1995 Blazer, 4.3L, VIN W, 210,000 miles.

My HVAC system is stuck someplace and pretty sure I have a vacuum leak.

In all modes, air is coming out windshield defroster and the floor vents. I believe this is "default" mode since disconnecting the vacuum line to the firewall results in the same behavior.

I have a vacuum gauge and pump.

Under the hood
1. all lines on the engine-side hold a vacuum. (assuming lines good)
2. check valve lets air out but blocks air coming in. (assuming check valve good)
3. vacuum reservoir holds a vacuum. (assuming good)
4. line going through firewall into dash will not hold a vacuum.

And disconnecting this line from item #4 above makes no difference... this seems to be the problem.

Ok, so now what? It looks like a vacuum leak someplace under the dash.

Does the fact that the problem occurs in all HVAC switch positions, confirm that the leak can only be in this main source line? i.e. if one of the branch lines leaked, then it would only occur in that one switch position, I think. From that reasoning, can I conclude that the actuators are not the source of this leak?

Is there any easy places to check or do I need to remove the whole dash to get to this?

Basically, the problem seems obvious but I'm looking for hints, tips, etc. that will help me narrow it down or get to it. Is there a common failure point for this line? Does it go straight from the firewall to the HVAC switch?

Thank-you!

 

This is kicking my butt...

I took off the HVAC switch and tested the vacuum there with my pump and gauge... every single line including the source line held a vacuum. I'm guessing the switch itself has a slight leak but I don't think this is the problem.

When the car is running, there is no sucking noise whatsoever coming from any lines. In fact, none of the lines at the HVAC switch appear to have a vacuum from the engine.

So I go back under the hood.

Tracing back from the firewall...

Source line from under dash goes into a T. One side goes to check-valve & reservoir. The other side goes behind the engine and down to the transfer-case/transmission. ?!?

Again from this T, while engine is running, there is no vacuum whatsoever.

I don't understand how vacuum from the manifold gets into this line when all it does is go down under the car. Shouldn't it be hooked to the intake someplace?

The diagram in my Chevy Blazer manual is not very detailed and does not show where it goes for the 4.3 vin W.

Any insight appreciated.

 

One leg off of the check valve should go to the intake.

 

That would make total sense and certainly explain everything.

That hose is hanging free and I cannot find the port where it came from. I'm assuming the rear of the intake manifold but I must be blind.

 

THANK-YOU! Yes-I am blind.

Thanks to a mirror, I found the port. And I have no idea how it popped off in the first place.