scorpion66772

Im sure the fix for my problem is listed somewhere in here but I cant seem to find it.. Ive seen mention of it elsewhere but no fix listed. I am having an evident vacuum leak or switch malfunction when the engine is under load, going uphill or acceleration, it goes into defrost regardless of previous setting. I have traced all vac lines underhood up to the firewall and have found no leaks with either visual inspection or carb spray. There may be a possible EGR boost valve problem unless it is supposed to run for extended amounts of time but I dont get a Vac leak code. Ive checked the vac valve just upstream of EGR Sol and it functions as it is supposed to. Ive plugged a vac meter into the system and it hovers just shy of normal under idle but regains fine at 1k rpm. Once again I havent found any vac leaks under hood but havent dug into the dash other than at the control valve. BTW its a 99 Jeep cherokee sport 4.0..

CCKen

EGR boost valve? EGR Sol? 1999 Jeep Cherokee, XJ body, doesn't have an EGR system.

Unless you connect a hand operated vacuum pump to the vacuum source tube at the intake manifold and pump the system down, you may be missing something.

Two common problem areas are a disconnect at the vacuum reservoir and a break/chafe in the tube behind the battery. Check these closely.

Lowrange2

Have you checked the plethora of emissions lines under the Jeep?

scorpion66772

Ive checked behind the batt and RnR the line to the Vac res and the res itself is in good shape. EGR boost, Im referring to the Pump that checks the vac-egr system before it goes to the vapor recovery canister under the vehicle.. Other than rusted the undervehicle lines seem to be in working order.. I even plugged them to see if it increased my vac pressure to no avail... Manual vacuum seems to be the next tool I need to buy.. unless there is a possibility of the selector valve being damaged.

salad

Cracked reservoir in the bumper can also cause this. Take the passenger side front bumper cap off and you'll find it.

CCKen

Here's the layout of the HVAC/Cruise Control vacuum system in the engine bay.



Here's a photo of the vacuum hand pump I use to check vacuum systems. Got it from Harbor Freight for around $27.00. It's a brake bleeder pump. Shown, is the pump hooked up to the vacuum source at the intake manifold. You can pump the system down to around 17-20 In./Hg. and see if it holds. If it does there's no leaks.



But if you look at the vacuum system layout you'll see two check valves. The check valve at the manifold is supposed to trap vacuum in the system when there is a vacuum loss when the engine is under load. The vacuum reservoir is there to tempoarily provide vacuum to the system. The HVAC check valve is supposed to trap vacuum in the HVAC control system in the event the manifold check valve fails or the reservoir is depleted. There is a possibility that both check vavles have failed if you lose HVAC control.

Check valve at manifold:



HVAC check valve:



The check valves can be checked. Remove the valve and blow into it in both directions. There should be free flow one way but not the other. If there's flow both ways the diaphragm is ruptured and the valve should be replaced.

Lowrange2

Great info! I'll have to check the valves when I get home. Mine does this as well.

CCKen

I should have mentioned it in my previous post; check that 3MM tube going from the HVAC check valve to the firewall for chafing, or being broken. I've read several posts that this tube was found broken.

If it is chafed or broken, a rubber tube splice can be slipped over the chafed/broken tube. I saw 2.8 MM ID rubber tubing at O'Reilly auto parts that can be used for this repair.

Lowrange2

I've visually checked all the lines in the engine compartment. I don't have a vacuum pump. I need to get one.

Big David

I have the same issue. Thanks for posting.

Lowrange2

Alright, I guess I'm going to have to go buy a vacuum gauge.

Checked the check valves. They're good. All the lines appear to be in good shape and working.

Still stumped.

cruiser54

The Mighty Vac is a good one and it's also a one-man brake bleeder.

scorpion66772

Been sick and someone stabbed all 4 tires on my Jeep so I havent been doing much. I have checked all the items mentioned so I too am looking at the vac. Mighty Vac seems like a good tool to have in the inventory. Will advise. Thanks for the info I hope it is helpful to many.

Gmartin

Hi all,

I am having a similar problem. I know that the 3mm tubing from the firewall to the check valve is cracked on my Jeep. Unfortunately its right at the connector to the check valve. Does anyone know the part number for the hosing? Im new to Jeep work and i want to replace the front half of the hosing with the connector but the Napa guy needs a part no.

Please help?

CCKen

Here's a pic of that broken tube and a fix using rubber vacuum hose from O' Reilly Auto Parts. Note that this XJ doesn't have an HVAC check valve.