what does this valve in the evap system do

[fastboatster]

This plastic canister/valve is located right upstream the n80 purge valve


same valve appears to be used in the A6 with 3.0T as well. I'd like to know what it does and what happens when it goes bad. Is this some kind of vacuum check valve? Audi very descriptively calls it a hose, and some listings call it expansion tank vacuum valve.

[RPMtech147]

Good question. I have a spare one at work. Maybe I'll cut it open for science. I'm guessing it's a filter as it's sucking vapor from the charcoal canister. Wild guess.

[fastboatster]

Good question. I have a spare one at work. Maybe I'll cut it open for science. I'm guessing it's a filter as it's sucking vapor from the charcoal canister. Wild guess.

Thanks, appreciate the response. Would be great if you could also check if it’s a check valve by applying vacuum to both ports

[Smac770]

What little I find about it says "expansion chamber". Until someone cuts one open, we probably won't know for sure. I assume it's some kind of expansion space for the fuel vapors in the line up to the N80 which would heat up after engine off.

[fastboatster]

What little I find about it says "expansion chamber". Until someone cuts one open, we probably won't know for sure. I assume it's some kind of expansion space for the fuel vapors in the line up to the N80 which would heat up after engine off.

Yeah, I’m not able to find much more, some sources also call it check valve hose set but then there’s something about expansion:
https://www.ecstuning.com/b-genuine-...BoCF5IQAvD_BwE
I think this valve has an arrow on the housing which implies its directional.

[Smac770]

No arrow on mine. Mine doesn't hold pressure or vacuum in either direction. It's light; hollow sounding. Believe it is nothing more than an empty cylinder.

[fastboatster]

No arrow on mine. Mine doesn't hold pressure or vacuum in either direction. It's light; hollow sounding. Believe it is nothing more than an empty cylinder.

thanks a lot for checking! My "a4 3.0t" has a slightly different canister, but the a6 with 3.0t has the same exact canister as a4 2.0t so the canister functions should be the same. Seems like it is some kind of expansion valve, mine does have an arrow on it but also doesn't hold any vacuum on either port. I suspected it was bad, looks like it does what it's supposed to but it might still have some cracks as I'm getting some pretty strong smell of gas from around that canister area at times.

[Smac770]

Yeah, saw your thread in the S4 forum, but as I don't have one on hand, I had no validated information to contribute. The 3.0T version has a part number on the actual cylinder. What is that part number? Searching by the component part number might yield some references.

[fastboatster]

Yeah, saw your thread in the S4 forum, but as I don't have one on hand, I had no validated information to contribute. The 3.0T version has a part number on the actual cylinder. What is that part number? Searching by the component part number might yield some references.

that part number unfortunately refers to the entire hose, I tried to check it already. just had to assume that that canister in the s4 bears the same function as a canister in the a6 which xrefs to a lot of cars.

[RPMtech147]

As promised:

There's nothing in it.

[NitroViper]

As promised:

There's nothing in it.

Just looks important.. but in all seriousness it could have a choking affect, sort of like how a 2 stroke exhaust works with the odd shaped pipes.

[RPMtech147]

Just looks important.. but in all seriousness it could have a choking affect, sort of like how a 2 stroke exhaust works with the odd shaped pipes.

Normally I'd say they put in there for a reason...but it's German, so it probably does nothing. You could install a straight piece of hose and I can say with 99.9% certainty it would change nothing and the system would still function normally.

[Smac770]

If it did have a purpose full time, not just something like handling thermal expansion, I'd guess it would be for slowing the air flow velocity. Like the doors on the intakes of a supersonic fighter close so the sudden expansion right after slows down the incoming air stream before hitting the compressor blades. Maybe the mix works better with a slower gas flow. There's any number of actual engineering possibilities. But you can be sure that VAG didn't approve anything that would add cost unless it was deemed necessary for some agenda. It's interesting that both the EA888 and EA837 have the object. Is there any similar construct on engines of other manufacturers?

[RPMtech147]

Is there any similar construct on engines of other manufacturers?

I've never seen an empty canister on any evap system like that regardless of manufacture. It could be some type of expansion buffer to keep a more consistent or smooth pressure change when the LDP is doing LDP stuff while the car is sleeping.

[fastboatster]

Thanks for an autopsy on this valve! It is called an expansion valve on the ecs tuning page. Perhaps it’s to allow for an expansion of the vapors trapped between the canister and the engine when it’s not running and can’t burn the vapors?

[fastboatster]

Got a new one today which I promptly installed right at the dealers parking lot. Vacuum testing the old one and there’s a vacuum loss of about 1.5-2 in Hg per 5-7 mins at 22 in Hg vacuum. This could have caused issues, couldn’t it?

[Smac770]

The object should most certainly be sealed with 0 loss over hours. If you are sure the two connection points were sealed against stray leakage, and the tools involved to create and measure the vacuum do not have inherent leakage, any leakage through the seams or pinholes of the object pieces would be a problem.

[fastboatster]

The object should most certainly be sealed with 0 loss over hours. If you are sure the two connection points were sealed against stray leakage, and the tools involved to create and measure the vacuum do not have inherent leakage, any leakage through the seams or pinholes of the object pieces would be a problem.

that is true, I guess I'm just curious how this vacuum loss corresponds to the size of evap leak (small, big or gross evap leak). I guess don't matter too much since the car appears to be running a lot better when the purge valve is open.

[RPMtech147]

I assume you're chasing an evap leak. Have you smoke tested the evap system?

[fastboatster]

I assume you're chasing an evap leak. Have you smoke tested the evap system?

yes, the evap and intake systems were "smoked", but I think the evap was smoked from the metal pipe in the wheel well, i.e., that "valve" was not smoke tested.

[RPMtech147]

Well, let us know how it goes. If you still get the p0455, I'll give you my diag procedure. Usually, it's the purge valve getting hot and sticking closed.

[fastboatster]

Well, let us know how it goes. If you still get the p0455, I'll give you my diag procedure. Usually, it's the purge valve getting hot and sticking closed.

Yes, you guys deserve an update, I drove the car a few times during the last 2 days, no evap codes and more readiness monitors have passed, including fuel system monitor, and catalyst readiness moved from “Not completed successfully” to successful completion. And the car just idles a LOT smoother. I’m very pleased overall, I would have been chasing my tail for a lot longer if not for this thread and forum section.

[Theiceman]

Thanks for an autopsy on this valve! It is called an expansion valve on the ecs tuning page. Perhaps it’s to allow for an expansion of the vapors trapped between the canister and the engine when it’s not running and can’t burn the vapors?

bingo ..its this..