Air defaults to blow out the defroster only sometimes...

My 97 XJ 4.0L auto 4x4 3.5" lift on 33x12.50r15 stock axle gears has its air blow out the defroster only sometimes in certain situations. It mainly happens at highway speeds 55+ mph going uphill and getting deep into the pedal but not too deep as to make the transmission kick down a gear. I have looked over the vacuum tubes and all seem well and the vacuum canister is not cracked. I know that with a vacuum leak the air defaults to the defroster all the time but my problem is it does it only sometimes. Any ideas on what this could be? It is really annoying and I would like to get it to quit.

If I remember right the XJ HVAC system is running off the vacuum system. Check for leaks near the top of the firewall.

Check the check valves in the vac lines.

Air defaulting to the defrost is almost *always* due to a vac leak somewhere. The only real exception is a problem with the linkage or a jam/blockage. The vac leak may not be due to a line breakage. Could be a connection to an actuator, a check valve or any other junction point.

Saudade said:

Check the check valves in the vac lines.

Air defaulting to the defrost is almost *always* due to a vac leak somewhere. The only real exception is a problem with the linkage or a jam/blockage. The vac leak may not be due to a line breakage. Could be a connection to an actuator, a check valve or any other junction point.

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Where would these check valves or actuator be? I haven't messed around with the vacuum system much before other than extending one hose when I put on the 99+ intake manifold which was also the first place I checked. I checked all the bolts holding the intake/exhaust on and they are all tight and there is no leaks at the gasket.

There's a small vacuum line that comes off the intake manifold, runs along the firewall, turns at the passenger fender and runs down to the front bumper and into a vacuum canister. If there is a break/disco in the line or crack in the canister, the system will default to defrost mode.

This is one reason they got rid of the vacuum disconnect axle, it would disengage at higher rpm because of vacuum loss. At higher RPM, (going uphill, higher speed, deep in the pedal etc) your throttle blade opens up a lot more, pulling more air to the engine, decreasing system vacuum over all. Usually, check valves are in place to help prevent a near total loss of vacuum to components like heater controls. Many are inline and some are in the vacuum reservoirs. Check for leaks like they all said. The line comes off the manifold on the drivers side and goes either along the firewall or over the engine to the passenger side firewall where it enters near the heater hoses (not too familiar with 96+ models.) Check all along that line, especially the rubber pieces.

Ben824 said:

Where would these check valves or actuator be?

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There is a check valve built into the vacuum reservoir. If it craps out, you can either replace the reservoir or put one in series with the reservoir.

A "LOT" of good information in this thread. Thanks folks from someone who also has this issue.

On my 97 xj the small vacuum hose that goes in the firewall next to one of the AC pipes was broken. The area around the break was brittle and too close to the AC pipe. Using the AC alot lately probably made it break. The control never worked very well and it was probably leaking for awhile. Put a little bit larger hose over the old one to repair it and it switches much better.