How to diagnose Jeep Grand Cherokee heater problem

I have a 1998 Jeep Grand Cherokee Laredo with no heat. This is a bad situation in Minnesota in December. The fan works fine. This vehicle has the simple single temperature control (not the fancy separate driver/passenger). I can make the air even colder by twisting the control. This makes me think I am not experiencing the infamous blend door issue. One other clue is I am hearing a lot of water rushing noise. My current best guess is I need to flush the heater core. Are there any other diagnostic steps I should try?

Do you have a heat gauge? Does it rise as the engine warms up? If it isn’t going up, or not much, you have a faulty thermostat and need a new one.

If the engine warms up, but the heater doesn’t, then you probably have air in the heater core and it needs to be bled out. I don’t know how the Jeep heater is laid out, so I can’t help you there.

The engine does warm up. I’m beginning to have hope this is as simple as adding coolant and air bleeding. Thanks.

When the engine is fully warmed up, grab the heater hoses and see if they feel warm or not. If they are both warm, fluid is circulating through the heater core-maybe not as much as you need, but probably a door not opening if both are hot. If the inlet is hot and the outlet cold, its not getting through the blocked heater core. If both are cold, hot water is not getting to the core, so suspect low coolant level, air bubble, or even a water shut off valve not opening. That’s a good start anyway.

Back to the blend door… Both heater hoses are warm. Coolant level was very low so I fixed that and the gurgling/water rushing noise has stopped. I did not air bleed, but as both hoses are warm that doesn’t seem to be the issue. I think my next move is to pull the glove box and check the blend door. Thanks for the help. I’ll let you know how it turns out.

Flushing the heater core seems to have done the trick. On my vehicle the blend door motor is not behind the glove box, but is near the floor just above the transmission tunnel. We could see the actuator turning when the temperature was adjusted. It was always possible to make the air colder. Sometimes the air would be warm and then go cold. The fact the air temp seemed under control led to the decision to flush the core. The flush pushed out a lot of crud.

For others reading this, here’s a link to further info on blend doors and their replacement:
Note for 1998 especially, your blend door motor may be located either behind the glove box or just above the transmission tunnel on the passenger side.

The Grand Cherokee has a common problem with the blend doors breaking and this is most likely the problem that you are seeing. You can read the HVAC codes from the center panel without any special equipment and get a definitive diagnosis. Unfortunately, this is an expensive repair from the dealer. However, there is an innovative solution available that can be done by the owner at significantly less cost. Check YouTube for a video showing the diagnosis and repair of the system. “Grand Cherokee heater” will get you to the right information.