EVAP System - how does it all work?
Thread Starter
Super Member
Joined: Oct 2014
Posts: 505
Likes: 39
2016 E63s Wagon - CCB & CF package
EVAP System - how does it all 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?
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?
Last edited by V-AMG; Apr 16, 2017 at 09:55 AM.
Thread Starter
Super Member
Joined: Oct 2014
Posts: 505
Likes: 39
2016 E63s Wagon - CCB & CF package
When is the Shut-Off Valve open and when is it closed? And why?
Dealer tells me valve only opens during filling up and remains closed otherwise. Online i read that its only closed when system self-tests for fuel leaks via vacuum test... tons of conflicting info...
MBWorld Fanatic!
Joined: Apr 2015
Posts: 1,486
Likes: 46
From: dmv
2009 E550 2000 Honda civic mash n' go
I searched here, on google, on my SDS WIS and even called the dealer... and no one can definitively answer:
When is the Shut-Off Valve open and when is it closed? And why?
Dealer tells me valve only opens during filling up and remains closed otherwise. Online i read that its only closed when system self-tests for fuel leaks via vacuum test... tons of conflicting info...
When is the Shut-Off Valve open and when is it closed? And why?
Dealer tells me valve only opens during filling up and remains closed otherwise. Online i read that its only closed when system self-tests for fuel leaks via vacuum test... tons of conflicting info...
Simple. purge valve is in between the charcoal canister and intake manifold. it opens and closes when ever the "ME" requests fuel vapors from the fuel tank. pulse width modulation.
charcoal canister shutoff valve, if it's faulty then vapors are escaping into atmosphere. That part is pretty self explanatory. pretty sure it's either open or closed, no in between.
Both purge and shutoff valve operate at random times whenever venting/de-venting of fuel vapors need to occur or when it's time to regenerate per-say.... hope this helps a little.
MBWorld Fanatic!
Joined: May 2015
Posts: 4,037
Likes: 1,012
From: Miami, FL
2003 CL 600
Shutoff valve is there to allow the system to vacuum test itself. Normally it's open, allowing engine vacuum to pull fresh air into the charcoal canister and move vapors into the engine for burning. The system closes the valve, opens the purge valve to draw vacuum, then closes it and watches the fuel tank pressure sensor to make sure the vacuum holds. If it doesn't, an evap leak fault sets, with the speed of pressure drop determining what size the leak is.
All this info is out there, what exactly are you trying to figure out?
All this info is out there, what exactly are you trying to figure out?
MBWorld Fanatic!
Joined: May 2015
Posts: 4,037
Likes: 1,012
From: Miami, FL
2003 CL 600
You say you're sure it's not the tank/pump, why? Have they been replaced? I just put a tank in a 2005 E320 today under the extended warranty. They leak ALL THE TIME.
The next culprit would most likely be a stuck open canister closed valve, but that will set fault codes specific for it.
The next culprit would most likely be a stuck open canister closed valve, but that will set fault codes specific for it.
MBWorld Fanatic!
Joined: Apr 2015
Posts: 1,486
Likes: 46
From: dmv
2009 E550 2000 Honda civic mash n' go
i would change purge valve, cheap part, and install a new gas cap. if neither of those suffice i would be looking at the tank and or something charcoal related.
but does it pass the fuel tank leak test?
test will tell you exactly what hpa loss is, max of 5hpa loss allowed in 120 seconds..... GL
U can smoke test all you want. Smoke is hard to see at times. The number data of hpa loss will point you in the direction of what component may be defective. Also if you are smelling fuel in cabin it may in fact be pump/pressure reg. seals.... just my two cents.
but does it pass the fuel tank leak test?
test will tell you exactly what hpa loss is, max of 5hpa loss allowed in 120 seconds..... GL
U can smoke test all you want. Smoke is hard to see at times. The number data of hpa loss will point you in the direction of what component may be defective. Also if you are smelling fuel in cabin it may in fact be pump/pressure reg. seals.... just my two cents.
