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Michael Warfield
Michael Warfield, Factory Mercedes-Benz Technician
Category: Mercedes
Satisfied Customers: 3135
Experience:  Mercedes-Benz Master Certified technician
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I have a 2003 Mercedes G500 with an M113 engine in it.

Customer Question

I have a 2003 Mercedes G500 with an M113 engine in it.
Recently the vehicle has been bogging down and sometimes dying on takeoff. The car does NOT idle rough at all, it's just rough on take off
I ran codes with my Autel Scanner and received back P0104, I have not ran codes with my STAR DAS but I can if necessary.
The code comes back as Mass Air Flow (MAF) Circuit Intermittent.
I have checked the voltage on terminals 2&3 and 3&4 as per ALLDATA's testing procedure. This test identical to the test procedure in my STAR DAS as well. Voltage came back within specs, so clearly there isn't an issue.
What else could be going on?
I should mention that if and when it happens, I can stop the car, restart it, and in most cases, the problem will go away after a restart. It also never starts with the problem, only after driving a second or an hour. This leads me to believe it could be unmetered air and not actually voltage.
But forget my assumptions, I'm looking for your help!
Things I have done:
1) Replaced the MAF (w/ OEM Bosch)
2) Checked voltage going to MAF (passed)
3) Checked wiring harness of MAF (passed)
Submitted: 6 months ago.
Category: Mercedes
Expert:  Michael Warfield replied 6 months ago.

The voltage tests for the M113 MAF sensor circuits are very inaccurate for diagnosing driveability/performance issues. I've found these voltages are basically to make sure the sensor is transmitting data, not necessarily the correct data. When these engines first were produced, we were getting a rash of mass air flow sensor failures and Mercedes soon required to have us have printed test data showing us the voltage of the Bosch MAF sensors. If the voltage was within spec, the warranty claim would be denied for any replacement of the sensor. Replacement of the sensor almost always resolved the condition and caused the car not to return thus creating a huge denial by Mercedes saying there was something else causing the problem. If there was unmetered air, the problem would return pretty quickly and also be accompanied by P0173 and P0171 self adaptation fault codes. Since you don't have any self adaptation fault codes, I wouldn't suspect any unmetered air leaks. The easiest way to check is to just replace the sensor and drive the car. I would suggest buying a Bosch sensor only since other brands aren't reliable. You could check for unmetered air leaks by spraying carb clean around vacuum and the intake manifold to see if the idle stumbles but I think you have found the issue already.

Customer: replied 6 months ago.

Hey Michael,

Did you read this part?

"Things I have done:

1) Replaced the MAF (w/ OEM Bosch)
2) Checked voltage going to MAF (passed)
3) Checked wiring harness of MAF (passed)"

It's already been replaced with an OEM Bosch MAF sensor... I did it two days ago, the problem still persists.

What do you suggest now?


Expert:  Michael Warfield replied 6 months ago.

I'm sorry I missed that part in my dialogue box. Is that the only code that is stored? If it is a little rough on take off, do you get any misfire faults?

Expert:  Michael Warfield replied 6 months ago.

Also, if you lightly take off does it stumble as much? There could be a restriction in the catalytic converters which can be checked by removing the two bolts where they each connect at the manifolds.

Customer: replied 6 months ago.

This answer is in response to your most recent question.

Yes, it does stumble but not as bad.

Also, the exhaust smells like eggs when the CEL is on and it only bogs down within 600-1.5/2k RPM, higher RPM is fine.

The answer below is in response to your previous question.

Hey Michael,

If I restart the vehicle it runs fine and the CEL will go away after 1 or 2 driving cycles.

So basically, when I park the car, pop the hood and turn it on (and the CEL is gone) it runs perfectly until I unplug the MAF sensor, at which time it'll bog down and rough idle - no misfire codes, should I be waiting a certain amount of time for them to appear?

Additionally, If I were to unplug the MAF sensor while the CEL is present (for example after the car starts to act up), nothing changes.

In short

  • When the car runs good, unplugging the MAF causes issues

  • When the car runs bad (which is what the issue is), unplugging the MAF produces no change to Idle or revving

It seems like when the CEL is present, the ECU is ignoring the MAF?

Any other ideas? It appears the voltage is not the only cause of this P code.

I'm basically trying to eliminate all possible factors that could cause this code.

I do have STAR DAS, can you tell me how to run codes with it, will that data help?


Expert:  Michael Warfield replied 6 months ago.

The misfire codes should set pretty quickly if they are going to set at all. For a self adaptation/vacuum leak code, it could take a while for them to set. When the check engine light does turn on, it would go ignore the mass air flow and go into a closed loop mode. It definitely seems related to airflow somehow. Have you inspected all the vacuum/brake booster lines for cracks leaks? You could try spraying carb clean around vacuum lines, connections, the intake manifold (but away from the MAF). Right now I would check for vacuum leaks and let me know what you find.

Customer: replied 6 months ago.

I have a smoke machine.

Do you suggest I just pump smoke into the brake booster line instead? Wouldn't that be easier?

Also, should the engine be running, or can I find leaks while it's off?


Expert:  Michael Warfield replied 6 months ago.

The engine should be off and yes, you can pump smoke through the brake booster line to the engine and inspect for leaks. A little smoke coming from the MAF sensor itself is normal.

Customer: replied 6 months ago.

Ok, doing that tonight will post back in a few hours.

In the meantime, does the ECU/ECM need to adapt to the new MAF or not?


Expert:  Michael Warfield replied 6 months ago.

No, it will eventually adapt. Immediately the new MAF sensor should of made a difference if it was the problem.

Customer: replied 6 months ago.

Just wanted to update you.

There are no leaks when I inject smoke into the brake booster or the other intake vacuum hose on the passenger side of the engine.

I was able to reproduce the issue this morning. If I feather the gas and slowly accelerate the car will drive fine. however, if I accelerate swiftly (not flooring the pedal) the issue will appear at next stop. The exhaust also smells like sulfur (egg smell) when the problem is happening. It does not when the issue is not present.

I did notice on the brake booster line there are multiple "t" connectors and there is also a filter. On the bottom of the filter "t" there is a tiny hose that is open. I'm assuming this is normal. it's extremely skinny, so i doubt anything should be plugged into it.

let me know


Expert:  Michael Warfield replied 6 months ago.

There could be a restriction in the exhaust. The rotten egg smell will come from a misfire (too much fuel going through the catalysts). My next step would be to remove the bolts at each catalyst pipe where it connects to the manifold. Separate it about an inch or so and then drive the car. It will be very loud but when you try and accelerate hard, it will tell you if it solves the problem or not. Does the problem happen every time you try and accelerate swiftly?

Customer: replied 6 months ago.

Hey Michael,

I don't know if this works with Benz or not. I plugged in my datalogger and logged very consistent O2B1S2 and O2B2S2 values.

Both remained steady at 0.6 - 0.7 V when the engine was revved at 2k RPMs. If I pressed the gas quickly and let it go, they would dip down to 0.04 - 0.1 V

Is this an indication of a bad cat? I asked because it appears that the cats are welded on thus I would have to remove the headers to really see if the cats are the issue.


Expert:  Michael Warfield replied 6 months ago.

Using the rear oxygen sensors to see if they are plugged isn't reliable since it can't measure volume of air, just concentration. The catalysts are welded in the pipes but there are two bolts for each pipe at the manifold. Here are some instructions for doing it (Use the third picture to locate the flange that connects the pipe to the exhaust manifolds. I wouldn't remove the entire exhaust, just the flanges at the manifolds and any mounts you need to get about an inch of separation between the pipe and manifolds.

Customer: replied 6 months ago.

Ok, will do tonight and I'll post back. Hopefully this is the issue!

In the meantime, I don't know if this will be helpful

But it appears as if others have this problem, there was another guy on here with an E500 that asked a similar question.

It appears in the case of the ML they took it off Just answer to solve the problem, so there is no telling if they ever got it resolved, but maybe that will help!

Thanks Michael!

Expert:  Michael Warfield replied 6 months ago.

Thanks for researching that, I did read through the post and "Marty" did bring up so good points. The purge valve under the hood could be causing the vacuum leak as well. You could plug off that line coming from the engine to the purge valve and see if the code returns. I have seen stuck open or leaking purge valves cause these issues also. Doing a fuel pressure test while idling and under hard acceleration could confirm that the fuel filter and pump are OK.

Customer: replied 6 months ago.
I'll be changing the fuel filter tomorrow morning, as far as the fuel pump, I'll reserve that job for worst case.. I'll check out the purge valve tonight!
Thanks Michael I'll keep you posted!
Expert:  Michael Warfield replied 6 months ago.

Great, I'll be available all day this whole week actually

Customer: replied 6 months ago.
That's awesome!
Also, in case I can't get the diagram off of alldata. Could you shoot me the location of the purge valve?
Expert:  Michael Warfield replied 6 months ago.

Yes, I'll post shortly