Last night my CEL came on, so when I got home, I plugged in my VAG-COM and pulled this code:
16840 - EVAP System: Very Small Leak Detected
P0456 - 002 - Lower Limit Exceeded - MIL On
I had this code last fall (intermittent, no CEL) and reseating the hoses on the N80 purge valve behind the airbox solved the problem. No such luck this time.
I leak tested using block 71 in Basic Settings. I removed and replaced the gas cap. I jiggled the hoses on the purge valve again. So far the leak tests have all come up Small Leak, except when I disconnected the left-hand hose from the purge valve and plugged it.
Here's the purge valve:
Here's the hose I disconnected and plugged, which resulted in NO leak detected:
What I'm wondering is, if disconnecting the purge valve and plugging the line stopped the leak, does that mean my purge valve is bad?
I'm going to buy some reusable hose clamps today, and do some more troubleshooting. So far that one connection was the only one I could remove easily last night without destroying the one-use clamp.
I found EricVonHa's writeup here and read through it, and the fact that my leak test passed with the line from the canister to the N80 plugged, that my N80 is allowing a small amount of flow when closed. Sound like a plausible theory?
BTW, can I buy the N80 purge valve at a Pep Boys/Autozone, or would I have to hit up an Audi dealership?
I doubt it's the gas cap, since the leak test passed when I plugged the hose going into the purge valve.
From what I've read, when the ECU tests for leaks, it pressurizes the system, then closes the purge valve for 15 seconds or so and a reed switch closes if the pressure leaks out.
When I plugged the line going to the purge valve, the test showed no leak. This makes me think that the purge valve is leaking, or letting pressure through when it's closed. If it was the gas cap, it would still leak even with the hose plugged or clamped.
I'll pull the valve and fiddle with it, and put decent hose clamps on, and try plugging the other side when doing the leak test, and see what happens. I have a new valve on order, just in case.
I dropped by the local VW dealer on the way home from work (much closer than the Audi dealer) and picked up a new purge valve. I put it in, cleared codes, and ran evap flow and leak tests, and everything passes now. Problem solved, and no more annoying yellow CEL on the dash.