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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
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