dhoerl

Well, I have good news and I have bad news. I am obsessed with this P2404 error code (because my wife complains daily about it!), and to that end I put her car on jacks (she is on a trip), and tore most of the EVAP system apart. I'm documenting how I am testing every component: pump, lead testing each part, etc. Also electrical tests, and how to actually verify that the Leak Detection Pump is working by pressurizing a small volume (yeah!). The procedure is going to be given to AudiWorld as a Tech Article in the next week or so.

So, what's the bad news? Well, every single piece of the system holds a small pressure (I had get a 4 PSI full scale gauge to perform this). The voltage is good at the pump, and the pump works. The lines going back to the ECU are not open - they both have some tens of thousands of ohms to ground. The vacuum line to the pump holds vacuum. Others have reported the ECU to be faulty, so in the end I may need to replace it - but not there yet.

There is only one oddity - when I go to pressurize the line connecting to the gas tank, I hear a loud "pop" on each press of the air pump, even if I do it very slowly. The pops begin immediately - that is, if the pressure is 0, and I go to say 0.05 PSI, "pop". Purse your lips then open your mouth wide to get that "pop" sound. So I'm going to guess that if I can get the Lead Detection Pump to actually pressurize the system to 0.25 PSI, there are going to be a lot of popping sounds. Maybe this is normal - who could I ever ask? If anyone knows about this PLEASE post something.

My final tests are going to be to cable into the car and connect a LED to the line activating the pump valve, and tee the low pressure gauge into the line connecting to the Purge Valve, to see if in fact the ECU runs the pump, and the pump can create pressure in the tank. If that works and I still have the error, then the problem MUST be the ECU.

Sigh. I spent a whole of last weekend (gorgeous) on this already, and now having no expectation of ever fixing this issue.

PS: I had already replaced that J757 relay and the Purge Valve.

A6Gary

I feel your pain. I've been tracking the P2404/009220 DTC that keeps appearing on my 2006 A6 Avant. After erasing it, the DTC now re-appears after about 100 miles. I've replaced the N80 valve, previously checked the LDP per the manual, and the vacuum line reported here on the left front of the engine that cracks. But I still have not found the culprit. Since it now is showing up very quickly, I going to re-check the LDP resistance and power supply to see if that is where the problem lies. With only 74K miles on it, I don't expect that the vacuum lines have been become brittle. So I'm looking for a failed component. BTW, in reviewing the wiring diagram, I can see any connection between the LDP and relay J757. Per the wiring diagram, the LDP connects directly to the ECM and body connection 5. Relay J757 connects to the ECM and body connection 6, which feeds tracks 154, 174, and 187. The LDP is connected on tracks 165, 166, and 167, and then to the ECM. So I am not sure how relay J757 affects the LDP, based on the wiring.

dhoerl

The reason that J757 comes up is that the power to the LDP must be pristine - if the power relay supplying power has some resistance, it could interfere with the LDP's internal valve from operating correctly - my guess.
I have a separate thread going on at Ross-Tech: Audi A6 2006 with EVAP problem (P2404)) - spent both days last weekend, and last Saturday. I am preparing an opus on how this EVAP system really works (in shade tree mechanics terms), and how to fault isolate every single piece of it - electrical, mechanical etc - with lots of photos.

Some of the tools that really help:
- MityVac vacuum pump
- bike air pump adapter (nipple transitioned into a piece of 3/8" section of PVC
- cascading sections of PVC, so you can adapt down from 3/4" ID to 3/8" or so (Ace Hardware or equiv)
- golf tee or equivalent
- low pressure gauge: 0-5 PSI works, 0-3 PSI better (look for H20 gauges too) (I got one on Amazon on sale - not cheap)
- Bentley's (helpful, not necessary)
- Ross-Tech VCDS (helpful, not necessary)

I'll update this thread when opus is published (will be tech article here).

The one piece of advice I have for you to try now - clear the code, then disconnect the input to the Purge Valve. Plug the hose you took off (the one leading back to the gas tank. Start the car, and use your finger tip (or a MityVac) to detect that the input of the purge value is not sucking - does not have ANY negative pressure. With the engine idling the vacuum on the input side should be exactly 0 - that purge valve MUST be completely tight and sealed.

If that looks good, then use a stub piece of hose, connect to purge valve, and plug it with a golf tee. Now drive the car normally for 100 miles and see if your fault comes back on. Please update thread then.

Good luck!

occhis

I'll try that purge valve test if (when) my CEL re-appears.

dhoerl

Well, in the end, I think I found my problem. I'm 99% sure its the PV. But, you ask, why didn't you change it. Well, when I first saw the P2404 code LAST year - one year ago - I did. Problem went away, came back when I found the broken vacuum line, then stayed around while other codes went away. So I was 100% sure it cannot be the PV. I did a quick pressure test on the inlet side anyway - it appeared to hold pressure just fine.

What really got me looking at the PV was when I was using VCDS to run EVAP test 071, and it would pass a few times, then fail a few times. So I tee'd in my low pressure gauge into the system, right at the PV inlet side, and wow - the gauge was oscillating hugely while the test ran (at least in one test that failed). No wonder the error code is "Implausible Signal" - the ECU must be reading the reed switch as almost random values. Once I changed the PV, the gauge behavior exhibited virtually no oscillation.

Since my EVAP tome is going to take me a while to finish, I knocked out a quick and dirty test you can use to test your PV. The other thing you can do, with virtually no effort, is to remove the hose from the PV inlet, plug it, then plug the PV inlet. You could reset your error code, then drive around for a few days and see if it comes back or not. If not, its got to be the PV. Or, run the VCDS test several times with the hose plugged, and see if it passes each time (open then close the gas cap to vent the tank before running subsequent tests).

I posted a PDF of a "How to test a PV" article on Dropbox:

Testing an Audi Purge Valve

The PDF has links to two ".mov" movies (QuickTime), of the Leak Test 071 with a new and old PV.

A6Gary

Thanks for the great writeup. I just changed the PV N80, and found that the old valve was leaking when I applied vacuum with my MityVac vacuum pump. After I replaced that valve, I noticed an improvement in the gas MPG. But I will check that valve again using your writeup, and run the VCDS test. Maybe the debris in the vacuum line to the valve resulted in another leaky valve, even though it has only been ~1.0K miles since I replaced it. Will let you know what I find (or not find).

milan_o

I recently have been suspecting a vacuum problem even though I was able to hold a vacuum measured at the pump back when I first started working on this 2 years ago. What I did not test at the time was the vacuum level during a test. Now if I can just find my MityVac I will go test it.

occhis

So assuming this test was successful and the code did not reappear, what harm would it do if one would leave the plugs in and not reconnect the hose?

A6Gary

Update on my EVAP problem: I checked the new N80 PV with my MityVac vacuum pump and found is was not leaking, so there is no problem with the valve. I then performed the Basic Setting test 071 and the test result was ABORT. I further performed the output tests (03) and determined that my LDP was not working when it was cycling. So, my next checks will be the power to the LDP and the LDP resistance. Since it appear not to be working, I suspect that either the pump is bad or it is not receiving power.

ILLusive

Awesome writeup! Unfortunately, the PV is not the problemo on my A6 since that was the first part I changed when the code popped up. I'm pretty sure there's an electrical gremlin happening with ours.