'98 XJ 4.0- I have a hard to diagnosis vacuum issue. The issue is all air goes through defroster. It only happens when pulling hills. I hooked up a vacuum gauge and when pulling the hills the vacuum goes to zero and stays there until I take foot off throttle. Once his happens maybe 10 seconds later all air goes to defroster. Once foot is off throttle air returns to whatever mode it is set for.

In garage I tested vacuum in all sections (at the tees) under hood with a manual pump tester. Every section held vacuum on the gauge. I then checked the vacuum lines under dash by removing glove box and disconnecting the quick disconnect that attaches the colored vacuum lines. All lines held vacuum except the black one. I was told by a shop who I also had check out the issue, the black line is not suppose to hold vacuum. So I assume this is true. It literally held nothing. Every pump of tester would immediately go to zero on the gauge.

Jeep idles at 14 HG and is steady. My thought is maybe the Jeep is not producing enough vacuum. I am thinking about adding a second vacuum tank to see if that will resolve my issue. Do you think this could help?

I have had a shop look at it and they have no idea. They do not think there is a vacuum leak. My Jeep appears to only have one check valve by the intake manifold. I replaced it for giggles. No improvement. I then added another check valve at the little vacuum line which goes into the fire wall and into the dash. Again no change.

I am just looking for some thoughts, advise and what else I should check or recheck. thanks in advance is you have any ideas.

 

It's called physics. You lose vacuum under a load, like going up a steep grade. There should be a vacuum reservoir behind the passenger side of the front bumper, check it for cracks. Fourteen inches of vacuum is kind of low for a healthy engine at idle. You should have between fifteen to eighteen inches of vacuum at idle.

 

I've had the same issue (although sporadically) so I'll be watching this for a resolution.

 

Yep, did physically check vacuum canister. No apparent cracking, and did a vacuum test as well which held. BTW I live at about 5500 ft, so i believe this is the reason my vacuum is at 14hg. Engine has 150K does not eat oil at all. I believe it to be healthy.

I figured intake gasket and throttle body gasket were not possible leaky culprits because the check valve by the manifold would hold the vacuum in the system.

Any thoughts on the effectiveness of adding a second canister?

 

Here's a vacuum function schematic.





When you say the black tube won't hold vacuum [at the quick disconnect ?], which way were you checking it, towards the engine or towards the Mode Selector? If towards the Mode Selector, what mode were you in?

 

I checked all the lines as shown in your chart both ways. Toward the mode selector, none of the lines held vacuum. I do not recall checking with mode selector switching positions so if this is important I will recheck.

Toward the engine, all the lines with the exception of the black (vacuum supply) held vacuum. I could hear the vacuum controlled doors moving. The Black line did not hold at all, the gauge did not indicate a slow leak it just went immediately to zero after I pumped the vacuum tester. I thought this was weird, but the Jeep idles fine as always, and there is no hissing. So I assumed this is normal?

 

If you look at the schematic you'll see that with the mode selector in Defrost all vacuum is cut off at the mode selector, so if the quick disconnect was connected and vacuum was applied at the small tube going into the firewall it should hold. If it does not hold, there is a hole in the black tube, the quick disconnect leaks, or the mode selector leaks.


The tubes take a tight turn through a sharp opening in the dash structure from the mode selector to the quick disconnect. You can just see the point above the power outlet connector in the pic. A poster on one of the forums found his black tube chafed through at this point.




Does your HVAC Tee setup look like this:

 

Got it. I am going to recheck vacuum with the mode selector in Defroster from the small tube in the engine compartment which goes into the fire wall. I will report back.

No my '98 does not look like the Tee you presented. My did not have the check valve. I did add a check valve there a couple of weeks ago to see if this was the problem, no difference.

 

I checked for vacuum at the little line which leads into the firewall with the defroster selected. It did not hold. I then plugged the little vacuum line in the engine compartment and rechecked vacuum from the disconnect behind the glove box leading into the engine compartment and same answer, A very heavy leak.

I guess I need to run a new little black vacuum line. Does it go directly from the engine compartment to the mode selector? OR does it make any stops? I would like to find a way to run a line without tearing the entire dash out. Has anybody done this? If so how?


Is there a reason such a large leak would not cause an idle issue or any other kind of issue? Also, as described above all HVAC modes work when driving the vehicle unless I am pulling longer grades.

Thanks for all the help.

 

Does your check valve installation look like mine - white out, black in?

 

I double checked the vacuum check valve and it does look like in the picture. I guess it would not matter because the line has a break in it anyway so it can't hold the vacuum??

Here is what I am going to do to fix the line. I am going to by pass the line completely. I will use the existing HVAC tee and run a new little black line into the fire wall either through the passenger side where there is a electrical harness in the upper corner, or through the driver side where I have run electrical wires before. I will then cut into the black vacuum supply line above the "quick connect" behind the glove box and solve the problem. I hope. If it works I will upload a photo of how i did it

Let me know if this is too smart.

Vacuum does not hold to the mode selector either when i tested it through the "quick connect" with the defroster mode selected. Should it? I might have two leaks.

 

It should not leak.

 

Thanks CC Ken, you information was very helpful. I hope my fix will work. I will report back. So the thread can be closed.

 

Okay, I ran a new HVAC vacuum line. Here is what I did: I cut the old line in the engine compartment and I pulled the other end off the quick disconnect behind the glove box. I than decided I would stick a coat hanger in the same hole where the existing cracked vacuum lines run. To my surprise I was able to fish the coat hanger through and I pulled a new vacuum line through. Now I have a new vacuum line without a crack and no longer have a vacuum leak. Very cool and the cost was about $5 for the vacuum line.

 

Good work.


Thanks for the follow up.