I have lost the heat in my 1987 XJ 4.0. Blower works fine, and if I choose full cool there is a good airflow at DEF, HEAT and VENT. If I go full heat I can barely feel any airflow at any positions. And That’s kinda tricky when it’s 0c (32f) outside… AC gives me some heat, but not as before. And not to my windshield… New coolant, new thermostat and flushed heater core.

I have read that on the post-97 you can check blend door operations by removing glove box and drilling a small hole into the HVAC, but as I understand you cannot remove glove box on pre-97 without pulling the dash? Is there any other way to diagnose blend doors or do I have to pull the car to pieces? The blend door pilot under the dash in the passenger footwell works as it should.

 

If you crouch down on the passenger's side and look right where the transmission tunnel comes up under the HVAC box, you'll see where the push-pull cable attaches to the blend door. You can easily see if the door is moving, unlike the later models, which use a motor. The plastic gears are the problem with those. The early models are pretty stout.

How hot are the hoses entering and exiting the heater core at the firewall? Does your XJ have a Heater Control Valve plumbed into the hoses or do the run straight to the front of the motor?

 

I can see the push-pull cable rotating the pivot pin for the door. The heater hoses get warm, I replaced the closed coolant system with an open one years ago. I also eliminated the heater control valve at the same time, so the coolant flow through the heater core at all times. Still can’t figure why the airflow stops when I move the lever to full hot…

 

Ok, so "Full Heat" may activate the door on the passenger's side that recirculates air versus pulling in fresh air from the wiper cowl. If that's not working, you wouldn't get any air flow. It's right above the kick panel w/ a small plastic cover over it...

 

Thanks, I have been investigating further.


Recirc vent open
The recirc vent opens at MAX AC and closes at all other settings.



Vent control diaphragms
In the drivers side footwell I found the vacuum valve for the floor vent to open and close as it should. And there is vacuum at least in the green tube to the defroster valve.



Heat valve
And the wire controlled blend door that select between warm and cold moved freely and in response to selector.

Next I’ll have to locate my vacuum gauge and try to see if there is a different vacuum if I go full warm or full cool.

 

Yeah I don’t get it… Vacuum is good measured at the vacuum diaphragms. No change from cool to warm.

Vacuum

All blend door actuators move as they are supposed to. But the flow is next to nothing in full heat.

Last thing I did before I lost the heat was flush heater core with a garden hose under low pressure. Is it possible I somehow expanded the heat exchange tubes so the air cannot pass through the core? There was a lot of gunk flushing through, but I cannot see any leaks.

 

TEST the temp variation between the 2 heater hoses at the firewall. They should both be nearly identical.

 

Yeah they are… Return a bit colder maybe

 

The HVAC box splits right where the vent door determines whether the air goes through the heater core or not. The front of the box, where the heater core tubes extend through the firewall is completely open, sealed only by the foam gasket. Have you tried running your hand around the box and where everything comes through the firewall to see if perhaps you have a bad leak? Generally speaking, over time, the foam seals between the various ducts and what-not break down, allowing air to escape the system. I'm not 100% clear which position you're calling "full heat" when you say in your OP that the "HEAT" location works fine, but perhaps it just a case of too many outlets for not enough air if leakage has good bad enough...

 

Full heat means the slider in the heat panel all the way to the right. The wire pulling the blend door in the passenger footwell moves in proportion with the temperature selector. I’ll try to check the foam gasket for air leaks

 

Also, are you sure all your heat isn't going out the defrost?

FWIW, the blend air door is adjustable via the cable. Should be easy to see if it is out of whack.

 

On mine, slider full to the right is defrost... Not trying to be difficult, just trying to understand. When you said "full heat" I wasn't sure if you meant MAX

https://i.ebayimg.com/images/g/kysAA...Vi/s-l1600.jpg

 

One lever for vent position and the other for hot/Cold.

 

The bottom slider.

When the lower slider is all the way to the right air stops flowing to the gaspers chosen by the upper slider. When the lower slider is all the way to the left there is good airflow.

 

Ok, got ya now... Well I think you know at this point that either airflow through the heater core is blocked or the air is leaking out somewhere. I guess it's possible for the Heater Core tubes to expand and block off the core. Radiator cap relieves at 16 psi or so, municipal water pressure is up around 60 if I'm not mistaken and can often times run higher than that.