What engine do you have? I had the 4.0 and this happened to me once. It is probably a vacuum hose off of the eng. On the 4.0 look on the drivers side of the eng at the intake manifold about the middle of the eng (around the 3 or 4 cylinder area). That is where I found my problem. You might be able start it up and hear a sucking sound. If not there then check all around and even under the dash for a vacuum line that is either off or broke/cracked.
Previous Jeeps: 84 CJ7 (A True Jeep), 2000 XJ (163,000 Miles/Totaled (Pictures of wrecked Jeep in gallery)), 2004 WJ (parting out die to major electrical issues)
If you don't have time to do it right the first time, how do you have time to do it over.
Lifetime member #13
2011 Tango/Mango JKU