HVAC Vacum Check Valve Where is it located

[ACR08]

My 96 GTS is having vacuum issues with the HVAC system. The air only blows through the defrost vents regardless of vent setting on the HVAC dials. This leads me to believe none of my vacuum actuators are working and the system is not getting proper vacuum. The shop manual says to first check the vacuum check valve when diagnosing vacuum issues. It says the valve is locate on the vacuum line coming off the intake manifold on the passenger side of the car. This line is easy enough to find and I can see where it runs through the firewall. It looks like there is maybe a check valve at that firewall, but you have to access it from inside (behind the instrument panel) to test and replace it. Is this the case?

I've read a few threads about people battling with low vacuum during hard acceleration and their vent doors shutting. Some people have fixed the problem by adding a check valve to the vacuum line coming off the intake manifold. This confused me. Since they system (at least what I read in then manual) should already have a check valve. So, why do people need to add one? I'm wondering if people have a bad check valve and are installing a second one instead of replacing the faulty valve.

Bottom line, I want to get my system working. I'm afraid I'm going to have to pull the dash apart to get to the problem, but want to make sure. If anyone has pulled the dash apart, how hard was it? Anything I should be careful about? Any good tricks? I'm always worried about breaking tabs and fasteners when doing interior work. Especially on a car with old plastic.

Thanks for the help!

[lane_viper]

Sounds like you are on the right track. I was lucky that the crack in my vacuum line was between the firewall and the manifold and the line was long enough I could cut that section out. Exactly the same symptoms.
I have not added a check valve, and mine does lose vacuum under acceleration. I'm assuming that's normal from what I've read, and the reason why people add the check valve.

If you don't see the line broken or cracked between the firewall and manifold, the next place would be before the actuators in the dash. Unfortunately taking the dash about is probably the next step.

I've taken the dash out before and honestly it was the easiest one I've ever removed. Just be very careful, as replacements are big money if you can get them.

PM if you need more help, as I think I can remember a rough step by step on how to take it out.

[GTS Dean]

I'll say it again - there have been a lot of these same HVAC complaints around here lately.

[goggles Pizano]

pop the vacuum line off the engine and use a manual vacuum to pull vacuum.

another way is buy a cigar and blow smoke into the line and look for smoke if you think a line is cracked or something

[Viper GTS-R]

I'll say it again - there have been a lot of these same HVAC complaints around here lately.

20-25 year old vacuum lines drying out!

[AZTVR]

The check valve on my 2002 GTS was behind the radio. I replaced it with one recommended on the forums years ago. I do not remember if it had failed or not; but, I had the "hot air blowing when on full throttle instead of cool AC air" symptom and that solved my problem.(but, different symptoms than yours.) Remove radio to access it and inspect hoses and connections there. Note that if you want to troubleshoot as suggested above, Autozone lists a vacuum tester in their tool loaner program.

[dave6666]

20-25 year old vacuum lines drying out!

Nylon 6 is not wet. It does not dry out. Nor does it flash residual solvents or outgas or anything like that. The deterioration process is called weathering. Oxidation, polymer molecular chain breakdown, and so on.

[8.0L>1.8T]

Nylon 6 is not wet. It does not dry out. Nor does it flash residual solvents or outgas or anything like that. The deterioration process is called weathering. Oxidation, polymer molecular chain breakdown, and so on.

You had me at "wet."

[ACR08]

I think I tracked down the vacuum leak. Looks like the line from the intake manifold that runs into the firewall and the back of HVAC control unit is cracked right where it enters the firewall. Looks like I'm going to have to pull the dash apart to change the line. Wish me luck.

[Viper GTS-R]

The deterioration process is called weathering. Oxidation, polymer molecular chain breakdown, and so on.

Whoa, slow down egghead!