audijim113

This is a follow up to the purge valve I posted about. I haven't replaced it yet cause took a week off and flew to FLA. I understand the two things might be related. (Correct me if I am wrong).

After the car has been running awhile and I stop get gas, should I hear an audible rush of air being sucked in? (whoosh?)

If I don't hear it, is my gas cap bad?

silverd2

In most cases there should not be "much" in either direction. A little or none is usually normal.

If you do hear a significant big whoosh, your cap is probably bad.

The gas cap DOES have a one-way check valve that only works inward...should allow air into the tank as it drains down. Tank is not supposed to retain a vacuum, but is supposed to hold pressure when needed (esp during leak detection pump test...normal on every start up)...the purge valve then takes care of bleeding off pressure (when present) and burning it.

There's an expensive testing device for vented caps, but there's another way too....laugh if you will, but little no more than the power of a normal human inhale will open the gas cap vent check-valve.
Being a self-proclaimed non-professional "stunt mechanic", I have tasted, inhaled and occasionally (unintentionally) swallowed every fluid present in an automobile...testing without special equipment

ryan_qttro

Your car is fine, it is just the fuel vapor fumes releasing into the atmosphere, that haven't been vented into the engine. If the gas cap was bad you would have a check engine light on with an Evap leak code

silverd2

Not necessarily...if the cap leaked outward (under pressure from leak detection pump) it would show a leak,then yes...therefore CEL.

If the vacuum release (inward) check valve on the cap failed shut, it would not be a leak...or detected by the LDP start-up test.

An un-vented tank (no vacuum release)can "apparently" run out of gas with lotsa gas in the tank.

Carpoof

Glad I found this thread. My '97 4.2 does the same thing. I have a check engine light on for P0442 (or P0441 can't remember) (Incorrect purge flow) that comes and goes. I also have a lot of vacuum at the cap when I open it. I thought the cap might be bad, but everybody told me the gas caps only job was to seal the tank, not to allow air in like I thought. I will try to check to make sure the cap flows air like you said, if not I will start with the gas cap.

ECS Tuning-Audi

I always assumed a little Pshhh when you open the gas cap when you are really low on gas to fill up was normal?

Jason

Carpoof

I think a little bit is, but at least in my case it takes about 5 seconds to fully release. Plus I get a bubbling sound in my gas tank after I shut the car off for a few seconds.

ECS Tuning-Audi

Seams more like a purge valve issue rater than the cap.

The tank doesn't seam to be venting properly. I thought I also read something about evap lines getting clogged or something that can cause an issue like this? It was a while ago I read it, it could be completely different just brain storming here maybe some one can chime in.

Jason

silverd2

Jason (and others),

The poster (Jim) started another thread a few weeks ago about needing a purge valve...I assume he had a code indicating that. He probably does need one for other reasons...a cap blowing off lots of pressure could be one symptom...maybe a canister problem, too(?)

This thread started with a question about normal inward whoosh upon cap removal.

My point..and an OEM supplied cap fact...and I repeat: the leak detection pump on a D2 puts extra pressure on the entire evap system (which includes all air space in the gas tank) every time you start the car It then measures (times) the leak down of pressure...if it leaks down fast, you get a large leak code...if it leaks slowly, but more than it should, you get a small leak code.

The gas gap indeed is supposed to hold in pressure, but (at least on 2000 and up), the gas cap also has the duty of preventing excessive vacuum. Possibly pre-facelift D2's have a vacuum release valve somewhere else (different part # for their gas cap...easy to check...suck on the inside of the cap and the valve should open ...does on a 40V cap). BUT a tank under too much vacuum can NOT continue to deliver gasoline to the engine...it will run out before empty. A small vacuum is OK, but a large one (creating a pronounced whoosh upon cap removal) can be an engine stopper, put unnecessary strain on the fuel pump and/or cause a lean burning condition.

The purge valve is there to release gas vapor under PRESSURE (NOT vacuum) to be burned off, rather than be vented to the atmosphere...pressure caused by rising temps, car exhaust pipe heat transfer, etc, when not enough gas is being drained (causing vacuum if temp is constant) to bleed off pressure. The charcoal canister is there to store pressure formed when car is not running (sun beating down, temp rising and purge valve not operating). If purge valve fails to let pressure pass through to be burned, there is a pressure release on or connected to the canister to release it into the atmosphere if absolutely necessary...but that's why it's a "charcoal" canister, so that any emergency release is vapor that has at least been partially "scrubbed".

The main point is that all fuel delivery systems MUST have a vacuum release somewhere to allow the pump to work...and, by law (anti-pollution) must hold pressure, which is released by a purge valve of some kind, to be burned (also by law).

There will be a quiz tomorrow...class dismissed

Cheers,
Sam

ECS Tuning-Audi

Sam,

Thanks for the informative post! A lot of good info.

Jason