Long story short…my jeep was overheating about 2 months ago…I replaced the thermostat and that did not fix it…so I took to my mechanic and he replaced the water pump. That fixed it, but now that it is getting cold, I notice I only have heat in the cab when the jeep is idling, once I get on the freeway at 60 mph the engine temp stays pretty low and thus very little heat comes out. Is it my thermostat cause I have heat when I am not moving? Thanks!!
I would try replacing the thermostat again. If the thermostat is stuck partially open, the ram air effect across the radiator will strip any heat from the coolant so no hot coolant reaches the heater core.
Tester
Not sure but you might have a vacuum operated heater door on this car. Basically the air system uses a blend of cold and hot air based on the temp control on your dash. This may be vacuum operated. At highway speed the vacuum drops a bit versus idle. Look for a leak in the vacuum system.
Assuming that water is circulating through the heater core and the core is hot, there are two common problems that can cause your symptoms. The easy way to check the heater core is to feel both lines going into the firewall from a cold start and verify that they are warming up at about the same rate and get uncomfortably hot.
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Re-circ door. This door chooses between external or internal air being “sucked” into the fan. It’s always one or the other, not a blend. The door tends to break, leaving both the internal intake port and external intake port open. When this happens, at highway speeds air will flow into the external port and out of the internal port…like having a window rolled down all the time. The air flow can actually be strong enough to create a venturi effect across the top of the fan, starving the fan for air. Check YouTube for a video on the problem and solution.
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Blend doors. These doors regulate the temperature of the air flow. Generally you would not expect there to be any difference based on car speed, but it’s easy to check the codes and verify the operation of the doors. Failure of the blend doors is a very common occurrence on the GC. Again, check the videos on YouTube. “Grand Cherokee heater” will get to more information.
I think Euryale1 may have hit on the answer to the trouble. I thought the same thing as I read the post about the trouble. See if the defroster works ok. If that seems to work then you most likely have a vacuum leak. I would start the check at the vacuum reservoir.