The HFM unit (hot film management) is the mass air flow sensor.
The usual symptoms of a bad mass air sensor is an uneven idle that fluctuates up and down, kinda like the engine is hunting for the right idle speed and can't find it.
There are many places to check the wiring harness to see if the insulation is flaking off. One place is right at the the terminations on the thermostat housing you'll see the insulation at the ends coming off. Another place is where the wires in the plug to the intake cam advance magnet are. It may be necessary to remove the plastic connector from the wires to see the insulation.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".