1997 E320 M104 flat at low RPMs, backfiring? - Page 2 - PeachParts Mercedes-Benz Forum

gmercoleza · #16 06-20-2007, 07:43 PM

OK, now I'm not convinced the MAF is the problem. I just drove the car again and noticed that the flat spot seems related to the position of the gas pedal more than anything. I can induce the flat spot even at a higher speed and higher RPM if I slightly press down on the gas pedal. If I just stomp hard on the gas pedal quickly enough, I can bypass the flat spot altogether. This applies even from a standstill. If I try to accelerate normally or slowly, I get the flat spot and a backfire or two and have to quickly let up on the gas. But if I stomp on it from the light and don't allow the flat spot to set in, I can skip the flat spot altogether and accelerate briskly.

I looked at my last gas receipt and somehow put 89 in there (not premium). Can that cause these symptoms? In any event I was 1/2 tank empty so I filled up with 93 and nothing has changed.

daveuz · #17 06-20-2007, 10:04 PM

Are these a "drive by wire" set ups?

gmercoleza · #18 06-21-2007, 12:01 AM

Quote:

Originally Posted by daveuz

Are these a "drive by wire" set ups?

I don't think so. I believe this one has a throttle cable. In fact, I'm pretty sure I saw it. Makes me wonder though - does it have a TPS?

I guess what's so confusing about all this is that the CEL has never even illuminated. Lot of good that OBDII is.

Update: I got the MAF cleaner and cleaned mine, reinstalled, and behavior is the same.

saumil · #19 06-21-2007, 12:33 AM

The CEL light not coming on is also a clue, none of the items that are routinely checked by OBD are malfunctioning, and if MAS were not working correctly, there would have been some error related to emission logged in the OBD. Would this not be the case ? Isnt the MAS a heated wire type in this car ? Or is it a potentiometer ? Damaged potentiometers can cause dead zones and they just cant be cleaned in any way.

gmercoleza · #20 06-21-2007, 09:28 AM

The MAF in this car is hot film and it can indeed be cleaned. New question: After cleaning or replacing the MAF, do the computer and/or adaptations need to be reset? I think I will disconnect the battery for 10 minutes to see if it changes anything.

david s poole · #21 06-21-2007, 09:42 AM

yes the adaptions do need to be reset.if you replaced a maf that had caused adaptions to move to the lean side and then fitted new one the car would be running lean for sometime until the adaptions straightened out.

gmercoleza · #22 06-21-2007, 09:52 AM

Quote:

Originally Posted by david s poole

yes the adaptions do need to be reset.if you replaced a maf that had caused adaptions to move to the lean side and then fitted new one the car would be running lean for sometime until the adaptions straightened out.

Thanks, so does disconnecting the battery for 10 min. accomplish this? Sorry for all the questions. I could fix my M103 with my eyes close but I am just beginning to learn the ins and outs of the M104 as it is a totally different beast.

david s poole · #23 06-21-2007, 11:43 AM

i don't know,i have a scanner to do that job but i doubt it.

gmercoleza · #24 06-21-2007, 12:44 PM

After some research, I found that disconnecting the battery for at least 10 minutes and touching the positive and negative cables together clears the adaptions, so I did this. I just got back from driving 40 minutes and 19 miles and the problem is gone. Am I out of the woods? How long before the adaptions are done cycling?

gmercoleza · #25 06-21-2007, 01:10 PM

After letting it heat soak in the garage, just took it out again for about 10 minutes and 3 miles. Really concentrated on the light to moderate throttle from a standing stop (since this was where the flat spot used to be) and the problem is absolutely gone. Again, am I out of the woods?

jcyuhn · #26 06-21-2007, 03:13 PM

You are definately out of the woods until you are back in the woods.

Sorry, couldn't resist. I've never seen any discussion or reports on how long a clean MAF lasts relative to a new MAF. You are officially the guinea pig. In theory there's nothing to wear out, so I would expect a reasonably long life.

Even if the MAF does go south it's not that big a deal. The ECU has a mode which ignores the MAF signal and guesses at the fuel injector pulse width. The car doesn't run great, but it runs, so it's not as if you can get stranded.

BTW, the car is drive-by-wire. The M104s since 1993 have been. The throttle cable is for limp mode.

- JimY

gmercoleza · #27 06-21-2007, 04:03 PM

Quote:

Originally Posted by jcyuhn

You are definately out of the woods until you are back in the woods.

Sorry, couldn't resist. I've never seen any discussion or reports on how long a clean MAF lasts relative to a new MAF. You are officially the guinea pig. In theory there's nothing to wear out, so I would expect a reasonably long life.

Even if the MAF does go south it's not that big a deal. The ECU has a mode which ignores the MAF signal and guesses at the fuel injector pulse width. The car doesn't run great, but it runs, so it's not as if you can get stranded.

BTW, the car is drive-by-wire. The M104s since 1993 have been. The throttle cable is for limp mode.

- JimY

Thanks for the input, yes I realize from everything I have read that this is only a temporary fix. I have read at VW, Ford, Honda, and Maxima forums that after cleaning a MAF the problems can reappear in as little as 6 months to, in some cases 2 or 3 years. (interesting that there were tons of threads about MAF cleaning at those forums but only a reference or two here at MBshop...)

My question about being "out of the woods" revolved more around the computer adaptions. I was wondering if 25 miles and 1 hour of driving were enough time to let the computer do its thing. I was under the impression that if the MAF were still bad, the behavior would return after some specified interval based on time and/or miles.

BTW - thanks for the info on the drive-by-wire. I keep learning new things about this M104 every day...

Very interestingly, I had a very slight but noticeable flare between 3-4 which has disappeared after cleaning the MAF - I know I am not imagining it. Now that shift is very clean like all the rest. I can't understand how the MAF influences shift quality, but apparently it does somehow.

gmercoleza · #28 06-22-2007, 06:14 PM

Problem came back after about 40 miles. I was on my way to work, and it occurred gradually, took about 1 to 2 miles to fully come back. Disconnected battery and reset computer again when I got to work; afterwards it was gone again. 40 miles later, it is back once more. So I think the adaptation interval is somewhere around 40 miles (to answer my own question).

It would appear, then, that cleaning my MAF didn't fix the problem. Evidently, resetting the computer restored adaptations to baseline which made it appear that the problem was resolved, only to return in 40 miles when the computer reached the limits of its adjustment. But I believe david s poole is correct, that I still need to replace it. Why? Well, in my research I stumbled upon this VERY HELPFUL document which outlines exactly how to test a MAF. No more guessing - it is a very exact procedure: http://www.x-desk.net/ximages/21302_pgsi0079en.pdf

The output voltage of a healthy MAF is supposed to be around 1.00 plus or minus 02 with key on engine off. Upon starting the engine, it should rise to 1.6 and then travel all the way to between 3.8 and 4.4 at full throttle.

When I tested mine, the output voltage was 1.462 with key on engine off, which is way high. When I blew a little bit into the MAF, the voltage would go both up and down (it should only go up) and was very erratic. Upon starting the engine, it was at 1.64 (a tad bit high). I didn't bother with the full-throttle test, as it was very evident to me that the MAF was malfunctioning.

At least I think I know what's going on now. The car is still driveable, you just can't accelerate slowly. Have to give it lots of throttle from a standstill or it will go flat. I will be ordering a new MAF as soon as I can.

Oh, and the document also settles the debate over whether or not there is actually a difference between the $130 MBZ element and the $59 generic (VW, Ford, Audi, etc.) element. Definitely different part numbers, so I wouldn't even mess with the generic since I like my catalytic converter the way it is.

gmercoleza · #29 07-03-2007, 02:37 PM

UPS delivered my MAF yesterday. I installed it and so far, 65 miles later, the car is running like new. I completely removed the upper and lower air intake boxes and cleaned them with a damp microfiber, then installed a new air filter to protect the new MAF. After resetting adaptations and connecting the new MAF, I measured 1.02 at the MAF with key on engine off which is right on target. I gently blew into the MAF and the voltage would rise in a linear fashion, unlike before where it was erratic and would actually drop sometimes. After starting the car, it measured 1.85 which was still high, but the reading should supposedly be 1.6 on a hot engine (mine was cold). I waited about a minute, during which I watched the voltage drop to 1.70 as the engine warmed up. I did not wait until the engine was fully hot, but I suspect it probably settled at 1.6. In any event, the behavior of this new MAF is totally different from that of the old one, and thankfully it has resolved my flat spot and backfiring. My car now accelerates very smoothly whether I accelerate gradually or quickly.