EVAP System - how does it really work?
 

Gents,

Im wondering if someone can school me on how the parts and pieces of the EVAP system operate. Im chasing down a P0456 small evap leak; code comes on sporadically but smoke test doesn't reveal anything. I also get a strong smell of fuel in the rear right occasionally. Its not the tank & pumps, I'm 100% sure. Here are a few specific questions:

1. Charcoal canister - what is its function? Whats in it?

2. Shut-off valve - what is its function? When is it open and when's it closed? I read that it only closes when testing the EVAP system and stays open all other times. Is this true? (it should've been called a "test-valve" if that was true)

3. Purge valve - When does the purge valve tick on/off? I know its function, but I'm not sure when the ECM commands it on/off and what does the shut-off valve do at the time the purge valve is turned on? off?

Did you replace the Charcoal Canister like Hulk was hinting at?



Pretty Sure this is close and should answer your general function questions. This diagram *may* not be 100% model specific to our w211 but general function of each are described in the description.

Perhaps others will be able to clearly define what you are looking for more if this does not answer your question.

***Thanks for the SC oil 1 down, one more SC to flush***

https://cimg8.ibsrv.net/gimg/www.mbw...f59bbb2101.png

I didn't replace it, no. I ran the tests with my SDS and it passed the major leak without any issues, but every so slightly failed the fine leak. Per SDS, it's suppose to lose no more than 5hPa over 60 seconds, and i lost 5.1hPa. I also did a smoke test with the Snap-On smoker, which didn't reveal anything. I dont think that it's a physical leak, I think that its valve related as its the only moving part in the system.

What i cant seem to find anywhere:
When is the Shut-Off Valve open and when is it closed? And why?

*someone correct me please if I am wrong*
From my reading and logic it sounds like during the test procedure is the only time the Shut-off (test valve) is closed then it is re-opened at the end of the test procedure:

"The activated charcoal canister shut-off valve (Y58/4) is closed and the purge control valve (Y58/1) opened. The intake pipe vacuum
arrives at the fuel tank and is detected via the tank pressure sensor (B4/3)."


In simple terms, Test valve shuts, system is sealed. Purge control valve opens and system should have x pressure within x(y1-y2 pressure within this value range).

With the system drawing through the purge valve onto a closed test valve from the attached doc it will show a pressure reading of x(y1-y2). You ran the test and it is .1bar off.

So that would lead me to inspection of where the smell is coming from. Can you run the test with the passenger rear wheel off and get out and listen for a small leak? There is probably a crack in something letting pressure escape or a leaking connection you need to find.

In example
In my engine bay off the back of the supercharger the metal hard pipe has all those crimped metal connections where it goes from metal to rubber hose back to metal... I discovered the hose crimps have loosened up and are letting go and just pull off. I have a feeling you have a wear item that has worn out. This problem area is just an example and not related to your issue as previously discussed in other threads.

Easier to read copy:

https://www.google.com/url?sa=t&rct=...FM8xgFkSuBbgsQ

Put a new gas cap on and run the test again. Usually good tests are in the 2-4 loss range.