Hi everyone. I think I have a vacuum leak somewhere with the HVAC system. I think the actuators are losing vacuum when I accelerate, which is causing the airflow to decrease out the dash vents even with the blower on max and sometimes there is a slight vacuum reduction at the brake booster when the brakes are hot/frequently used and the pedal doesn't have the same travel (but the car stops fine. Sometimes the brakes stay like that as long as they are hot even after a shutdown for 5 minutes and restart, so I am not sure it is a vac problem. It holds vacuum for days. Might be the A/C only, so I am going to diagnose that first.). The airflow restores when I bring the engine to idle, meaning a low vacuum situation at the HVAC system that corrects itself when the vacuum increases again.

Also, this might be a bit off topic, but where is the vac hose that goes to the transfer case switch for the 4x4 connected to at the engine side? The actuator is no longer connected to the axle, making the hoses, actuator and cable totally unnecessary. For my 4x4, that is what the Posi-Lok is there for. I was thinking of disconnecting the hose and plugging the ends to rule out those hoses first since the fix is simple. What is the best way to cap off those lines to prevent leaks and dust intrusion?


I replaced the 7/32" main vac hose from the engine and a few of the smaller ones that connect to the A/C under the hood (between engine and check valve, between check valve and vac tank, and a couple little ones that go to the hard line to the evap system) but that didn't solve the problem. Now it is even worse and the airflow only is improved at higher rpm when I am moving at a high rate of speed forcing air into the truck. Means there is a vac leak somewhere else.


On Friday, I drove the truck from the capital region in NY to Northeastern PA (via I-88 to Binghamton, NY and then I-81 into Scranton, PA) on a camping trip (Both nights I slept in the truck. First one was hurricane like rain, second was only because I didn't want to waste time putting up a tent to sleep 4 hours before making the trip back to upstate NY. Saturday/Sunday had no rain.) in rainy weather that included the windshield fogging up. I had to accelerate to like 2500rpm to make the vacuum actuators get it into defrost mode to clear the windshield. With the vents on dash mode, accelerating above 1800-2000 made the airflow decrease without lowering the fan speed. With defrost, I had the opposite problem (worked better as I increased power). Obviously I am dealing with a vacuum problem, but the truck runs just fine and I am getting between 18-21 highway mpg throughout the trip depending on the speed I drove. Where else could vacuum be escaping other than the lines or the intake manifold (Replaced 2/2009, no oil/antifreeze contamination, no drips on block.) that I can look for? Could there be a line loose behind the dash that I need to look for? Could the PCV valve be a problem even though I just replaced it and it makes vibrating noise sometimes?

 

THIS POST I made back in January explains the routing fairly well.

You will loose vacuum to the HVAC & 4wd during acceleration if your check valve is not functioning properly, the reservoir isn't in the loop, and/or there is a sizable leak (leak doesn't have to be that large if the reservoir isn't in the loop and/or the check valve isn't functioning).

 

Check valve? Is that the "T" near the firewall with the oversized round part on one side that takes the vacuum from the engine and gives it to the HVAC/4WD and other stuff?

If that is the case, I will go to Advance after class and see if they have it and replace it. Thanks

Also, I checked out that vacuum diagram. Would it be safe to disconnect and plug the hose going to the transfer case since it is no longer used to reduce the amount of possible leak points?

EDIT: Is this the part? http://shop.advanceautoparts.com/web...7CGRP2037_____

Ok, I replaced the check valve with a part from Advance that is the closest thing to it in the HELP! section (but it only goes up to 96) and that didn't solve the problem. I even found the hard tubing going to the A/C system and disconnected and capped the other one (4WD?) and that still didn't work, so that limits it to between the engine and the vacuum actuators of the A/C. I even ziptied the lines where they were slightly loose on the connections. I installed the valve in the same way as the other one, but for some reason, I think it might have been backwards to begin with. For starters, I disconnected the tubing between the check valve and the fender (vac tank?) and there was no vacuum out of that outlet on the valve. I am thinking the check valve has to be BEFORE the split between the vac tank and the A/C and 4x4 hoses. I am attaching before and after photos of the setup as tested.

 

The post I linked to spells out the routing, but if that wasn't clear enough, here's the progression; Engine, inlet (single) side of check valve, 'T' side of check valve out to another 'T' that connects the HVAC, 4WD, & reservoir.

The black part of the check valve should be towards the engine. If the system is intact, you should be able to disconnect any line from the white part of the check valve and hear a rush of air into the system.

You can test the check valve by plugging the two white outlets & sucking on the black inlet. You should be able to pull a vacuum, stop, and there should still be vacuum holding your fingers on the two white outlets.

 

Well that is the problem then. The check valve was reversed all along. I will go out now, reverse it, test drive it and post the verdict. Thanks!

 

Ok, I reversed the check valve and it seems that the airflow isn't dropping as it did before (It is much much reduced if I notice any drop around 3000rpm). I think the problem is solved, but I will need to do a longer drive to see if that is a fact. Thanks.

Now I know why the shift fork on the axle was destroyed when we bought the car because it was partially releasing itself because of low vacuum. I just hope that between 12/2007 and 1/2011 that it didn't damage itself again since it is hard to engage with the Posi-Lok after I had that installed. It takes a special trick.

EDIT: I drove it around today in town trying to get it above 2500rpm on acceleration to see if the problem is still there and I think it is solved. New lines, check valve and reversing it solved all this. Maybe it will improve my MPGs a bit (doesn't low vacuum decrease MPG?).

 

Does the 4x4 line serve any purpose at all except for activating the actuator? Does it serve as a vent? I pulled one of the caps off the line before and found it either pressurized or under vacuum (heard a hiss) even though there is no vacuum source attached to it. See attached photos.

 

bump?

 

I'm going through the same sorta thing. When in 2wd there is still slight vacuum to the 4wd actuator, but not enough to pull the cable. When in 4wd the vacuum is stronger and actually pulls the cable... I'm wondering if my vacuum lines are mixed up at my transfer case.

 

You probably have a leaking vacuum switch on the transfer case. Common problem on these trucks. In your case, it is probably letting partial vacuum through in 2WD. New part is $15 at Autozone or Advance in the Help! section.

Only thing is in my case is that the transfer case is no longer connected to a vacuum source since there is no need for it since I have a Posi-Lok system. I had rotted vacuum lines all over the place and the transfer case one is the worst to replace so I replaced all the rest and removed the remaining one from use since it is not needed since the cable that is attached to the actuator is disconnected.