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Old 01-30-2006, 08:46 PM
Join Date: Jan 2006
Posts: 16

I just purchased my first Mercedes ('00 ML320) from the original owner, drove approximately 100 miles of the 130 mile trip home and stopped at the grocery store when the Check Engine light came on in the parking. I had the code read the next day (P0170) and had the light turned off, thinking it could possibly have something to do with going from sea level to 5K ft. in altitude. The light came on again within 5-10 miles, at which time I researched the codes and ordered new 02 sensors and replaced both on the left cylinder bank.

The car has been running fine the entire time the light has been on. However, about a week ago, I just jumped in about mid-day and took off to do a quick errand without warming it up as I usually do. There was an obvious misfire upon acceleration for approximately one mile, upon which time I pulled over and stopped and restarted the vehicle, which cleared up the problem. It has done it one more time under similar circumstances since.

I had the codes read at my local Auto Zone today: P0170, P0173, P0300; P0302; P0306 (the latter three being the obvious misfires registered). Before throwing more parts at this vehicle (MAF sensor would be my next step), does anyone have any insights as to what I might have going on? I was told by the previous owner that this vehicle has always been dealer maintained, which was substantiated by the CarFAX report that I ran prior to purchase. However, my husband was a Jeep mechanic for 20 years and can easily perform any repairs, but is unsure of European diagnosis.
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Old 01-30-2006, 11:03 PM
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Normally P0170 code can be fixed by replacing MAF sensor.
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Old 01-30-2006, 11:09 PM
Join Date: Jan 2006
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I forgot to mention that we did remove and clean the MAF sensor, which didn't appear to be dirty at all, reset the light, and ended up with the same results/code sequence reported above. I don't want to go to the expense of completely replacing it if there are some other diagnostics that we should be doing to be sure that is the problem.
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Old 01-31-2006, 06:18 AM
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In more detail....
Youre symptoms and solution are very clear although you do not give mileage.
P0170 advises a Fuel Trim problem which means that for some reason the ECU (Engine management computer) is sensing an error with the fuel trim compensation value.
P300 etc (misfire) are the result of the Fuel Trim (weak) problem.
You need to get a bit of additional info before replacing parts namely STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim) readings from the onboard ECU which can be read by most scan tools whilst the engine is running.
These readings show what compensation the ECU is making as a result of the sensor inputs it receives. I'm prepared to wager that you will find the LTFT very negative whilst the STFT will be not far from 0.
The STFT is the mixture compensation value in REAL time. The ECU will work hard to get this within an allowable range in order to meet legislation.
The LTFT is a compensation value to allow for wear and tear of system components and is the average value over a period of time. This value must lie within limits (I think +/= 25) at which point it turns the CEL on.
This is where a reset of the CEL (Check Engine Light) can sometimes make you think a problem has been cleared when it hasn't.
The key input to the LTFT is the MAF although every other part of the system can also effect this reading.
P0170 + negativeLTFT + high mileage 50k+ = (almost always) faulty MAF.
However the MAF can fail in a number of ways. Some situations require replacement of the MAF (broken film) but usually the problem is that the Hot Film sensor in the MAF (Black plastic sensor with 2X security Torx screws) has undergone a resistance change due to surface contamination. This leads to the P0170 code.
Clean the sensor CAREFULLY in Toluene (some carb cleaners work..others don't. Petrol doesn't. Not sure about acetone.) by soaking and careful air blast.
On reassembly you should see a reduction in the LTFT which will continue to move towards 0 over a period of time.
Problem solved.
Let me know mileage, what cleaner you use and how you get on.
My experience is that MB dealers do not understand how the EMS works. They just do what their STAR diagnostics system tells them. Mind you STAR is an excellent system if you have one!
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Old 02-24-2006, 04:39 PM
Join Date: Jan 2006
Posts: 16

Thank you, Stuart, for the information. After cleaning the MAF again, per your recommendation, the CEL came on again. I have finally replaced the MAF sensor (bought new on eBay for $155, including shipping costs - not too terribly bad), and the light has turned itself off after less than 100 miles of driving. I cannot tell you how happy I was to see that light finally go out!!! Thanks so much for the help. This forum is a wealth of information.
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