HEAT HELP!!! Please help me figure this out!! Im missing my jeep this winter!

OK, so I've done a lot of reading on this subject.

My 98 4.0 auto cherokee has very lttle heat. Now at one point it wouldnt change from hot to cold, but i feel like we got that fixed... so long as the HVAC system isn't on I can change temperature so that is tolerable. My radiator is new, the thermostat works, and my coolant flows well. I'm not too worried about a blocked heater core.

When I start the jeep, it takes FOREVER to get any warm air. Now when it finally does warm up, the air is only luke warm. Now i know it could be the blend door, but i feel the problem is related to Vaccuum, which the blend door isnt, right?

The air always stays luke warm. When i start to drive the jeep, the air may warm up, just a tick, but barely noticable. Here is the real problem... once i start going down the road at highway speeds or even slow speeds up hills or something where i have to give it a lot of gas... all the air like goes into the dash. Thats the best way for me to describe it. It has a vaccuum leak. When this happens no air comes outta the vents or the floor, maybe a little bit out of the defrost vent, but it's mainly just a LOT of fan noise and the air seems to be getting trapped. Once i let my foot completely off the gas for about 5 seconds, you hear the air build up behind whatever door that had shut when my foot was on the gas and I hear like air pressure build up, a loud pop(like the door swinging) and then i can feel air blowing from whatever vent i had selected. The HVAC system worked great when i got the vehicle, so i know it's not blowing air out of the vents with the pressure it should. and i definitely know its not as warm as it's suppose to. I think the temperature difference may be related to the vaccuum leak when im going down the road.

When we took the jeep to the garage and had it tested for vaccuum leaks they told me there wasnt one. Hell, they revved my jeep to the point of blowing it up sitting still and it would never make the vents switch over.. so it has to be under a load for it to do it, but since the temps cool sitting still i dont know what to think!!!

I miss my jeep!!! someone please help!!!

thanks,
Brian

Brian, I wish I could tell you I had the same problem and "here's the fix" but I can't. The fact that your forced air is changing locations is a sign of a vacume problem. The fact that the heat doesn't get hot is, well sounds like another separate issue. The blend door is cable operated on your model (check to see that the cable is connected and moving the door, this can be done from looking under the dash without removing anything)but the selcetor is vacume operated. You didn't mention where your tempature guage reads and how long it takes to get there. I would be tempted to first change the theromastat (even new ones can be defective) and look closley at all hoses for a vacume leak. It is possible that the selector stwich could be faulty and have an internal leak but most of the time the leak is elsewhere.

they revved my jeep to the point of blowing it up sitting still and it would never make the vents switch over.. so it has to be under a load for it to do it, but since the temps cool sitting still i dont know what to think!!!

Click to expand...

Vacume is lost under load, when the throttle plate is opened and air rushes in. That's why you notice it when in a pull or going up a hill.

I too hate when guys rush my stuff harder than I drive it. I was under my 1985 c-10 when a guy was at a local muffler shop was checking catalitic coverter back pressure. I thought I was about to see all 8 connecting rods at once. Let us know what you find. Interseting sysmtoms. Al

they revved it in place to no end... My temp gauge reads up to a tick below halfway, seemed before it might have been right at half way... i could try that.

I have to figure something out..lol id hate to have to sell this one and find another

You have more than one problem. You have LOW HEAT and HEAT DISTRIBUTION problems. The low heat is addressed on MANY threads. Burping the air out of the system seems to be a very common fix, followed by thermostat, and then heater core flush, and then blend door. You seem to have narrowed the heat distribution issue down to a vacuum problem. You should look over the vacuum lines especially where they connect to the control and the distribution doors. There is a vacuum reservoir under the hood that sometimes can have leaks. If you can't see anything, you could test the system with a Mityvac gage, pressurize with a small pump and look for bubbles with soapy water, or blow smoke into the system.

x2 on the vaccum bulb. my 87 was doing the same thing exactly when a vaccum line going to the vaccum reservoir had a hole rubbed in it. yours may be inside the bumper under the passenger side headlight.

well i think i fixed the vaccuum problem, i found a T in the line right behind the battery and the T part was blocked off with a piece of rubber line with a screw in the tip.... i could hear air leaking after i turned the jeep off.... sealed that up and so far it seems to hold my vents in the selected position

next is to address the temperature...

However, Why is it that i can change the temperature, you know, move the blend door while the HVAC is turned off, but when the air is on and i try to change the temp selector it wont change it

It won't change in regular AC or is it just in MAX AC? MAX might block it out?????

may just be a bad head unit, however i am at a loss as to a way to test it. hopefully someone else will chime in. congrats on finding your vaccum leak by the way. they can be a real pita to find sometimes.

I have a 90 XJ 4.0. If I had an engine at normal operating temperature, and no matter where I set the heater/defrost controls all I could get is luke warm air, I would put my money on a problem with heater hose blockage, heater core blockage, or bad heater coolant valve. Now, in the FSM for the 90 it states that the heater coolant valve is reguired to close off the flow of heated coolant to the heater core--i.e., if you have normal operating temperature and unblocked heater hoses, a full-flowing heater core, then a bad heater coolant valve will either cook you in the cabin or it will redirect the heated coolant to the engine.

As we all know, never AssUme anything. Check the flow of the heater hoses and test the heater coolant valve. If both are good I would definately suspect the heater core. Once you have heat in the cabin, you can fix whatever problem is causing the vacuum motors to move with the engine load.

Nuff said.

You say you have a new radiator, Did you have the thermostat replaced aswell? Was your heat working prior to this?

Does it take such a long time for the engine to get up to temp, or does it appear to be just the heat issue?

Check the vaccum line on the vaccuum canister in the front bumper.... You will probably find that it has come loose.

Oh yeah, hopefully the garage you used didnt charge too much for telling you that you DIDNT have a vacuum leak before you found it for yourself!!!

Check your actual coolant temp, don't just rely on the dash gauge. My 87 dash gauge read 200 degrees but was actually 140 degrees. It had a bad sender & thermostat. Changed sender & thermostat also flushed heater core & had heat again. It appears you have the vacuum leak under control so now you have to adress the heat temperature issue.

Why is it that i can change the temperature, you know, move the blend door while the HVAC is turned off, but when the air is on and i try to change the temp selector it wont change it

Click to expand...

Are you saying your temp selector won't move when the air is blowing?

I have a problem with my 97. I think someone has been into the heat box and didn't get it together right. I relpaced the cable but it's still hard to move the temp. I have to put pressuere on the knob and wait for the door to slowly move to change the temp....HTH....Al

yes, when my HVAC system is on, i cant change the temp setting. I have to first turn it to off, and then change my temp.

the vac problem seems to be fixed. My jeep has two "T" fittings right behind the battery and both of them have the third part blocked off????


the temp problem.... I intend to buy a new thermostat and flush my system today!! hopefully it works....

yes it has it's third new radiator ...lol

Gotta love a Jeep!!!

Try the complete flush and make sure your system is full when youve finished by burping it. Dont rely on the feel of the hoses.
THEN try your heat.

If no joy, THEN try changing the thermostat and see if that works.

It would be really helpfull if you done it one at a time so we could see if either of these was causing the problem.
Get back to us and let us know how you got on