Chrysler Repair: dodge carvavan heating, master brake cylinder, dodge caravans

Question
I've owned 3 dodge caravans and have encountered the same problem on all 3, After the vechile is a few years old or reaching higher mileage(100000), the heating system does not function properly. on a cold day the van will not produce sufficent heat at idle, It does warm up when the engine RPM increases but at best you could say warm not hot. Over the period I have tried changing heater cores ,flushing, thermostats and water pump changes to correct the problem to no avail. I've talked to several mechanics on the issue and no one seems to know how to fix the problem.If you adjust the idle up it does get warmer but you are only masking the problem not repairing it. Any thoughts

Answer
Hi Gord,
You touched on a number of possible reasons for poor performance of the heater. The other three possibilities I can mention are:
1. The heater by-pass valve may be open or partially open when you are asking for heat. That valve is supposed to be open only when you have the system on max-cooling, but if it were by-passing some of the coolant that should go to the heater then that would be a reason for low heater output. It is vacuum operated and it is located in the engine compartment usually near the master brake cylinder (it has several hoses attached to it including the ones that go thru the fire wall to the heater core). You could look for a lever that should change positions between when you have it set for max AC (recirc) and when you have it set for heat (while the engine is running, ask for this change). You may need a vacuum gauge to verify that you are getting vacuum to the valve for operational purposes. The vacuum line begins at the big vacuum hose to the power brake canister, near the firewall, and a thin plastic tube goes thru the firewall to the heater control panel which then either sends or stops the vacuum from reaching the by-pass valve. That vacuum also is responsible for shifting the air output to various locations, so if that weren't happening then that would be a sign of a crack in the vacuum line. If there were a crack in the hose or it was not connected tightly you could have a situation where the heater by-pass control valve was not closed when you were seeking heat.
2.air trapped in the heater core. This is particularly a problem with the 4 cylinder engines where a specific protocol has to be used when draining and refilling the cooling system in order to avoid that problem. Let me know if you have a 4 and I'll refer you to the instructions.

Both of these possibilites would be manifested by a there being a significant disparity in the temperature of the incoming and outgoing heater hoses that pass from the firewall to the core when you weren't running the fan. So feel the two hoses and determine if one is much cooler than the other (a sign of restricted flow of hot water thru the core). Typically they should be pretty close in termperature, and more so the case if you aren't using the heater fan at all. If the heater were really doing its thing well on a cold day with the fan on then of course the two hoses would have a fairly siginifant disparity in temp.

3. Something unrelated to water flow would be a maladjustment of the blend-air door in the heater unit. Because of the need to handle both AC and heat, there is a door attached to the temp lever that has to direct the air to the heater core when you want heat and not so when you want cold. There is a cable attached to the door's level that has a clamp to make it work properly, but if that cable clamp slips then the air may not be directed over the core when you want heat. You need to remove the part of the control panel that has the temp slider and examine the cable on the temp slider. In the manual for '89 and for '94 it says to position the lever on the cool end of its travel range, then allowing the self-adjusting clamp to slide, rotate by hand the blend air door crank to its full counterclockwise position.
Roland