M112 Rough Idle When Cold - Fixed

09-10-2014 | 03:05 AM

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krazzdav

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2017 GLA 250, 2011 C63 P31 (p30 w/ lsd) (in repair), 2005 C230 Sport Eurocharged, 2003 C320~sold

M112 Rough Idle When Cold - Fixed

Hi everyone...I just wanted to share my experience with my car having rough idle only when it was cold with NO CEL.

For me my fix was the spark plug wires which I was able to get a Bosch set part# 09848 for all 12 wires off of Amazon for less than $100.

History and trouble shooting--
My car is an '03 C320 with the M112 V6 and has 119k miles on it. I had recently changed the spark plugs to Bosch Iridium before the trouble started as well.

Before the car reached normal operating temperature the car would run so rough it would jerk when stopped at a stop light but then after it warmed up it would smooth out. After fixing it I realize that there was also a definite decrease in power and a slight misfire as well even when it was warmed up. It wasn't near as extreme as when it was cold so didn't really notice until fixing it.

I have an SDS / DAS and there were no codes thrown so spent some time trying to figure it out. I have worked on cars for over 20years, built my own motors to race and in general pretty well know what I'm doing but have to say this was a pain in my *** lol

Since the problem smoothed out after it warmed up threw me for a loop. First I scoured this site and internet for suggestions on what the trouble could be.

The most common thing that came up was the MAF so tried cleaning it then replaced that to no avail. (Bosch unit from Amazon again was the best price - about $120)

The next thing I came across was the throttle body and throttle actuator. Well I sure wasn't about to spend over $1k for one from the dealer! I found some new ones for less than $300 (rockauto) but lo and behold there was a used one on ebay that I got for less than $60! This was from an '05 w/ only 38k miles so was definitely worth a shot-- and yes, definitely REPLACE the o-ring if you are doing this. It is only a $5 piece for Pete's sake!! FYI The torque is 9nm. There is a spot in the SDS to recalibrate this. When I came across this I reset everything because I had changed the CPS (crank position sensor) some time ago and come to find out I should have reset the computer then too. Anyhow after replacing it, it still didn't fix my problem.

Well I came across something in the SDS to help me finally fix this gremlin.

I should have done this first and recommend anyone having similar problems to do so!!!

Under the ME-SFI control unit in the Actuations menu towards the bottom there was one for the ignition. Run this for both the A and B sides. You can jot the numbers from the readings down but for the most part if it is so bad it will now have a code you can pull! For me the readings with the cylinder that was bad read about 15 vs 0. After running the tests I pulled the faults again for the ME-SFI and had 3 cylinders showing up with a misfire -- 1A, 2A and 5B.

- Now that I had something to troubleshoot the first thing I did was move my coil packs around leaving the plug wires on the original cylinder. The problem did not follow the coil packs so I went ahead and bought new wires. One other thing that I should have done was also move the spark plugs to see if it followed but since I had new ones thought it highly unlikely they failed in such a short time. I should have also move the wires to ensure it did follow but wth...I already had over 100k on them so was needed in any case just for maintenance.

I hope this helps someone else having the same problem and at least can have them start by checking the ignition system for misfires by running the test in SDS.

Thanks to nlpamg for his experiences on his thread even though mine wasn't the same fix. https://mbworld.org/forums/c-class-w...ml#post3537209

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