97+ vacuum system

[Red7delta]

Could someone here explain the system on the 97+ xj? I get what engine vacuum is, but I don't understand the various devices the vac lines run to. I have an issue too where if I press the gas enough the interior vent switches to defrost until I let off and it'll return to where I have the control set. The vacuum also isn't strong enough, if the fan speed is at max, to go from defrost to vent.
As I understand it the cruise control is vacuum controlled, another problem area I have. I press the set button and the xj will accelerate a few mph higher before coming back to where I set it.
I checked the lines and the one check valve, found one leak but fixing it didn't solve anything. Possibly one of the various thingys the vac runs to could be bad?

[4.3LXJ]

That is generally caused by a vacuum leak or more often a bad vacuum reservoir located behind the front bumper under the battery area. Low vacuum will cause the HVAC to default to defrost

[Red7delta]

The reservoir seems to be ok, though it's no longer on the bumper as I relocated it. No visible cracks, vac line is hooked up

[4.3LXJ]

If it has no leaks, then there is a leak in the lines somewhere

[Red7delta]

I conducted a test today using carb cleaner to check the intake and throttle body gaskets, then used propane to check the vacuum system. It turned up nothing, no increase in rpm or any hint that the gas or carb cleaner was being sucked it through a leak. I think I'll hook up a vacuum gauge next.

Where is the vent door behind the dash?

[NW99XJ]

Interested in the solution, I have the same issue goin on.

[Red7delta]

I also heard that even with the motor off, the vacuum system should still hold a vacuum for a while, and mine does not hold

[4.3LXJ]

To hold, it has to have a check valve in it near the manifold. But it won't hold forever. The vacuum switch in the dash is leaky by design and so is the central axle disconnect by design if you have one

[Red7delta]

Alright, it's been fixed. I used a thick glue along the reservoir just in case it was leaking. The main culprit was the check valve not doing its job once engine vacuum dropped from acceleration.

[4.3LXJ]

Good to hear

[NW99XJ]

Alright, it's been fixed. I used a thick glue along the reservoir just in case it was leaking. The main culprit was the check valve not doing its job once engine vacuum dropped from acceleration.

Where is said check valve located, and do you have a part number that you used for the replacement?