I have a 1998 Grand Cherokee with 155,000 miles on it. Still runs well with one problem. When I have the AC/Vent system on, when I step on the accelerator, the ventilation get redirected from the panel vents to the defroster vents on top of the dash. For a while, if I took my foot off the gas momentarily the airflow would go back to the panel vents, but that no longer happens. I first thought this could be a vacuum problem, but haven't been able to find any leaks. Im not getting any indication of a problem like the check engine light on or anything else. If anyone has any ideas, I sure would appreciate any suggestions of how to proceed.
That is a symptom of a vacumn leak. Check all the hoses carefully. Where it goes through the firewall especially. The is a small check valve in the line coming off the manifold, see if you can blow through it one way but not the other. Eyeball the vacumn resevoir carefully. Lastly, you could have a leak inside the cab in some of the tubing or a vacumn motor, selector switch. I have a hand held vacumn pump I got from harbor frieght I use for stuff like that.
x2. It's a vacuum leak. There is a small cluster of vacuum lines under the hood between the battery and the overflow tank. I had two tiny plastic lines broken here. I used tiny rubber hose that fit over the lines and then inside a larger diameter rubber hose to fix mine. That took care of the vent problem for me.
I Have a similar problem, but for me the vents turn off when I accelerate hard, and when I let off the gas I hear a "pop" and the air starts coming through the vents again.... Vacuum leak or the mode door ??
This is EXACTLY what happened to mine. pop yer hood, look on pass side near firewall and u should see a vacuum line with a smaller rubber port with an even smaller vacuum line attached. For me, this was the line that went into my cab and controlled my selector. When i would get on the interstate, it would default to defrost, then when i got up to speed, it would go back to normal. Found there was a tiny crack in this VERY small line. went and got some new vacuum tubing and replaced the section and whaa-laa air works great. Hope this helps
Agree with others, it's a vacuum leak, and check the hoses along with vacuum reservoir as coralman said. This reservoir is actually in the hollow bottom plastic battery tray. Good idea to pull this tray and check the rubber hose which attaches to the bottom since sometimes it tends to rot/loosen.
Most likely the plastic line will break/crack at the "T" where the small black plastic line goes through the firewall to the control switch or where the line by the right front fender attaches to the cruise control servo. Also check where the plastic line attaches to the intake with a short section of hose.
Once I had a leak in one of the small hoses/lines behind the dash and was able to pull it down to work on. Took an old bic pen apart, cut 1/2" of the empty ink tube, cut the crack/hole out of the vacuum line, heated both ends slightly, slid the ink tube in as a splice, and covered with electrical tape. Worked great.
Sorry to take so long to reply to all of your advice...been busy with work and just a few family issues. I did find the problem ...a break in a small vacuum hose just in the engine compartment side of the firewall. Of course I found this only after tearing half the front of the engine compartment apart trying to get to the vacuum resevoir and tracing all the different vacuum lines. Needless to say the last place I looked (and the easiest to get to) I found the break. I cut the line and using a 2 1/2" piece of vacuum line and all is good. Thanks for all your replies and for pointing me in the right direction.