Fuel evaporation leak check questions

[Morph_986]

I am hoping someone can answer some questions I have about the automatic fuel evaporation leak check. This appears to be a problem for many owners with relatively little concrete information available.

I had a CEL a few weeks ago for a small leak detected in the fuel evaporation recycling system. I reset that and changed the fuel filler cap. Now several hundred miles later and a full tank of gas later, the Durametric software shows failed for the leak check and pass for everything else. However there are no pending or active codes showing for a vacuum leak.

1) is it likely that the evaporator vacuum leak test has run since the CEL was canceled and failed, or that it has not run?
2) if it has failed, how many fails before the car shows a fault code?
3) does anyone know of a way of forcing the leak check to run? It appears that the fuel level must be between 3/4 and 1/4 full, but that is all I have established for searching. I cannot find anything in the software, although it appears you can cycle some of the valves.

4) does anyone have a good diagnostic procedure for ruling out the usual suspects, filler cap, filler valve and carbon container valve? Is there any easy place to T in a vacuum pump and check for piping leaks?

Advice much appreciated.

Philip

[Morph_986]

Just a follow up in case anyone stumbles across this thread in the future.

There is very little available about what the 986 does about the evaporation leak check. I have drawn a blank after extensive searching and the lack of responses to my question indicates that few others are knowledgeable.

In any event, I think I got to the bottom of my issue. Recall, I got two engine check lights for minor leak warnings on my 986. I was suspicious because in both cases the fuel gauge had just dropped below 3/4 and I had also just dropped about 1500ft in elevation (about 1 psi). The rumor is that checks only run at fuel levels between 3/4 and 1/4.

When I reset the warning, my status checks for the environmental compliance checks were pass for everything, except for the leak check. The problem was that the Durametric software just said failed, not tried and failed. I could not tell if the test had run since reset.


Over the last few days I made a concerted effort to see if there was a leak or just a failure to run the checks. It appears it was the later. The Bentley manual had a short section about what a drive cycle actually means for a car to complete the emission checks. It has a few odd things that a normal driver like me does not regularly do. Specifically, a drive cycle starts only with the engine cold or very nearly. The biggest oddity is that the engine must idle for up to 5 minutes after coming to full temperature after reaching a destination. I hardly ever do that.


To test things out, I specifically added just enough fuel to the car to be a little over 3/4 tank when I stated from cold, but that I would be below when I arrived at my destination an hour later. The destination was in a flat area and so the initial elevation drop would be long forgotten. I then idled the car for 5 minutes after reaching the destination. Lo and behold, next time I ran the environmental status check, I had all passes.

It appears therefore the Porsche engineers did not account for the scenario of dropping in elevation at the same time as the fuel level dropping below 3/4. The differential pressure measurement cannot account for a change in ambient pressure while performing the check and it fails. I was just exceptionally unlucky twice and then owing to the type of driving I do and the location, the system never got the chance to run the test again for months, until I finally forced the situation.


Philip

[Lowski]

I found this in my files form the 1998 Service Information document. Hope it fills in some blanks.