PO455 evap problem 1996 s420

[ac680]

I am diagnosing this 1996 S420.

Sets code PO455, ofcourse checked fuel cap, purge valve, and vent valve, canister.
Checked all vent lines, and purge lines, all are fine.
Keeps setting PO455 but only after extended idle.
I did remove the fuel tank, part number on it is
140 470 37 01.
Called dealer . they tell me this car needs a tank with 140 470 47 01.
This car was hit behind the rear door, evap, canister etc, is all new, maybe the tank was replaced with another one.

I have tried other tanks, from a 1995 1996, 1997 S420, apparently every tank I have looked at, and has 140 470 37 01 on it.
The dealer cannot reference 140 470 37 01.
I did notice the earlier versions (1995 and early 1996) have a vent valve underneath the tank.
However mercedes tells me this car I am working on does not require this...
I have noticed that the tank does not build up pressure when left outside on a warm day.
However the tank absolutely has no leaks, nor do the lines.
Odly enough, I noticed too that the alternator fails to charge at idle.. during extended idle the voltage drops to 12.3 volts..(650 RPM)
will replace alternator tomorrow.

any idea's, ??