EVAP system

**T0M3K** · 07-14-2008 08:03 PM

I noticed that I know nothing about it, with the exception it is required to pass emissions and it has something to do with secondary air injection or something like that. Can somebody explain to me what it exactly is and how it works?
Thank you.

**mxrz** · 07-14-2008 09:07 PM

Simple explanation: The EVAP system takes care of fuel vapors, which are routed to a charcoal canister, and from there, they're routed into the air intake.

**brewmastr** · 07-14-2008 10:06 PM

more informative explanation: ok can u remember when you would walk into the garage as a kid or maybe young adult depending on your age and the frikin thing would stink ass like gasoline? this is what evap is for. okay so in the world of vehicle emissions you have HC's (hydrocarbons) CO (carbon monoxide) NOx (oxide of nitrogen, or ground level o-zone), ok the one we will be discussing here is hydrocarbons. HC's are unburned fuel. now evap is pretty much exactly what it is. as im sure you know, gasoline evaporates over time, before evap these vapors would simply escape out of the fuel tank and into the enviroment, causing a high HC emissions from vehicles, so what was needed was a way to catch these vapors and vent them into the engine to be burned (as even though they are vapor now, they still are unburned fuel) so what was created was the evaportive emissions system, which consists of a pressure sensor which is essentially a "I'm telling mommy device" to monitor the system for leaks and proper operation, a charcoal canister to capture the fumes, a fresh air vent for the canister so during purge operation it can flow, and a purge control solenoid up near the engine to to purge fumes into the engine. hopefully this has been the informative answer you were looking for.

**T0M3K** · 07-15-2008 06:02 AM

Thanks, it helped a lot. So now it makes sense why we have little air compressor that works when engine is cold, to push all those fumes out of the canister to the air intake. It's dumb but does it result in higher rpm at colder engines? I'm asking all of those because my friend looked at used A6 (w/ 4.2) and it had lots of codes related to evap system. Myself I had some problem with a valve that didn't open and gave me MIL/CEL.
Thanks again for the info, won't mind more information for curious mind.

**kpatz** · 07-15-2008 06:13 AM

Originally Posted by T0M3K

Thanks, it helped a lot. So now it makes sense why we have little air compressor that works when engine is cold, to push all those fumes out of the canister to the air intake. It's dumb but does it result in higher rpm at colder engines? I'm asking all of those because my friend looked at used A6 (w/ 4.2) and it had lots of codes related to evap system. Myself I had some problem with a valve that didn't open and gave me MIL/CEL.
Thanks again for the info, won't mind more information for curious mind.

The secondary air injection pump has nothing to do with the evap system. Nor does the fast idle on startup. These are to help the engine warm up faster so the catalytic converter can start doing its job faster. The evap system simply captures the vapor from the fuel when it evaporates in the tank, and releases it to the engine when it's running in a controlled fashion, rather than venting it into the atmosphere.

The evap system is conceptually very simple, but thanks to our wonderful government "looking out for us" it also has a lot of self-checking that can cause CELs for minor issues. I've had to replace the purge valve twice in mine due to CELs.