Tnk Venting issue, where to start?

[Henn28]

Twice in the past month the X has triggered the SES light with only one meaningful code, and both times on an identical drive to the airport in hot weather. In fact, the car is so regular, it triggered at exactly the same point in the drive each time.

after a 20 min/ 14-mile drive in 90+ degree heat I drop my wife off at departures, only to get back in the car and see the SES light. My trusty, if outdated Foxwell 520 pulls two DME codes. The first is for the activation (or lack thereof, of the Inlet Flap that my 4.6 DME is looking for but my 4.4 intake track doesn't have. This code never resets and has been present since I did the motor swap. The second code is for "DME Function, tank venting system" and does reset, extinguishing the SES light with it each time. It doesn't seem to return until the next time I drop my wife off at the airport!

Clearly, I must have a vacuum or vent leak somewhere, or some other part of this system is failing under specific circumstances. Any advice on where and what to start troubleshooting? I also have gotten in the past, but not too recently a check fuel cap warning, so I bought a new cap about 6 months ago in response to this issue. It didn't stop this very, very occasional warning.

foxwell pic to follow...note the codes don't seem to be BMW codes, but the 520 isn't supported any longer so I can't update it.