Fuel pump check valves don't play a part in running. They keep the pressure in the system after stopping which prevents hard hot starts.

The airflow potentiometer is rather easy to test. There is a power wire and a ground and the signal wire. The signal wire will have about 1-4v depending on the airflow. The critical value at idle is around .8v. The important thing is consistant readings. Values that change by more than .1v with constant airflow are probably bad. If you remove the device from the airflow meter the semiconductor that the wiper rides on can be seen to be worn through on the ones that can't be cleaned. As Donnie pointed out the traces can be cleaned with an eraser if the path isn't worn through. The area worn through will always be in the idle section.

One other alternative on your early 300e is a service bulletin changing the idle speed. Basically you get to a connector under to fusebox. You get the wire that comes from term 50 on the starter to the KE- controller. Separate the wire at the connector and ground the side that goes to the KE controller. Separate and insulate the wire that goes to the starter. This will increase the idle speed while in gear by more than 50rpm. It can make the difference.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician