Here's what's been going on with this car in case anyone is following.
Bought the car running badly at idle, the usual idle surging but otherwise running great at loads. I knew nothing about the HFM-SFI system at the time. I mistakenly thought this would be an easy repair..Boy, was I wrong.

Step 1 I check the plugs. It had some incorrect autolites. I replace with FR8DC+ copper core. Would have liked to have non resistor but they were out of stock.
Step 2I also checked the wiring harness and it was in bad shape. I replaced with an updated one. NO change..car runs the same.
Step 3 I check the regen/purge valve..it's pintel is moving but not shutting off vacuum. i bench test and it passes. Turns out it was installed backwards!!I installed it correctly but still no change.
Step 4 Pull codes again, release codes and 49 at pin 8 keeps coming back. I had already checked the OVP to be the latest version, so I pull it out and bench test..fails to close the contacts on 4-6 relay. I take it apart and there is a tiny particle holding the small relay open. I fixed it and passes all test. reinstall, checks codes all clear now. WOW! Big change in the running of the car, idles now like it should, I think great.. all fixed..till i take it for a drive. falls on it's face on acceleration. pull back in the shop. What I think I've learned on the hfm for all those that have the surging idle problem, I think that just means it's in limp mode?
Step 5Now, I'm getting really frustrated..pull codes again, get code 4 on pin 8. I clear it, and it comes back.I pull the HFM/maf and bench test. Seems to perform. Starts with around .9Volts with no air flow. As blow air across it, the voltage rises smoothly up past 2.5Volts. I repeat this a few times. I never cleaned it. I reinstall it and now the code hasn't come back, but car still runs the same. starting to think, i'm chasing my tail now..lol
Step 6Decide to check the O2 sensor..seems to respond ok. I check the heater voltage...none! but, bad news it it gets it directly from the hfm ecu...no fuse or relay to check. i give up for the day..now today I get the car up to operating temp, recheck the heater voltage and still zero. I plug th harness back in, and with a sharp probe recheck and haha, it's there..over 12 volts.That's odd, so I unplug and it's still there.
That's where I'm at so-far with this step child of mercedes(300e 2.8HFM)..I'm still wondering about the maf, and also the throttle servo, since this is drive by wire, and now that it's out of limp mode it's in play.