Hello. I went to Moab two weeks ago, when I drove home the heater worked fine. I came back . . . installed two Brown Dog motor mounts and a trans mount. Was driving back to Moab and my heater was only blowing cold air. Do you think I just have a plugged up system that a flush would fix? The fan blows air out of all the ports just fine, unfortunately it is cold air.

Thanks!

 

Put coolant in radiator.

 

Yeah, I checked the coolant level and it is full. Thanks though.

 

Did the hoses to the heater core get pinched somehow while you were changing the mounts? Or maybe kinked?

 

Make sure you didn't detach the vacuum line from the heater control valve while working under the hood, assuming of course that you still have the valve.


When the Jeep is warmed up with the heater on, are both of the underhood heater hoses hot?

 

which is this vacuum line cause im having the same problem?

 

I don't believe the '99 has the heater control valve -- I know it was eliminated on later models. Someone will confirm.

 

Newer thermostats fail open. could be problem .

 

Hmm...this is interesting. I went to start my XJ. The coolant was a little low so I filled it up, started the engine to see if the lines were hot, let it run, the lines started to heat up, so I went and turned the heater on and it seems to be working fine. It might just be one of these issues that only happen when you are driving in a snowstorm in the middle of nowhere and don't reappear in the comfort of a garage. Fun times.

MoparNJeep, I think you might be right. I had a problem where the upper radiator hose wasn't full one time (I had no idea I was just crawling around on some slickrock) and my engine overheated and shut off... I am going to pull it out and check it. Any ideas on how to diagnose a faulty thermostat?

 

One pot of water, one thermometer, and one suspect thermostat.

Heat the water until the thermostat opens and then take note of the temp.

 

And since u added coolant, watch hot & cold coolant levels

 

Thanks everyone! I found out what is wrong . . . not sure the best way to fix it. Probably a flush?

When the engine is running, I go under the hood and squeeze all the hoses that run in and out of the heater core a couple times. It fixes it until I shut the engine off. When I restart the air blows out cold again. Then I squeeze the hoses a couple times again and it works. At least this is how I got through a cold weekend trip to Moab.

Has anyone had that happen to them?

 

U may have a bad heater control valve or possibly a very weak water pump. Also sounds like air is some how re-entering the system as the motor cools. Something ain't right....squeezing hoses and it temporarily starts to work sounds a lot like air in the system, the HCV is sticking closed, or flow from the water pump is weak. What cooling system TLC has been done?

 

^^^^^Any ideas on how to diagnose a faulty thermostat? There u go.....and if the stat that's in there is one of those goofy, and failed, "fail safe" stats, u'll be able to tell it the second u pull it out of the head.

 

Thanks djb383. I am slowly replacing one bolt/part at a time on my XJ. I recently replaced the entire cooling system (radiator, clutch fan, water pump, thermostat, hoses, etc) you actually gave me some awesome advice, I ended up picking up the same radiator as you . . . you mentioned it in one of your posts). I also installed a trans cooler.

I will look into the heater control valve. I recently replaced the water pump. Any troubleshooting ideas on how to tell if either one is bad/not working right (water pump)? Or maybe like you said, there is air in the system and I just didn't burp it right.