Originally Posted by HikerJeep
1999 Cherokee Sport 4x4; 5-spd; 4.0L - so now in the past two weeks my Jeepster's heater has decided to
1. only produce mildly warm rather than hot-at-the-top-of-the-dial as it used to, and
2. fog up my windshield when I'm running heat-to-my-feet ... when I change to defrost, the windshield clears up nicely, but ...
Any suggestions how to go about whupping this thing back into shape?
Many thanks! Appreciated.
Cooling system: (leaking, other problems)
Make sure your cooling system is fully functional, thermostat is not sticking open or closed, right amount water and anti freeze, good psi (no leaks), good radiator cap, lower hose is not collapsing , etc. . .. average temperatures can be from around under ~200 to ~210 deg. F.
Heater fan: (CFM air flow)
Inspect the heater fan making sure its putting out enough cfm, does it sound as it did when the heater was working? These sometimes become worn and loose cfm (air flow) after years of use. If the fan is just worn out it might be running at a lower CFM, from the lowest to highest setting.
Sometimes multi speed fans develop problems where they are able to run at a lower speed, when on the lowest and highest setting. You should be able to notice a variation in fan noise while changing fan speeds from the lowest to the highest setting.
Vacuum lines: (leaking) (heater door sticking)
Vacuum lines and heater door. When there is not enough vacuum the heater controls might not work as expected causing e.g. the heater door not to work as it should . Sometimes heater doors start to stick patially open or closed for other reasons. Does the heater door open and shut as it should?
Heater core: (clogged, leaking)
If the coolant system level and psi is ok (air in the system and/or coolant leak(s) effects the psi), vacuum is ok, heater door not sticking then the problem may be with the heater core has become clogged (or leaking)
These are some of the basic things related to heater problems to diagnose.
To get the heater back into shape I would start with the cooling system, which includes the heater core. Test or observe one thing at a time such as coolant level, radiator cap & leaks. Repair any problems.
Then test the heater again. If everything starts to work again as expected then that's probably all that's necessary.
Adding coolant regularly indicates a leak somewhere. The color of the coolant, air bubbles and/or oil in the coolant sometimes indicates other leaking problems such as with the engine.
If the problem(s) are both heater and a/c related then there may be other things to diagnose.