1999 Jeep Cherokee SPORT heater problem.

Your problem may be a simple as checking the amount of water in your radiator. If it is too low, the heater and defrost will only work a short time, until all the water pressure and level decreases then it won't work. When you add coolant to the radiator level your problem will be gone. If not, then it could be you need the system reverse flushed with pressure as the fins in your heater are plugged with gunk. hope this helps

Have the heater "reversed" flushed, you probably have a build-up of rust and other particles clogging the tubes.

I did this on a Dodge Dakota, and removed a considerable amount of rust/debris from the heater. After it was flushed, it really warmed up the interior.

As already mentioned, back flushing is a good method. Once this is done and the problem is still there, check your top hose temperature after the engine has been running for say 10 minutes. The hose to the heater should have the same temperature, if not, something is still blocking the water flow....

It is possible that you thermostat is broken and not letting the engine achieve its proper temperature, this will also cause the air temp from the heater to be too low....especially in winter time.....

It is also possible that the water valve for the heater is partially blocked or incorrectly adjusted....

If you have rust and debris in the system, then the whole water system needs to be cleaned and good quality antifreeze to be used as rust and debris come only when the antifreeze is too diluted, or of bad quality or simply missing completely.....

Keep us informed please.

All other options should be considered as appropriate and very much in line to an expected solution to Ur problem. Also have a look at the engine thermostat and temperature selector switch. The defrost switch could also be bad or having a high resistance contact.

Dickson

Sounds like your engine thermostat is stuck open.

On a 1991 Olds Silhouette I once had an engine thermostat stick open and not allow the engine to warm up sufficiently. This problem exposed another; a cold air blend door that was stuck mid-open.

I had a very long cold winter drive of 400 miles where I immediately dropped the van off for repair while I went to a relatives to thaw out.

Hooker

I reciently had the same problem with my car, it turned out to be corrosion in the heater core. The heater would not put out enough heat at idle but the heat woud increase when the engine rpm's were increased and cool back down at idle. I had to replace the heater core on my vehicle.

1st- Does your vehicle have a temperature gauge? If so, does the needle go up to somwhere near the middle of the range? If it does, then check the heater hoses after engine warms up. They should be pretty close to the same temp. if not.blocked heater core- clean/replace heater core. If the temp gauge doesn't go up to at least the center of the range, Replace the thermostat. Its that simple.