Defroster only on the highway

My Heat/A/C blows out the defroster only on the highway. I am aware that when there is a vacuum leak that the air will only come out of the defroster but I am confused to why mine does it only at highway speeds. My XJ is a 97 4.0L auto 4x4 with 183,xxx miles on it. I have looked over the vacuum system and I cannot seem to find a leak and when I removed my front bumper about a year ago I did not see a crack in the vacuum canister. Any ideas on where I should check next?

On the freeway at part throttle or more for long periods of time there will not be enough vacuum stored if there is a leak. I ran without the canister and it would do it easily.

my guess is that the heat is RPM dependent, not speed dependent... try sitting in your driveway and holding the throttle at typical RPMs of highway speeds and see what happens.

beyond that, idk where to begin fixing the problem. just wanted to make that distinction.

Rockslut said:

On the freeway at part throttle or more for long periods of time there will not be enough vacuum stored if there is a leak. I ran without the canister and it would do it easily.

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Yes, this is it. Mine did exactly what you describe. I eventually tracked it down when I started replacing vacuum components to a softened elbow connector on the intake manifold. Replaced it with a new one and no more defrost unless I select it.

Low vacuum default is "defrost".

Get a vacuum pump, check the reservoir, move back past the battery, check again, and so on until you find a point where the system won't hold vacuum--the leak is between where you are at and the last known good point.

So I tried the sitting in the driveway and revving it thing and it was fine no defrost default. I then got out on the road and from 55 i stomped it to shoot the RPMs up as high at 4200 and still no kick to defrost default so now I'm just confused. Any ideas?

Yes. Do what Joe suggested. You have a vacuum problem between the vacuum reservoir and the heater control. Look for it.

You definitely have a vacuum leak. It's probably on the vacuum sub-harness that serves the HVAC and cruise control, with the reservoir at the end.

This specific behavior usually indicates that the outlet selection sub-harness on the back of the HVAC head unit has a leak. That sub-harness only pulls a few hg inches of vacuum so any localized leak will cause it stop working. Here's a pic of mine from when I had the same problem. All the colored lines go to different outlets and the head unit basically moves vacuum among them to open and close the different flappers. I took the clear plastic thing and cleaned it off then smeared a light coating of silicon dielectric grease to get a better seal. No problem since.



Basically you need to get yourself a vacuum gauge ($20ish at any auto parts store) and measure vacuum pressure at different places. Normal engine pressure is 15-20 hg inches at idle after fully warmed up. Test for pressure then disconnect and plug off the HVAC vacuum line (the nylon hose coming out the firewall by the A/C expansion valve) and see if you can isolate it.

ehall said:

You definitely have a vacuum leak. It's probably on the vacuum sub-harness that serves the HVAC and cruise control, with the reservoir at the end.

This specific behavior usually indicates that the outlet selection sub-harness on the back of the HVAC head unit has a leak. That sub-harness only pulls a few hg inches of vacuum so any localized leak will cause it stop working. Here's a pic of mine from when I had the same problem. All the colored lines go to different outlets and the head unit basically moves vacuum among them to open and close the different flappers. I took the clear plastic thing and cleaned it off then smeared a light coating of silicon dielectric grease to get a better seal. No problem since.

Basically you need to get yourself a vacuum gauge ($20ish at any auto parts store) and measure vacuum pressure at different places. Normal engine pressure is 15-20 hg inches at idle after fully warmed up. Test for pressure then disconnect and plug off the HVAC vacuum line (the nylon hose coming out the firewall by the A/C expansion valve) and see if you can isolate it.

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Is all this the same on a 97? I noticed in the picture that yours is a pre-97.

Back away from your keyboard. Go over to the Jeep. Start it. Find the vacuum line exiting the intake manifold heading across the firewall. Keep pulling the hose connections and putting your finger over them to check for vacuum. If there's vacuum, plug it back in and go to the next connection. Repeat until you find no vacuum. There's your problem.