97 Grand Cherokee vent/acceletation issue

I have 2 1997 Jeep Grand Cherokees. One with the 6 cyl and the other with the 5.2L. Both have an identical problem, which leads me to believe this is a common issue BUT every mechanic, dealer, etc. treats it as the greatest mystery on the planet.

Both Jeeps will begin blowing out of the defrost vent under acceleration. This does also affect the cruise control's ability to maintain speed. Obviously this is a vacuum issue. I've traced the lines and cannot find any leaks. If I disconnect at either the T that sends vacuum to the heater controls or to the cruise control unit, they suck my finger very strong.

Again, with 2 Jeeps doing the exact same thing there has to be a common component that fails. It's not a fluke that they just happen to have the same symptoms. Does anybody have any insight?

You have leak. Put a vacuum gage in-line- when you shut engine off- does vacuum hold? If not listen for leak. vacuum should be near 15 inhg

Sounds very likely the lines that run to your vacuum canister is where the leak is. Or the canister is cracked.

Thanks... I'm certain there is a vacuum leak, the problem is locating it. It's just suspicious to me that 2 Jeeps are acting the exact same way. To me that rules out random issue in each of them and points me to a common failure. I've been turning wrenches long enough to "listen" and in past experiences I could always hear the leak. Are you sure it will hold pressure after the engine is off?... my logic would tell me that vacuum pressure would release once the intake stops creating suction.

The canister does sound like a likely culprit, but since it's under the battery (must remove battery to access) I don't have the slightest clue how to test it. Am I going to see a potential visible crack or is this a part to just swap and hope for the best? Without a battery, I can't start the motor to create vacuum. Is there a procedure for testing the canister?

Most of the times I have seen issues. The line/lines that plug into the canister is cracked. You won't hear anything. It's not that type of a vacuum leak. You hit the gas, and the airflow from vents changes. Either all goes to the lower vents, top vents, etc.... Had a cutlass once. No matter where you had the selector, hit the gas, everything blew out from lower vents. Cracked line under the battery running to the canister.

Thank you. I'll have a look in the morning. Do you know if these model Jeeps can be tested with either a spray bottle or carb cleaner? On my older vehicles I used to be able to mist around vacuum lines and notice a drop in idle when a leak was present. I tried that old trick to show a friend on a 2000 Jeep 4.0 and the computer compensated, so there was no change in idle at all.

Well, here's the update on troubleshooting. I was able to borrow a friend's vacuum pressure tester:
I pulled everything apart, tested each component (hvac, cruise servo and canister under the battery) All hold pressure from the vacuum line that supplies them.
I was able to narrow it down to the the plastic vacuum tube that runs from the manifold to about the power distribution center (under hood fuse box). The plastic lines with "T's" plugged would not hold pressure... so I replaced the entire thing with rubber hose, new "T's", etc. all the way down from the manifold to the canister. The old one had no visible cracks or imperfections.
A reasonable person would assume that when you can test/verify each component controlled by vacuum, introduce new vacuum lines that hold pressure in pace of ones that weren't, etc. that you've now fixed the problem... NOPE! I have the exact same symptoms on BOTH Jeeps with all new vacuum lines.

have you checked what your vacuum reading is while its running? its possible your engines just have low vacuum while running. another way to check for a leak is find a shop with a smoke injection machine, they pump "smoke" into the intake system and look for it coming out of leaking parts.

I have checked the vacuum pressure while running. Everything checks out running at idle and while revving the motor... problem is, my issue does not show when in park or neutral and revving the motor. It only comes up when actually driving the vehicle, which makes it difficult to run a hose through the window and keep on the road watching a gauge while playing with the throttle to cause the issue. The gauge never drops below 15

What I find really weird is that the cruise will hold the vehicle at speed, up a hill while the vent switches to defrost now that I have replaced the plastic lines under the hood. I have the exact same result with the cruise off. If I had weak vacuum in the motor(s) I'd have other running issues, error codes for running too lean, etc. It has to be a very minor leak somewhere that only appears when there is additional suction from the motor. For example the piece of vacuum line that I replaced would not hold vacuum with the tester, but holds positive pressure just fine and there isn't even the slightest visible imperfection.

I really don't want to spend the $$ to take to a shop. I'm definitely selling one and have already sunk a little too much into them to ensure everything is perfect on them. Another couple hundred in them and I'll exceed a reasonable asking price.

To give another update... and beg for some additional insight...

Took one Jeep to a shop ($289 fee) for the smoke test. The results show no leaks.
Had the professionals hook up their vacuum gauges ($147 fee) and the motor is well within range of the vacuum it is creating.

I did find a service bulletin from Jeep calling for a vacuum check valve (part #0467720) to be installed between the "T" and the HVAC control. This is a common problem and was part of a recall. So I ran out and bought a Dorman vacuum check valve. Installed (yeah, I'm proud of myself at this point) and took for a test drive. It does the exact same thing!!! Is it possible there is something special about the Jeep check valve above the Dorman generic check valve?

It's getting way past frustrating. With new vacuum lines, shop fees, etc., I'm creeping up on $400 and several days wasted to be at ground zero. Please, somebody has to know! Jeep actually issued a recall for this problem so it can't be a great mystery.