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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53007
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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My 2000 ML320 suddenly wont accelerate. It starts easily

Resolved Question:

My 2000 ML320 suddenly won't accelerate. It starts easily and idles wonderfully, but when you press on the gas, it hesitates and won't accelerate.

My Code Reader collected these:

- P0170 Fuel Trim (Bank 1)
- P0173 Fuel Trim (Bank 2)

This morning, I replaced the MAF but I still have the problem.

The problem is present all the time (engine cold or engine warmed-up).   Just about two months ago, I successfully replaced the crankshaft position switch and until now, the car has run great.
Submitted: 7 years ago.
Category: Mercedes
Expert:  Marty replied 7 years ago.

Hi JDC, it's going to take about 30 minutes of driving for the fuel adaptations to adjust to the new sensor. Your problem could be as simple as that. In the shop we would reset the values with the Mercedes SDS tester. If that isn't the problem then I would perform a fuel pressure and flow test. Low fuel pressure can be the cause of the P0170 and P0173 also. You should also smoke test the intake system to check for vacuum leaks. The fault codes only indicate that the computer can no longer control the fuel mixture. This can be in the lean direction or the rich direction. It would be helpful to know what the fuel trim values are to see whether the computer is trying to add fuel due to an overly lean condition or take away fuel due to an overly rich condition. Also if you didn't use an OEM MAF your new one might also be defective. I'll be logging off soon but I'll reply to any questions you might have in the morning.



Marty and 3 other Mercedes Specialists are ready to help you
Customer: replied 7 years ago.

Thanks for the quick response Marty.


I started and let the car run for 30-minutes, then I limped up and down my street for an additional 15-minutes. But after a total of 45-minutes of operation, the problem seemed to get worse.


I purchased the MAF from a reputable supplier (GermanStar in AZ). They are endorsed by a number of people who post in the forum. I purchased the whole MAF (housing and sensor), and it is an identicle replacement from Bosch - same manufacturer as I took out of the car, with the same model numbers in exactly the same places. It lookes exactly the same as the unit I removed. New Bosch box, with Bosch protective paper and dual caps (one blue and one white) over the housing ends. I made sure I had a solid connection with the MAF cable (heard that nice "Click" when the cable was fully seated).


While I limped up and down my street, I captured a live view from my OBD2 Reader. Which data would you like to see (I captured 19-sets of these numbers as I drove):


Short Term Fuel Trim Bank 1 (STFT B1)

Long Term Fuel Trim Bank 1 (LTFT B1)

Short Term Fuel Trim Bank 2 (STFT B2)

Long Term Fuel Trim Bank 2 (LTFT B2)


Short Term Fuel Trim B1 S1

Short Term Fuel Trim B1 S2

Short Term Fuel Trim B2 S1

Short Term Fuel Trim B2 S2


I can imagine how to do a smoke test (cigarette in front of air intake port) - and watch for smoke to appear from a part of the intake system, which would indicate a leak.


Can I use the Actron Fuel Pressure Tester for both the pressure and flow test? Can you say where I can find the fuel line to connect? Since I can't start the car with the fuel line connected to the testor, how do I initiate the tests? What kind of values should I expect?


Thanks again for your quick response!




Expert:  Marty replied 7 years ago.

Can you go ahead and post those #'s first JDC?


Short Term Fuel Trim Bank 1 (STFT B1)

Long Term Fuel Trim Bank 1 (LTFT B1)

Short Term Fuel Trim Bank 2 (STFT B2)

Long Term Fuel Trim Bank 2 (LTFT B2)


Customer: replied 7 years ago.

Before I role the numbers, keep in mind that because of the issue I have (no acceleration), the fastest speed I could attain during this live view was 2-MPH - the max RPM was 1131 (and that was down hill). I took 19-frames of data during my short and slow drive. Here are the minimums, maximums and averages:


STFT B1 Min 9.3% Max 25.0% Avg 17.4%

LTFT B1 Always 34.3% (never changed)


STFT B2 Min 2.2% Max 25.0% Avg 10.9%

STFT B2 Always 34.3% (never changed)


STFT B1 S1 Min 9.3% Max 25.0% Avg 17.1%

STFT B2 S1 Min 0.70% Max 25.0% Avg 10.1%


Even if my issue might not be directly connected with the fuel pump or fuel filter, I know that after 8-1/2 years and 120,000 miles, these two items will need to be replaced soon - and I don't want this car to die on my wife again. In addition, my early 2000 ML320 needs the update to the large capacity fuel filter and pressure regulator. So I plan to replace these today.


Thanks again Marty - it's great to have your expert perspective!

Expert:  Marty replied 7 years ago.
It looks like it's running out of gas JDC. I'd say the pump and filter is a good idea.
Customer: replied 7 years ago.

THANKS MARTY! I replaced the fuel pump and upgraded to a high capacity fuel filter and pressure regulator - and the acceleration problem is GONE - it runs well.


The computer threw me a couple of random misfire codes during my test drive:


P0300, P0301, P0302, P0303


I cleared the codes and will see if they come back, but if they do, I might need new spark plug wires or plugs. Hopefully I won't need new coils or injectors.


But THANK YOU AGAIN - You were spot on with your diagnosis! You are the EXPERT!

Expert:  Marty replied 7 years ago.

Glad you got it going JDC :-)