SCL

I'll have to check the gas cap. Is it common for the pump to fail? Or could there be ripped or leaking vacuum hoses in the rear wheel well? I know it's possible...but have the hoses been failing? Or is it more common to be a leaky gas cap? I know that the CA Smog test includes pressurizing the evap system and/or checking the gas cap... and mine has passed each time (when I clear the fault code just prior to taking it in for Smog). I can't remember exactly what the guy did to check it all -- I remember clearly on my other Audi that he pinched off a vacuum hose at the evap canister and (I think) maybe pressurized the tank. I don't remember how he did it on the A8, which I had taken in only a few weeks earlier. Maybe he didn't check the gas cap... I'm not sure now.

So... just so I understand... Is the pump in the rear wheel well supposed to pressurize the tank so that the fumes are sent up front? And then burned in the engine or sent to the evap canister? Is the evap canister like a buffer or temporary storage, or is it rather a filter of some sort? I had checked the parts under the hood, and the hoses to (what I believe are) the evap components were all in tact. I am more familiar with the evap system on my S6 w/2.2 turbo...and aside from tightening a few clamps or hoses (and cleaning out the PCV valve), I've never had to replace or change any of the evap emission components. Is the canister supposed to be a regular maintenance item?

SCL

One more question -- which way does the gas cap check valve work? Does it allow air in, or does it relieve excess tank pressure by letting the pressure escape?

silverd2

The leak detection pump is just a diagnostic device which runs periodic leak tests...at start up and retests if pressure drops too fast..then intervals later controlled by ECM (don't know how often). The LDP is also activated by VagCom diagnostic to check evap system functions, leaks and the leak det pump itself. Cars without this have to be pressurized from the outside to check evap and leaks...maybe how the state does it? ...no inspection here in TN.
If the system was pressurized through VagCom (or similar) using the leak det pump or outside pressurizing, the gas cap would have been checked automatically...if it had bled off pressure, it would have showed in the test. The machines they use have a seperate gas cap test hook up, if a leak was detected.

The evap system is an anti-polution device, in place to handle gas tank pressure built up in normal operation, (without the LDP)to keep as much as possible from venting to the outside air. The evap purge valve (controlled by ECM) allows it vent into the intake to be burned while car is running . The canister is storage and filter...when engine is off and pressure builds beyond a point (or purge valve fails while running), it is vented through the charcoal to the atmosphere.

The gas cap vents inwards only (if needed...valve opens under suction only)...all gas tank pressure is normally released through the purge valve or out the charcoal canister atmo release.

Don't really know how often LD pumps, hoses or gas caps fail. I've only had to tighten loose fitting hoses as I find them, but have never gotten a code.

SCL

Thanks for the detailed explanation. I cleaned the gas cap and flipped the o-ring seal over... maybe that will help. I blew into the check valve and it seems to be sealing just fine...but cleaned it with solvent anyway. And reset the fault code... so now I'll wait a few days and see if it returns. It usually takes about 2-3 days for the CEL light to come on again.

thanks again!

silverd2

If the fault is seen during 2 driving cycles of 3 minutes or more, it will trip the code again.

SCL

My gas cap cleaning didn't work. I drove it a couple times and the light stayed off. But as soon as I restarted the engine again, the CEL immediately came on again. I must have a leaky hose, or a non-working pump.

CoreyDL

The problem with mine - which cost me quite a bit at the shop for them to get to - was the cap on the sending unit.

I first had the small EVAP leak code P0442, and then a small and large one (Gross leak). They did a diagnostic and found that the cap/seal on the sending unit (on top of the fuel tank, related to the LDP) was sucking in air.

I hadn't realized it before, but when you started the car, you could hear that pump in the rear left fender, sucking at air.

This took care of my small and large EVAP leak code - $328 for labor and the sending unit repair kit. Apparently the sending unit "part" has been discontinued.

Hope this helps. The entire contents of the trunk had to be taken out in order to get to it. I do not think they had to remove the CD changer and the GPS however.

SCL

Hmmm... that could be the problem with mine too. I have noticed a slight gasoline smell in the trunk when I remove the black metal plate that covers the access to the fuel pump assembly.

Are you referring to the left side? I have not taken that cover off yet. I thought the sending unit was the tube that slides into the fuel pump assembly on the right. Is the one on the left for the fuel gauge and the one on the right for the low fuel light?

I have the same errors -- both minor leak and major leak detected. Why did inside of the trunk need to come apart? Isn't the sending unit you mentioned accessible by removing the other metal plate on the left side of the trunk?

silverd2

The cover on the right is the fuel pump and fuel guage "sender". The left cover is for the fuel level "sensor 2".

silverd2

P.S. All related parts (cap nut, seals, etc )are readily available from http://www.genuinevwaudiparts.com/pa...?siteid=214407

The seals are $22 & $27 R & L respectively.

Punch in car make/model and look under "fuel system components".