Chrysler Repair: No heat or very little, oil filler cap, plymouth voyager

Question
I have a 1994 Plymouth Voyager V6 3.3L. Just wondering what are the causes of no heat or very little? Where is the thermostat located?

Answer
Hi JP,
The lack of heat could be the thermostat is stuck open or opening at too low a temperature, and that should show up as a cool running engine on your temp gauge at the instrument cluster (if you have a gauge). Otherwise you might want to check the temp (with a thermometer) of the coolant in the radiator before it pressurizes to see what it it reading when the engine has warmed to close to its usual level of temp. The thermostat is in the hemispherical manifold into which the top radiator hose is connected, directly in front of the oil filler cap. You could remove it and see at what temp it begins to open using a pan of water on the stove top and a thermometer. It should begin to open at 195F. Check the condition of the gasket or o-ring before you reassemble the manifold, and replace if necessary (usually can be bought separately at a good parts store).
Other possibilities include: a partially clogged heater core which you could clean out by removing the hoses at the fire wall and back-flushing with a garden hose. Test for this by feeling the temp of the ingoing ve outflowing heater hoses at the firewall; they should be similar in warmth (and hot to the touch), if the return is much cooler it means there is little flow of coolant through the core. If the problem began just after you drained and refilled the system, you could merely have air trapped in the heater core. If that might be the case, then you need to redrain the system partially collecting the coolant and then refill. But before you drain, start the engine and set the controls for heat so that the valve in the water line (if there is one) will be open. Then turn off the engine. Then drain off about half the coolant and refill through the radiator opening. Fill the overflow bottle to the indicated level on the side.
Or there could be a fault in the temp control door of the heating/ac unit itself. That would require removing the control panel and checking the mixing door cable and its attachment to the unit under the dash. Let me know if you believe that is the issue for futher info.
Sorry to give you such an extensive list of possibilities but that is how it is. Diagnosis is the first step.
Roland