2007 S550 consumer battery replaced - Very rough idling now.
Thread Starter
Newbie
Joined: Apr 2022
Posts: 12
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w221 S550
2007 S550 consumer battery replaced - Very rough idling now.
Hi,
(I tried to search for a definite answer in this forum, no success, sorry if this is a duplicate).
I replaced the consumer battery, the car was operating flawlessly before. The old battery was quickly dropping to 8-9V when car was turned off. The battery was disconnected overnight, the new one put in the next day. Right now, the car starts, but various codes appear. From misfires on some cylinders, to issues related to air intake. The exhaust smells on unburned fuel, and is white-ish. If I erase codes, engine runs fine briefly, and then again, codes start coming (different cylinders now) and is idling very rough. My mech (with limited merc tools) thinks misfire codes are consequence of incorrectly set air intake parameters, and the ECU fails to adjust to it. Before I take/tow car to Merc Service, where they might charge value of the car for the repair, I would like some opinions, which one of the following is:
1. Removing the consumer battery causes ECU to lose settings, and the car needs professional programing, costing $200-500-4000 etc
2. Removing the consumer battery causes ECU to lose settings, and the car will re-learn and readjust itself after several minutes/days on the road.
3. Totally unrelated, true errors, and the repairs will cost arm and leg.
Thanks everyone.
(I tried to search for a definite answer in this forum, no success, sorry if this is a duplicate).
I replaced the consumer battery, the car was operating flawlessly before. The old battery was quickly dropping to 8-9V when car was turned off. The battery was disconnected overnight, the new one put in the next day. Right now, the car starts, but various codes appear. From misfires on some cylinders, to issues related to air intake. The exhaust smells on unburned fuel, and is white-ish. If I erase codes, engine runs fine briefly, and then again, codes start coming (different cylinders now) and is idling very rough. My mech (with limited merc tools) thinks misfire codes are consequence of incorrectly set air intake parameters, and the ECU fails to adjust to it. Before I take/tow car to Merc Service, where they might charge value of the car for the repair, I would like some opinions, which one of the following is:
1. Removing the consumer battery causes ECU to lose settings, and the car needs professional programing, costing $200-500-4000 etc
2. Removing the consumer battery causes ECU to lose settings, and the car will re-learn and readjust itself after several minutes/days on the road.
3. Totally unrelated, true errors, and the repairs will cost arm and leg.
Thanks everyone.
Senior Member
Joined: Mar 2018
Posts: 493
Likes: 182
2006 W221 S500L M273
What year is your car?
By consumer battery you mean the big battery in the trunk or another one?
Are you 100% sure the new battery is connected properly?
Are you 100% sure the new battery is good?
By consumer battery you mean the big battery in the trunk or another one?
Are you 100% sure the new battery is connected properly?
Are you 100% sure the new battery is good?
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,322
Likes: 1,678
S500
reconnect, start the car check the errors - if it says cyl misfires replace offending coil pack if it says CAT 02 sensors replace them
you can have flat batteries all day long on the early ones once volts are normal the car is normal..
on the later cars (built after aug 2009) the baby battery under the ignition switch for the ISM might throw a tail spin due to low volts - its ONLY there to allow you to play PNR and D when the front battery gave up
Last edited by BOTUS; Jul 1, 2025 at 05:49 AM.
Thread Starter
Newbie
Joined: Apr 2022
Posts: 12
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w221 S550
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Joined: May 2018
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clk320
How long have you driven it after the battery replacement? I can tell you from experience that sometimes these things loose their minds after battery or major component swaps. What has worked for me every time has been to reset/ clear codes and drive it HARD for a few miles. Clear codes again, if it repeats, do it again. Not saying that it works for everyone, but it has worked for me. They do not like parameter changes. Needs to relearn.
Super Member
Joined: Jun 2025
Posts: 784
Likes: 249
From: USA
It sounds like the ECU lost some adaptations when the battery was disconnected. This is common, and the car may need some time to relearn idle and fuel trims. Drive it gently for a while (20-30 miles) to see if it improves. If not, a professional reset/recalibration might be needed. The white exhaust and fuel smell suggest it’s running very rich, likely due to reset adaptations. If it doesn’t clear up, a dealer or specialist scan tool will be necessary. Unlikely a major failure if it ran fine before the battery swap.
