Heat problem

So this weekend I set aside some time, replaced my thermostat and gasket, and coolant pressure bottle on the 90 cherokee. I also replaced some hoses and other stuff. Well I refilled the system with coolant, and started the jeep. The cherokee got over 210, so I shut it off, waited, and then sure enough, my thermostat kicked in and coolant started to cool the motor down. I had the heat on full blast when I did all of this. My motor warmed up but my heat was cold, even after coolant started circulating through the system. I have the old closed system btw. When I took it out driving tonight I noticed my temp guage wouldn't stay at a constant temperature. It would get to 210 and then drop dramatically. I didn't burp the system either, which would explain why when I shut it off the first time I filled the system up and the thermostat had kicked in it made a bubbly noise. So basically what I'm getting to is I'm getting luke warm heat, not a constant engine temp. Could this be because I didn't burp the system? If so how would I go about burping the system at this point? Possible heater control valve problem? What exactly does the heater control valve do?

Heater control valve either allows the hot coolant into the core when you want heat or bypasses it when you have the temp slider/selector all the way to the cold end.

what could cause my inconsistent temps then?

megabyte5689 said:

So this weekend I set aside some time, replaced my thermostat and gasket, and coolant pressure bottle on the 90 cherokee. I also replaced some hoses and other stuff. Well I refilled the system with coolant, and started the jeep. The cherokee got over 210, so I shut it off, waited, and then sure enough, my thermostat kicked in and coolant started to cool the motor down. I had the heat on full blast when I did all of this. My motor warmed up but my heat was cold, even after coolant started circulating through the system. I have the old closed system btw. When I took it out driving tonight I noticed my temp guage wouldn't stay at a constant temperature. It would get to 210 and then drop dramatically. I didn't burp the system either, which would explain why when I shut it off the first time I filled the system up and the thermostat had kicked in it made a bubbly noise. So basically what I'm getting to is I'm getting luke warm heat, not a constant engine temp. Could this be because I didn't burp the system? If so how would I go about burping the system at this point? Possible heater control valve problem? What exactly does the heater control valve do?

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Have you tried using the search feature of this board? There are many answers to these questions.

The heat problem (if your talking about your Jeeps Heater) would be caused by either a malfunctioning heater control valve, or a clogged heater core. Im betting the core is clogged rather then the heater control valve not working. The "HCV" slows down the flow of coolant to the core when the AC is being used helping to aid the AC in being as cold as possible. Some people bybass it, i personally feel its best kept in place.
Flush the core following these steps:
Unplug the hoses from the heater core, and flush starting with the bottom spout from the core. I suggest hooking up a extra hose to the core, and attach the other end to the garden hose. Wait till the water is clear, and then move to the other spout and do the same thing. Alternate like this a few times, and then hook the two heater hoses back up to the core.
Dont try to use the garden hose directly to the core as it will spray out the sides into the "foam" gasket on the firewall soaking it, and making your passenger side floor very wet. Also the amount of pressure obtained hooking it up directly could also push a hole in the core causing a huge huge headache.
Its likely to get clogged again, or even worse spring a leak since whats clogging it is corrosive. It may not, not very many complain of it leaking after a flush, just want you to be aware of what could happen.
Enjoy heat now!


The fluctuating temperature would be caused by the existence of air in the system. Make sure you flush the core before you evacuate the air. This is how you can remove trapped air (Should be done on a cold engine):
Jack the rear of the vehicle up and support on jack stands. Make sure the Coolant bottle is at least half full. Unplug the coolant temperature sensor (its the sensor almost directly below the MAP sensor on the Drivers side rear of the head) and unscrew it slowly. As your loosening the sensor, you should notice coolant start to leak from the threads. Let some coolant escape the system making sure that the coolant bottle does not get low on coolant. Once you get a fairly constant stream of coolant from below the coolant temp sensor, tighten the sensor and hook everything back up. Remove the jack stands, lower the vehicle etc. This should remove most if not all of the air that was trapped in the system.

I hope that helped.

this irritates the heck out of me, when I sit at idle it heats up fine and stays constant temp, heat is actually pretty warm, but when I start driving heat feels luke warm. I'm guessing air is trapped in the system then

Have you checked for vacuum leaks. The heater control valve is operated by vacuum. If you have a leak then you would have even less vacuum while driving due to higher manifold pressure. Just a thought.
This is assuming that the heating problem was there even before you did the work this past weekend.

I actually did notice one vacuum line was snipped in half behind my airbox, I think it goes to the EGR solenoid. I'm going to have to replace that. When I first bought the car, it always had the problem of not staying at a constant temp, blew good hot air. The temp thing got worse when winter rolled around though.

I have almost exactly the same conditions. Same vacuum line gone as well. My mechanic tells me that the EGR needs to be hooked up pronto or it will start pinging and tearing shit up...I'm not so sure about it, but I'm hooking it up anyway because that's my only vacuum leak! I'm hoping that helps my heat. Post your results!

my egr is hooked up, just one part of the egr solenoid got cut in half. I'm not sure about pinging, but emissions will suffer. I'm going to burp the system today, to see if that helps, I probably will play around with the heater control valve to see if that helps also.

any suggestions would be helpful

Suggestions? I think there are plenty in this thread. Once you fix your egr solenoid vacuum line, and burp the system of air like i explained in my previous post, i am pretty sure your problems will be solved.

alright sorry, I just got impatient with it. It was like 10 degrees here this morning and I basically had to drive with luke warm heat. Anyways is it alright if I back the jeep into my drive way and burp the system that way? Will this work?

Depends on how steep the driveway is. Your going to want to get the rear as high as possible.

As long as when you back it up, the back of the head is the highest point in the engine bay (relatively speaking of course .. its not like its going to come off the motor mounts lol!)

last night I was trying to get the temp sensor off, so I pull off the map sensor entirely in the process of this I cracked an old vac line, put some new hose on to it and connected the pieces back together, however my idle is acting funny now. When I was driving this morning it would idle up and then idle down and when I had the blower on high it would act like it wanted to die. I'm pretty sure this had something to do with last night's fiasco with the map sensor. I was just checking to see if this could be a possibility. I never did get the temp sensor out because I didn't have the right size deep socket

The XJ will also stumble too. The new vacuum hose I used was some extra hose off my EGR solenoid. I bought this hose two hours before I even attempted this. I already checked the solenoid and everything is hooked up correctly for the solenoid part. Can this happen if the vacuum for the MAP sensor isn't hooked up right?

I'm talking about the sensor that is mounted above the firewall also.

Ok So let me get this straight.
You repaired the EGR Solenoid vacuum line?
Your cracked the line going from the MAP sensor to the throttle body, then repaired it with some extra vacuum hose that you had lying around?
The vacuum line that you used .. is it soft or hard? If its soft, how soft is it? Very flimsy, or only moderatly flemsy?
Sometimes vacuum lines will collapse when under vacuum if flimsy vacuum line is used.

The line from the EGR to the throttle body could be the culprit here. Triple check this line and make DAMN sure there are no leaks at all here.
If this still doesnt fix your idle running problems, un-fix the EGR solenoid, and see if that helps...
Let me know

I only started getting these problems after the hose cracked. It's not flimsy hose. In fact when I fixed the cracked egr hose my idle was very good. when i cracked the map sensor line and used some that extra hose idle performance and acceleration degraded. The extra hose that I used is identical to that of the hose coming out of the MAP sensor, until it reaches the hard plastic molded hose.

megabyte5689 said:

I only started getting these problems after the hose cracked. It's not flimsy hose. In fact when I fixed the cracked egr hose my idle was very good. when i cracked the map sensor line and used some that extra hose idle performance and acceleration degraded. The extra hose that I used is identical to that of the hose coming out of the MAP sensor, until it reaches the hard plastic molded hose.

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make sure you have a good connection between the vacuum line from the MAP into the side of the throttle body and that its in the correct hole on the throttle body. IIRC, the rubber grommet on the side of the throttle body has what appears to be 2 holes when in fact there is only one hole that it can go in. or pull the vacuum line off the map end and see if the line is pulling vacuum. my 89 runs like crap with the MAP vacuum line disconnected.

89xj said:

make sure you have a good connection between the vacuum line from the MAP into the side of the throttle body and that its in the correct hole on the throttle body. IIRC, the rubber grommet on the side of the throttle body has what appears to be 2 holes when in fact there is only one hole that it can go in. or pull the vacuum line off the map end and see if the line is pulling vacuum. my 89 runs like crap with the MAP vacuum line disconnected.

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This is why i said to be DAMN sure that the line is free of cracks etc. There is only 1 way the the line can go into the Throttle body, but it is possible its not pushed in all the way. This has got to be whats causing the problem...

You need to get that system burped of air also... if anything just spend 4-5 bucks on socket at your local parts store and get-r-done...