RE: Check Engine issue- 2001 ML320
First, you may want to fill your tank all the way full, and see if this happens again. If you fill it half way, the ML will need more time to pressurize the tank. More if the gas cap is not sealing correctly.
IF your vehicle is just working fine, you may want to change the gas cap first, before going to more expensive solutions. Be careful with MB dealers, they can take you to an unecessary several thousand dollar fix up solution if you are not careful.
You may want, too, to take your ML to autozone, or any other parts store, where they can read, using an OBDII, any DTC's (Diagnostic Trouble Codes) set on the computer. I wonder what is the computer's DTC error code and particular engine parameters set when the gas cap is loose.
On my wife's Honda, we need to replace the gas cap. Somehow, it needs to be tighten up every so often because it looses it's sealing properties. We took it to a local mechanic and he mentioned just to get a new gas cap from the dealer.
2005 ML 500
Black, 18" AMG Wheels, 285/60R18 Yokohama Geolander HTS + Bi-Xenons(+ Drew's e-code mod, though the reflectors rusted off and do not give out much light anymore), Yakima roof bars, Akebono pads all around. Former vehicle 2001 ML 320 with 108K miles, when traded, and a large repair/defect history. After all these years of owning my current 2005 ML 500, it is a major unreliable vehicle with a very large/expensive repair/defect history and has been like this since brand new.