leaking rear head gasket, and yet have no symptoms of this. This was in December, and here it is mid-April, and the car is still running beautifully. No white smoke from the tail pipe, no smell of coolant from the tail pipe or vents, no slimy stuff on the oil dipstick...I took it to an independent repair facility, who told me that it might have been condensation from the ceramic valve on the water pump. At any rate, since March, there has been no change in the level of coolant in either the jug or the radiator, until today...coolant jug was below min, and radiator was 3/4 full, but still no evidence of a leak.  The only thing that changed since then, was it was chilly enough to require the heat to be turned on. Nobody thinks it is necessary to pressure test the system...HELP!!!


ANSWER: Hi Kathleen,
Turning on the heat does not change the dynamics of the water flow in the system, the coolant continues to flow as usual through the heater coil under the dash. The only change is that the air is diverted to pass over that coil instead of by-passing it to pass over the ac coil. Unless you can find a leak somewhere, and you have listed all the places to look except perhaps the rug under the heater/ac housing if there were a leak in the cabin that doesn't get in the air stream of the ventilation system. That, or a leak in the engine compartment, or possibly along the hose pathway to the rear heater unit (if equiped). I believe that the relationship between 'turn on the heat' and the perceived leak is coincidental and not causally related. Feel free to disagree and let me know why.
Roland

---------- FOLLOW-UP ----------

QUESTION: Hi Roland, according to the mechanic we saw, the symptom we described was the key to diagnosing the problem, it actually was one of the ceramic seals on the water pump - he said it reacted with the weather change which is why we were intermittently seeing the coolant levels dropping. Says he sees this a lot when the water pumps get older, and showed us where the stain was on the engine while it was up on the lift. He replaced the water pump with one that has a lifetime guarantee, will see what happens.  So far, so good...you are still my hero, though!

Answer
Thanks for reporting on what appears to be the cause of the intermittent loss of coolant. You first noticed that the loss occured only when you turned on the heat, but what was really the case is that you turned on the heat when the weather was cold and it was the cold weather that caused the pump to leak. So it was a mis-diagnosis of the true cause of the leak.  
Thanks for the endorsement!
Roland