Misfiring and rough idle after spark plug change
Thread Starter
Newbie
Joined: Apr 2022
Posts: 1
Likes: 0
2009 C300 4matic luxury
Misfiring and rough idle after spark plug change
Hello,
here’s a classic if it ain’t broke don’t fix.
changed the spark plugs on my 09 c300 158k kms from the oem platinums to denso iridium plugs(pre gapped), car started idling very rough dropping rpm’s getting a code for misfires on different cylinders. So went ahead and changed back to the oems still have the same issue rough idling, sputtering and misfire. Took it to my mechanic changed the old ones back to the new ones still having the same issue. Checked to see if it was the coil by switching and still same issue.
from what I’m reading it can either be a cracked intake manifold or something with the fuel injectors. But I should mention that I had zero engine issues with the car before hand.
it’s only when I come to stop or am idling that it does this once I pass 1k rpms and go from 1st to second it runs normal.
super weird that this would happen after a spark plug change. Wondering if anyone has gone through something similar or has some insight.
here’s a classic if it ain’t broke don’t fix.
changed the spark plugs on my 09 c300 158k kms from the oem platinums to denso iridium plugs(pre gapped), car started idling very rough dropping rpm’s getting a code for misfires on different cylinders. So went ahead and changed back to the oems still have the same issue rough idling, sputtering and misfire. Took it to my mechanic changed the old ones back to the new ones still having the same issue. Checked to see if it was the coil by switching and still same issue.
from what I’m reading it can either be a cracked intake manifold or something with the fuel injectors. But I should mention that I had zero engine issues with the car before hand.
it’s only when I come to stop or am idling that it does this once I pass 1k rpms and go from 1st to second it runs normal.
super weird that this would happen after a spark plug change. Wondering if anyone has gone through something similar or has some insight.
Member
Joined: Jun 2019
Posts: 142
Likes: 26
What you describe often means one of two things...
-Wrong plug/temp range plug. Or bad plug, such as one that has been dropped, resulting in the porcelain cracking. Going to guess this is not your issue.
Or...
-Vacuum leak. Make certain your secondary air pump hose (front half engine cover) is properly attache. Make certain you didn't crack/split any of the hard vacuum lines around the intake. Make sure the hose on the oil separator is attached.
It could also be something like a plug/connector that isn't clicked fully, but doesn't really fit with what you describe about behavior.
-Wrong plug/temp range plug. Or bad plug, such as one that has been dropped, resulting in the porcelain cracking. Going to guess this is not your issue.
Or...
-Vacuum leak. Make certain your secondary air pump hose (front half engine cover) is properly attache. Make certain you didn't crack/split any of the hard vacuum lines around the intake. Make sure the hose on the oil separator is attached.
It could also be something like a plug/connector that isn't clicked fully, but doesn't really fit with what you describe about behavior.
Joined: Nov 2016
Posts: 7,847
Likes: 3,497
From: Maryland
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Can you confirm your engine designation? Did you have to remove the intake manifold to get to the plugs?
EDIT: OK, apparently you do not need to remove the intake manifold like you do on the M276 engine. This video shows it to be a pretty simple job.
You said multiple misfires. Which cylinders? If you move the coils around, do the misfires follow the coils or stay in the same cylinders? Is this the first time these plugs have been changed?
Since you have put the old plugs back in and still have this problem, I would suspect that it is either the coils, the boots, the wiring, or a combination.
EDIT: OK, apparently you do not need to remove the intake manifold like you do on the M276 engine. This video shows it to be a pretty simple job.
You said multiple misfires. Which cylinders? If you move the coils around, do the misfires follow the coils or stay in the same cylinders? Is this the first time these plugs have been changed?
Since you have put the old plugs back in and still have this problem, I would suspect that it is either the coils, the boots, the wiring, or a combination.
Last edited by JettaRed; Apr 10, 2022 at 11:54 AM.
