Hello guys,

I have a 1995 Mercedes S420. The car runs terrible sometimes, and it seems to be Air Mass Sensor related. Sometimes the idle is fine, but sometimes it just randomly surges, and stalls. Sometimes it doesn't stall, but RPM goes very low and the battery light flashes for a second, the car then recovers and RPM is normal again. This repeats itself randomly. The spark plugs are fairly new, as well as wires, air filter. I don't think it is ignition related, I will explain why later.

I tried disconnecting the MAF and the car runs smooth when I do that. The car idle's perfectly, no misses, everything seems perfect with the MAF disconnected. This is why I believe its related to the MAF being bad. I tried taking the MAF out, and cleaning it with MAF cleaner, used a whole can and that didn't fix anything. Symptoms are still there. I can drive the car without the MAF fine, it seems flawless, except an annoying Check Engine Light.

I had spoke with a Mercedes Tech via justanswer.com and they had told me that MAF sensors do not usually go bad. He mentioned the Ground strap wire might not have a good contact, and to replace/clean the ground strap wire. I then followed up by telling him about my unplugging and the car ran fine afterwards, again he says it may just be the ground strap. I have never heard/knew about this ground strap wire. Does anyone know where it is? He said it is located on the right side of the engine near where the transmission and engine meet. He says it can only be accessed from underneath the car. Anyone have any experience with this?

The fact that the car runs fine without MAF plugged, flawlessly (besides CEL), even MPG seems the same, I don't suspect its the distributors, rotors, etc.

So what do you guys think? Bad Air Mass Sensor? I also noticed my Check Engine Light does not go away when I plug the MAF back in. I think I need to clear the code? My car came with the little LED with a small button, but it doesn't seem to respond (I read about how to pull codes, holding the button down for a few seconds and releasing I don't get any blinks whatsoever, with key in engine off or on it doesn't work). If I buy a new sensor would I need to reset the code before the new sensor will work properly? I also noticed next to the LED box theres the 38 pin plug under the cap.

Please help if you can, thanks in advance!

 

The best way...without guessing....go to a shop and pay them 20 bucks or so to hook up their scanner and get the codes. Just a suggestion.

 

Not throwing any codes, except when MAF is unplugged. Any experts??

 

I'm sure you'll find that the insulation on you wiring harness have deteriorated. You will need to replace it.

 

You can buy sensor cleaner at a Auto parts,to clean it first.Might fix your problem.

 

A new MAF will work if that’s the problem but the code light will stay on until cleared I believe. Without reading the codes I'm not sure how to know for sure but it could also be the fuel pressure sensor or Cam positioning sensor, they can cause the same symptoms. Both fairly cheap and easy to DIY. For what it's worth I'm no pro but have experiance simular issues.

 

Swap out the cam sensor should stop the surging.

 

Just had same issues with another vehicle, just idled the car and tapped on the MAFS and it would idle rough and stall, tap on it again runs smooth. Replaced MAFS and it was fine never had a problem since.
MAFS do go bad.

 

I know its kind of late to update this, but the solution was a bad MAF. Replaced the MAF from a re-manufactured unit from AutohausAZ back in March. Idle perfectly now, more power, all good now. It wasn't the Wiring Harness, although I still need to change that since its never been replaced.

 

Figured that was the problem..