Misfire after coil replacement
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Misfire after coil replacement
Car in question is a 2018 S63 that ran fine prior to coil and spark plug replacement. To my knowledge these had never been done and the car was at 80K miles so I did them.
So I used Bosch plugs, and I used AFTERMARKET coils. Coils on these cars are one for every 2 cylinders so 4 coils total.
Car pulls hard and runs great - except every few days it throws a CEL - codes show misfire on cylinder 7/8 (which share a coil incidentally). What is crazy about this is that if you leave the car sit for 5 minutes it all goes away. Which is bizarre..
I already replaced the offending coil with a new aftermarket one and it keeps doing the exact same thing. Oh, and it tends to do it most when the car is cold and just started.
THOUGHTS?????
So I used Bosch plugs, and I used AFTERMARKET coils. Coils on these cars are one for every 2 cylinders so 4 coils total.
Car pulls hard and runs great - except every few days it throws a CEL - codes show misfire on cylinder 7/8 (which share a coil incidentally). What is crazy about this is that if you leave the car sit for 5 minutes it all goes away. Which is bizarre..
I already replaced the offending coil with a new aftermarket one and it keeps doing the exact same thing. Oh, and it tends to do it most when the car is cold and just started.
THOUGHTS?????
MBWorld Fanatic!
Joined: Nov 2015
Posts: 1,691
Likes: 826
From: Newcastle, WA
2019 S63 4Matic+, 2018 E400 Cabrio, wardens car.
These spark plugs need to be indexed. Theoretically if you torque the MB plugs exactly to spec then they should land in the proper position. Not sure if Bosch plugs are designed to the same standard. Regardless, if it was my car I would pull all the plugs and make sure they are indexed by marking the tops of the plugs with a small dot that corresponds to the electrode direction and make sure they are all pointed in the right direction. Apparently this effects the way the detonation happens and can lead to valve and or piston damage if not correct. If they are off a little that could be the cause of this random misfire. One of the many reasons I chose to pay someone to do this PITA service.
Last edited by superpop; Jul 30, 2025 at 09:59 AM.
MBWorld Fanatic!
Joined: Aug 2007
Posts: 1,836
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From: Pacific Northwest
2001 S600 V12 Sold, 2011 Jaguar XFR Sold, 2017 S550 4-Matic, 2018 S63 AMG Sedan
I agree with @superpop about indexing, there are a few YT videos showing how to index the plugs on the M177 motor, moostly on E63s. I've had nothing but trouble with aftermarket engine control products. Not saying for sure that's your problem, but it's not worth the risk IMHO.
