PeachParts Mercedes-Benz Forum - View Single Post - '97 S420 Acceleration and trans jerking issues

tinypanzer · #1 03-22-2017, 07:04 PM

Hello all,

There's a lot of variables here, so I'll try to break this down into a sane set of parameters.

Background: Just bought a '97 S420 with 265k on the clock from my local indy. Yes, that's a lot of miles however the body is in absolutely stunning condition, and my indy just did a complete rebuild on the transmission. It's a car he wound up with after the previous owner decided they didn't want to pay for a new trans.

A host of performance issues plagued the car when I got it, for starters it had been sitting for a while and was just starting to feel the effects of lot rot. My main complaints with the car so far are these things:

- Poor acceleration, weird and inconsistent throttle response
- Transmission sometimes causes the car to jerk back and forth a little bit when coasting down to a slower speed
- Lifter tick, likely the oiler tubes although 8oz of MMO made it go away for a while

I've done several things to the car, and some improvements have been made. The throttle body was badly fouled, so I cleaned it. I put a new MAF sensor in. I reset the adaptive transmission data using the standard key/throttle method

The poor acceleration issue is a lot better, but not 100% cured. Sometimes it feels as though the transmission has simply shifted too soon and doesn't want to kick down unless I give a LOT more throttle. Overall however, the MAF sensor replacement made a very big improvement.

I very much doubt that the actual transmission is the issue here. For starters, my indy used to be a dealer tech and he has done lots of these. His work is excellent, and I do not yet think that the problem lies in the transmission because to me, it feels much more like the ECU is confused about how much throttle I am giving, or is getting bad sensor data somewhere.

The jerking motion I feel during coasting down to a lower speed is intermittent, and it almost feels like the ECU can't decide if I'm stepping on the gas or not, as if it's failing to allow what I will ignorantly refer to as 'overrun'.

My main suspects at this point are the throttle position sensor and the electronic throttle assembly.

Can anyone offer any suggestions and/or test procedures to help me isolate the fault? Is there some other part I didn't mention that needs to be checked?