When ever I use my heat no matter if it is on defrost, vents or floor it always and only blows from the defrost vents.. figured id give it a shot to post it maybe some has same issue. It blows normal amd it is hot just only through def.

 

not common, something is disconnected.....take a peek under the dash

 

Under what conditions does this exist?...

Sitting still, idling, or while driving and loading up the engine (like accelerating or climbing a hill) and the engine vacuum drops off from its normal cruise vacuum of around 17-22 in./hg. to near zero?

If loading the engine, it indicates a loss of vacuum to the HVAC Heat/AC mode selector, and the defrost door actuator reverts to its default Open position due to the loss of control vacuum (same as selecting Defrost on the mode selector). This may be from a leak in the vacuum tubing in the engine bay.

If it does this while sitting still and the engine is at its maximum vacumm, it may mean the defrost door actuator is defective or the door linkage is broken and leaving it in the Open position.

 

probably a vacuum leak.

default goes to defrost so you at least can see out your window.

 

Thanks yea it does it both at idol and driveing. Is it crucial to replace? I literally just bought my lift today and the money I have left is for tires. But if it will lead to vorse things I will replace right away.. thank you for all this information

 

I take it then that if you are sitting still and idling or if you are cruising this happens, and not putting the engine under load. Correct?

Under these conditions you may have a complete loss of engine vacuum to the AC/Heater Mode Selector, which determines the position of the HVAC unit doors, or...if you have no luck, the defrost door is stuck open.

Here's a preliminary check you can do in the engine bay. Visually check the HVAC/Cruise Control vacuum system.

Start at the intake manifold. There is a rubber elbow connection to a check vavle, then its connection to the plastic tube whiich feeds the rest ofthe vacuum sysem. See pic. Check to see if the rubber elbow is connected or is weather checked (leaking).



Next, move to the area by the brake poer booster where a bunch of tubing gangs together. Tubing in this are chafs on each other. Check for a hole chafed in the HVAC/Cruise tube.

Next follow the tube along the firewall to a tee off to the HVAC Mode Selector. THere's a smll (3 MM) tube that comes off the HVAC tee that goes through the fire wall to the Mode Selctor. This tube has a habit of getting broken or a hole being chafed in it. Check the tee coonection, check valve and the 3 MM tube.



Move along from there to the tube that extends from the HVAC tee to the Cruise Control Servo, check that length of tubing for holes.

At the Cruise Control Servo, check the tee connection for being secure or for rubber deteoration.



From the cruise Control Servo, the vacuum tube runs behind the battery. You'll have to move some wiring harness around to check the tube behind the battery. This area is prone to causing damage to the vacuum yube for some reason so check it carefully.

After passing behind the battery, the tube goes to the Vacuum Reservoir, located under the front bumper end cap. The connection at the reservoir has been known to break loose, or the reservoir itself, cracks. To check this area, lay on your back with you head situated under the end cap. Pull the right wheel well liner back enough to look up at the reservoir connection. A flashlight may help.



If you find absolutely no defects in the HVAC/Cruise Control tubing system, there may be a defect in the defrost door system. To see if the HVAC Mode selector is doing its job, you can check the only accessible HVAC door in the system to see if its working, if it is, vacuum is availble at the Mode Selctor.

Study the vacuum circuit diagran below first to get familiar with what this Mode Selector does. Look at the Defrost circuit. You can see that when defrost is selected, vacuum to the defrost door actuator is vented from both sides of the actuator (the defrost door actuator is the only actuator with two vacuum control tubes going to it).



To see if vacuum is in fact getting to the Mode Selector and it is capable of doing at least one task, you'll be checking to see if the Recirculation door is working.

First, roll down the glove box and look in beyond the clutter and you will see the Recirculation door, covered by a cage.



Refer to the Vacuum Circuit diagram

With the engine running (to provide vacuum), select OFF (O) or A/C -Recirculation on the Mode Selector, then selct some other selection besides these. You should see the Recirculation Door open and close. If it does, the Mode Selector is receiving vacuum and is capible of doing, at least, this function.

Check these things and get bck.

 

CC, just when you think it isn't getting any better, you do this. Copy this to the write ups for vacuum leak checks too!
Oh, and add a little section on using little sprays of thottlebody cleaner ( or in your case propane) as a fast way to locate possible vacuum leaks too he he

 

wow ken awesome pics/diagrams.... simply amazing..

I just replaced my front bumper and I don't remember seeing this vacuum reservoir......

I'll have to check again tomorrow just to make sure, if I don't everything seems to operating fine is that possible?

 

If you don't have a reservoir it may be possible for the HVAC Mode Selector to hold its settings, provided the HVAC Check Valve is working properly. The Cruise Control setting may drop out however, even if the check valve at the intake manifold is working. This check valve is used to trap vacuum in the reservoir, and the rservoir is used to maintain proper speed control through the Cruise Control Servo. The slightest change in vehicle speed conditions would call for a change in vacuum to the servo diaphragm, so you can see if you have no reservoir the Cruise control would drop out pretty quick.

 

I have no cruse control in my xj. Its the se model, crank windows, no cruise control, but have the 4x4,

 

You will at least have the vacuum tube from the intake manifold to the HVAC Check Valve.

 

Im am going outside to check it out now, again thank you for all the info again

 

Thanks ken !!! Is I was inspecting the lines I saw a crack in a part of the line , fixed tgat keep looking further the line was completely disconnected from that plastic ball thing in tge front.. some electrical tape over the crqck amd re connected the line to the ball secured it with electical tape and fixed it all right up. Working great now thanks again!!

 

Good work.

BTW, RTV works a little better to secure the tube to the reservoir than tape.

 

Ok.. Looked but dont see a post about about the first problem you listed. I have the problem with the heater vent selection that cuts out or to def. under load, up hill or under acceleration.. Ive checked all the vacuum lines several times as well as check valve and related hardware under the hood. I have looked in cab but not as hard. Any suggestions would be greatly appreciated. It isnt a priority fix just an annoying one.