Hello,

I'd like to ask some questions:

1) Can (Snap-on MT2500 + Mercedes module) be used to RETRIEVE and RESET fuel trim adaptation values on a 1994 c220?

2) If that combo cannot be used to RESET fuel trim adaptation values, is there any "hack" method that can be used to reset them without using SDS?

3) Do new MAF replacements (such as those from Fastlane) also use thin-film instead of hot wire as the sensor element? I bought a non-Bosch MAF for my 1994 c220 from Ebay, and it uses a hot wire. It seems to be the constant cause of the DTC P1070 that persists after the MAF replacement.

4) Can long-term fuel trims values that are obtained through OBDII be interpreted as approximate fuel trim adaptation values?

5) After the MAF replacement, the code P0170 keeps coming up every time after I clear it using an ODBII scanner and drive for about 10 minutes. The car runs very well on highway as well as at idle. However, its only symptom is that it often sputters or hesitates for a fraction of a second when I give it gas to accelerate from a stop (usually after exiting a highway and stopping at the red light). This problem only happens when the car has warmed up after driving for about 5 minutes. It does not seem to happen when the engine is still cold. I have checked the fuel pressure regulator, EGR (it has a red dot indicating it was replaced in the past) and made sure there's no vacuum leaks in the engine compartment. BTW, the car's PSE pump is inoperative and I blocked both of its vacuum and pressure lines. I wonder whether this non-working PSE pump may play a role in the aforementioned symptom and DTC P0170?

FYI, I also replaced both pre-cat and post-cat oxygen sensors.

Thanks in advance for all your precious information.

Best regards,

Eric

THE mass air flow sensor IS designed as a hot film on the C220! Never seen any thing other than that!....

That sensor is most likely your problem..

OBD2 on that car is only talking to the diagnostic module... the computer that needs reset is the HFM-sfi unit...

__________________
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Retired Moderator

You MUST use exactly the same MAF as the engine managemetn system is DESIGNED to use--no substitutes. A hot wire WILL NOT substitute for HOT FILM MAF. You are running on 'open loop' fuel ratio control.

I think the Snap-On MT2500 with MB module should set adaptations, I'm relatively sure it should allow one to view them in actual values.

It is very unlikely that a MAF for your car could be anything but the hot hilm that it came with, but there is no scienticfic reason for that fact. The output of hot wire sensors and hot film are both voltage values between about 1v and about 4v. It's the "about" that causes problems. The situation is precise and the calibration can only be a ways off before the adaptation of the system runs out of range. You also may be faced with an early control unit that has the software that could only make an 18% correction. The new units run to 32% correction, either rich or lean.

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

Wow I learn something new every day :-)

Dear Mr. Brotherton: your reply really solves one of the puzzles that I faced, because I always got the long-term fuel trim value of (-18%) using my OBDII scanner. I thought it might be a bug in my OBDII scanner. In some of your posts in other threads, you mentioned something about the ability of Mercedes control units to make a correction of as much as -32% or +32%, while I could only get a LTFT of -18% from my car. Now everything is clear: my car must have an early control unit that can only make a correction down to -18% and up to 18%. Thank you very much for that precious piece of information!

Many thanks to M.B.DOC, Kebowers, and Steveblf for your expert replies. I will send my complaints to the Ebay seller regarding the HOT WIRE MAF. I should expect a HOT FILM MAF only (anyway, that's what HFM stands for :-)

From your extensive experience working with Mercedes cars, what are the average corrections on a new Mercedes and an used working Mercedes? Is a 0% correction very commonly seen? If a car is running with a correction of as much as -30% or +30% (supposedly with new control units), is it still considered as being in good shape?

Are you aware of any "hack" that allows the resetting of the HFM-sfi unit without using a Snap-on MT2500 or a SDS?

Many thanks again.

Best regards,

Eric

>

You can reset/re-activate the memory of the HFM/SFI module with a home made led tool.

http://members.aol.com/ajdalton7/ledtool.jpg

This tool will let you get flash codes at HFM at pin #4 of the 38 pin connector. After you retrieve the codes [ write them down] , you can clear them, one at a time .
After all codes have been cleared , you will get a Single flash, verifying all stored code have been erased.
Now the trick..
After you get the single flash, turn the key OFF for 30 secs .... Key back On and get the single flash again.. wait 2 secs and hold button on tool for 6-8 secs.. turn key Off for 2 secs , On for at least 10 secs , and then start car.
The memory on the HFM module will now be reset to zero/mean...