Surging Idling and CEL
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Joined: Oct 2021
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2008 C300 4Matic Sport
Surging Idling and CEL
Greetings,
I have a 2008 C300 M272 with 147k miles. At 144k miles, I replaced the intake manifold, PCV high and low sides, oil drop pan, spark plugs, and ignition coils. The MAF and throttle body were cleaned at this time. That cured all my ills until about a week ago. I was getting random rough idles which cleared up when adding accelerator pedal. Now I am getting check engine lights, which give me p2015, usually p0305, and also other random p030x misfires. I am also experiencing random idle surges up to around 2k RPM which clear up with a throttle blip.
I can certainly feel a miss at idle, but not during acceleration - it drives perfectly normal. Odd. Might have another dead coil on #5, or bad plug. Seems unlikely given the short installation life, but stranger things have happened. The addition of other random cylinder misfires makes me think theres something else afoot.
I have some theories, but don't want to start throwing parts at it. I wanted to see what you all thought about it. Thanks.
I have a 2008 C300 M272 with 147k miles. At 144k miles, I replaced the intake manifold, PCV high and low sides, oil drop pan, spark plugs, and ignition coils. The MAF and throttle body were cleaned at this time. That cured all my ills until about a week ago. I was getting random rough idles which cleared up when adding accelerator pedal. Now I am getting check engine lights, which give me p2015, usually p0305, and also other random p030x misfires. I am also experiencing random idle surges up to around 2k RPM which clear up with a throttle blip.
I can certainly feel a miss at idle, but not during acceleration - it drives perfectly normal. Odd. Might have another dead coil on #5, or bad plug. Seems unlikely given the short installation life, but stranger things have happened. The addition of other random cylinder misfires makes me think theres something else afoot.
I have some theories, but don't want to start throwing parts at it. I wanted to see what you all thought about it. Thanks.
Last edited by seafartfiretruk; Feb 20, 2025 at 06:24 PM. Reason: additonal info
Super Member
Joined: May 2024
Posts: 506
Likes: 124
From: Northern Indiana
2014 c300 w204.080 3.5 v6
I dunno what those codes are but at idle a couple easy things to check are the throttle body and throttle blade could prolly use a good cleaning. Also if there is a EGR valve on your engine it could be plugged with carbon also.
After that if it was running goods before you did the work that you did you should double check all your work for mistakes. Incurring pulling all the plugs too bee sure the gap is good.
After that if it was running goods before you did the work that you did you should double check all your work for mistakes. Incurring pulling all the plugs too bee sure the gap is good.
Out Of Control!!
Joined: Apr 2019
Posts: 11,993
Likes: 6,827
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
Greetings,
I have a 2008 C300 M272 with 147k miles. At 144k miles, I replaced the intake manifold, PCV high and low sides, oil drop pan, spark plugs, and ignition coils. The MAF and throttle body were cleaned at this time. That cured all my ills until about a week ago. I was getting random rough idles which cleared up when adding accelerator pedal. Now I am getting check engine lights, which give me p2015, usually p0305, and also other random p030x misfires. I am also experiencing random idle surges up to around 2k RPM which clear up with a throttle blip.
I can certainly feel a miss at idle, but not during acceleration - it drives perfectly normal. Odd. Might have another dead coil on #5, or bad plug. Seems unlikely given the short installation life, but stranger things have happened. The addition of other random cylinder misfires makes me think theres something else afoot.
I have some theories, but don't want to start throwing parts at it. I wanted to see what you all thought about it. Thanks.
I have a 2008 C300 M272 with 147k miles. At 144k miles, I replaced the intake manifold, PCV high and low sides, oil drop pan, spark plugs, and ignition coils. The MAF and throttle body were cleaned at this time. That cured all my ills until about a week ago. I was getting random rough idles which cleared up when adding accelerator pedal. Now I am getting check engine lights, which give me p2015, usually p0305, and also other random p030x misfires. I am also experiencing random idle surges up to around 2k RPM which clear up with a throttle blip.
I can certainly feel a miss at idle, but not during acceleration - it drives perfectly normal. Odd. Might have another dead coil on #5, or bad plug. Seems unlikely given the short installation life, but stranger things have happened. The addition of other random cylinder misfires makes me think theres something else afoot.
I have some theories, but don't want to start throwing parts at it. I wanted to see what you all thought about it. Thanks.
Read the fuel pressure data with your scanner to confirm no fuel starvation.
