2002 ZR2. Fighting the typical issue of the vents switching to default under engine load. Me and Mr Mighty Vac have been chasing leaks for 2 months now. I have taken 4WD out of the loop because of a known issue with the switch at the transfer case that I will address when I get a chance to get under the vehicle to replace all the vacuum lines and switch. Just replace the intake manifold gaskets and seals and addressed a few issue there. The vacuum reservoir and check valve check out good and all lines in the engine compartment to the plastic line thru the dash replaced. Installed a new stereo and alarm system which gave me the opportunity to check vacuum lines and pots for the HVAC system. Tested through the multi port connection at the back of the switch where I could operate all pots and assure they held a vacuum. The only issue I found was at the switch itself that I can actually hear leak down after about 5 seconds of engine shut off and can't get the inlet port to show any vacuum with mighty vac. Pulled the switch off of the main control panel and disassembled it to find a pretty simple arrangement. Port disk with 6 ports, a rubber disk with veins and a spring to keep the pressure against the 2 disks. Stretched the spring to create more pressure, cleaned the disks up and put a light coating of grease between the contact/rotating points. No change still leaks down. Is this supposed to leak down at the switch?

 

Did you open up and work on the springs of the vacuum solenoid valve? It sounds like you did.

(BTW, this is what I'm calling the vacuum solenoid valve More Information for ACDELCO 1572182 )

I've wondered if mine was leaky too. However, I only notice that my air delivery will change when driving up a hill (engine under load for a while) if I have not driven the truck for at least several days or a week.

So to answer your question, no, I do not think it should leak down so quickly.

It sounds like you did check the lines that are controlled by the vacuum solenoid valve and the ones in the engine compartment. Did you also pull a vacuum at the end of the line that supplies vacuum to the solenoid valve? This would determine if there was a leak along that line between the vacuum solenoid valve and the engine compartment.

On my 1999, for as many of the hose to nipple connections I could reach in the engine compartment and under the dash, I used small zip-ties as hose-clamps as an extra layer of insurance against leaks. You might try that.

Assuming all the hoses and connections check out, the only thing to do might be to replace the vacuum solenoid valve.

 

I don't believe my 2002 has the same valve, mine is attached to the rear of the switch and is round. Thanks for the reply.

 

Ah, yes. Got it.

Did you test the ability of the line that supplies vacuum to the control switch to hold vacuum?

Did you bench test the switch?

 

Tested the supply from the check valve to the back of the switch and bench tested the switch itself. the vacuum line held and the switch did not.

 

Does anyone know if the mode switch is supposed to "leak down" or should it hold vacuum after engine shut down.

 

I am waiting to see if someone knows this as well.

man0steel: question....after engine running/driving for awhile, then stopped/engine off...is there vacuum in the reservoir after say 15 min or so?

 

Complete loss of vacuum in about 5 seconds. I tested this with a gauge after the check valve, took the HVAC out of the loop and it held vacuum.

 

I would think that total loss of vacuum in 5 seconds means there is a bad leak or a component is bad. In this case it sounds like you have isolated the leak to the control switch.

 

My leak was at the mode switch in the A/C- heat controls on the dash. The rubber veined disk inside had shrunk and was leaking vacuum. OE switch from Rock Auto ($80) solved the problem. Now when under a load the heat doesn't blow in my face anymore when on floor setting and 4WD will stay engaged.