1991 Cherokee XJ, Limited w/ AC

How long should the HVAC vacuum system hold a vacuum with key off?

I just replaced my blower motor and when I tested it the other day, after the Jeep was off for a while, the air came through defrost. Which I believe is due to a vacuum leak, and would be consistent to us losing cruise control (it used to be intermittent).
So I have been working through the vacuum system. I think a small improvement has been made but I have a ways to go.

• I have relocated the old vacuum canister to a location next to the coolant overflow bottle.
• I have replaced the vacuum lines and elbows from firewall pass through to canister and to cruise control - with new, silicone lines and new tees.
• I have left (so far) the line from the Tee on the firewall to the intake
• I have slid silicone hose over top of the hard line that goes through the firewall for HVAC controls - from the Tee to the firewall. I didn't want to try removing this without pulling the dash and sleeving it with a new silicone hose over top should stop any leaks in that location if there were some. I can't access the part that passes through the firewall, or the part of the tube behind the dash.
• I have done a very scientific "blow and suck" test on the check valve by the firewall (check valve acts on the HVAC controls). Seems to be fine. I haven't replaced it.

When I run the engine, I get full HVAC controls.
When I turn off the engine, I get full HVAC controls.
When I come back to the engine after 1.5 hours, it goes to defrost. Lose vacuum.

I don't hear any hissing in the cab or engine bay.

This seems too fast to lose vacuum.

Things I have done, but yet to test:

• I noticed the vacuum harness behind the HVAC controls had some pinched spots, but no visible cracks. I put some black RTV over those spots in case there's a small hole. This is likely from wires/ stereo crammed in there getting pinched.
• I will use dialetric grease before testing again to re-install the vacuum harness to the HVAC control panel. Make sure it's been seated well.

I have planned:

• Do a better repair on the recirc door hose that was chewed by a mouse one day. It has a bit of a backwoods fix on it - larger hose RTV'd onto it. I have a possible "barb" fitting coming in the mail that that tiny vacuum hose will fit over, and some new hose.
• I have a new vacuum canister coming in the mail. The bumper was punched in from previous owner, right where the vacuum canister was mounted. It's been years since I bought the XJ but I am getting a new canister in case there's a small crack on the canister somewhere from that.

I want to figure out if my expectations are too high, and if I should stop obsessing over this. Or if I should keep going.

 

I would think a rather long time. That is the reason for check valves. Having said that.., two things...one, is the silicone tubing you are using rated for being a vacuum line and two, have you performed a smoke test on your vacuum line connections?

 

Thanks. That's what I thought.

Yes, it's silicone vacuum line.

No smoke test yet. Don't have access to a machine, don't smoke, and live in an apartment so need to be careful on what I do in the garage. But a smoke test or similar would be on the list if I should proceed.

 

Just mentioning, if you run the HVAC controls with the engine off, you'll bleed off vacuum pretty quickly. Don't believe there's a spec for how long it should last. I assume functioning returns as soon as you start the engine.

Another "scientific" test, is to start the engine, turn the air to the vents (or floor), then either climb up a long hill (if one is nearby) or find a long road where you can give it a good amount of throttle to accelerate (like getting on a freeway/highway) and see if it flips to defrost before you have to let up. The high load on the engine will greatly reduce engine vacuum. If you have a leak somewhere, the doors should flip fairly quickly.

 

Yes, as soon as I start the engine everything works. Then I shut the engine off, and it works for a while (I dunno how long, I ran the fan on acc for maybe 2-5 minutes and changed modes a few times). Then after sitting for 1.5 hours it was not working. Then works again when I start the Jeep.

Good idea about the high load test.

 

I had a vacuum line for the recirc door that I had a dodgy repair done on it a couple years ago to get it all working (a piece of silicone vacuum line slipped over the original line and RTV'd in place). This is that really tiny vacuum line off of the HVAC control manifold. Mouse chewed through a couple years ago and I had to get the line fixed quickly, so that's what I did. So today I spliced the proper vacuum line size in, used a small metal tube (2mm OD) and slid that into the vacuum line as a barb. Then used adhesive lined heat shrink over top. They don't make barbs that size. It's tiny, delicate, and a PITA. But it was a decent connection and when blowing or sucking on the tube after I couldn't find any leaks after the repair.

I also replaced the vacuum canister, because mine was all banged up and missing the mounting tabs from the PO getting into an accident and punching that side of the bumper in.

Interior fan now works properly in the "vent" position when the was key off for a couple hours. Not half a day or anything, but more than it was. I'll do another test tomorrow. I also have a new check valve that I might throw in and give it another try.

I know I should do a proper smoke test, but I'm happier with this outcome so far. I'm trying not to make a mountain into a mole-hill. If it's working well enough I'll call it.

Will do a full load test on the weekend when I have the time.