HVAC No Vac

[SSB]

Deuce's air flow to the interior falls up to the defroster when under boost regardless of where the HVAC panel is set (except defrost, of course). I've checked the vacuum canister and it holds vacuum and followed the vacuum line to the HVAC. OK, vacuum operates the diverter and the HVAC directs the vacuum. Where is the check valve? There has to be a check valve otherwise the vacuum canister becomes a pressure vessel when under boost.

Is the check valve internal of the canister??

What say you Collective?

[andrethx]

not sure about the check valve...

IIRC, though, if there is no vacuum going to the HVAC control panel (or if no buttons are selected), the system routes to the defrost position by default...so perhaps there is a leak or a small crack in one of your vacuum lines?

andré

[SSB]

Thanks Andre

The system works correctly up until the intake pressure goes positive, then it falls to the (as you say) default position of defroster. I've applied vacuum (or removed as the case my be) to the system from the hose at the intake manifold, before the canister. Then going into the car with the engine not started the air flow will go to wherever I select and stay there. So there is no leak anywhere but the vacuum is being replaced by pressure when the boost comes up. So somewhere there has to be a check valve and I have looked and not found anything. It wouldn't make any sense to have a check valve after the canister as then the canister would, under boost, become a pressure vessel. As I have two 505 N9TE's I have checked both and found nothing that looks like a check valve.

[Bean]

If you create a vacuum in the reservoir, and then open it to atmosphere does it continue to hold vacuum or does it vent? Have you then applied pressure to see if it's stops or if it goes in?

To me it would make sense that the valve would be integral into the reservoir, but I've never checked it. It not working and allowing boost to pressurize would explain everything - so I would bet on the tank / check valve.

I know certain cars have check valves for the brake booster - so maybe it's inline somewhere.

Rabin

[SSB]

"If you create a vacuum in the reservoir, and then open it to atmosphere does it continue to hold vacuum or does it vent? Have you then applied pressure to see if it's stops or if it goes in?"

If I use the engine to supply the vacuum and turn the key off it only holds for a short while. I have not tried to add pressure into the system as it didn't leak down when it had vacuum from a vacuum pump (how does one have a vacuum pump?). I'd guess adding pressure into the system might determine if the check valve is working but I think it has already shown itself not to work.

You know me, I don't like "shot gun" diagnosis but it makes little sense for a check valve in that type system to be anywhere else but in the canister. I guess I was looking for confirmation.

Anybody got a canister? (plastic style)

[Bean]

I don't know if you read my diagnosis debacle when my car was overboosting... Turned out to be the simplest of all things and I didn't catch it right away because I didn't use a proper boost signal to test the vacuum / boost hoses.

In your case I'd still want to know if that's the issue for sure. It really has to be that the check valve is toast - but it can be integral to the tank or it could easily be in the hose that feeds the tank. Any vacuum run accessories would have to be behind the check valve.

Sadly mine isn't easily tested - but if you can find a simple inline check valve it would be an easy fix to install.

http://www.autozone.com/autozone/accessories/Help-Universal-one-way-vacuum-check-valve/_/N-255s?itemIdentifier=102999

Rabin

[SSB]

That is a good idea !!! The hose between the manifold and the canister has no devise on it but we could add one. Not sure about the physical size other than the ends being able to plug into the existing line.

OK, anybody got a check valve (lol)