w204 rough idle but normal at higher rom
Thread Starter
Joined: May 2025
Posts: 4
Likes: 0
w204 c200
w204 rough idle but normal at higher rom
recently i changed one of the coils in my w204 c200. afterwards the car started to idle roughly like one of the cylinders was not working or is having a missfire but as soon as i go over 1000 rpm it smooths out and becomes normal. now i have tried changing all the coils and spark plugs. scanned the car with an obd reader (although nothing comes out) and cleaned the air pressure sensor, throttle body, even the top of the piston where the spark plugs plug in, and still i have the same idling issue. i brought it over to my mechanic and checked the compression for the cylinders . all the cylinders were at a range of 720-760 l/min except for cylinder 3 which was at 680 l/min. so i thought that would be the issues, but surprisingly when the voltages of each cylinder were checked all the cylinders had 0v exept for cylinder 1 which was at 2v. now i dont know which cylinder is the issues but im assuming that the compression wont really lead to a rough idle just smoke coming either from the exhaust or where you check the oil level, so cylinder 1 is the issue but i still dont get what the 2v means and why that would happen.
if anyone had somewhat a similar problem and has managed to fix it, it would be nice to know how. because currently im lost 😊
if anyone had somewhat a similar problem and has managed to fix it, it would be nice to know how. because currently im lost 😊
Junior Member
Joined: Feb 2025
Posts: 18
Likes: 4
From: NC
2014 W204 C250
Hi, welcome to the forum!
What year is your c200?
Why was the coil changed in first place and how do you know the new coil is good? These days new doesn't necessarily mean good. It could be weaker than the OEM coil. Have you tried to unplug coils while the engine is running to see which one makes no difference?
I have no idea what the voltage test is either, sounds very generic, could be from coil ground to car ground, which would indicate a bad ground, but that's just a wild guess. Maybe ask your mechanic to explain what was measured.
What year is your c200?
Why was the coil changed in first place and how do you know the new coil is good? These days new doesn't necessarily mean good. It could be weaker than the OEM coil. Have you tried to unplug coils while the engine is running to see which one makes no difference?
I have no idea what the voltage test is either, sounds very generic, could be from coil ground to car ground, which would indicate a bad ground, but that's just a wild guess. Maybe ask your mechanic to explain what was measured.
Joined: Nov 2016
Posts: 7,847
Likes: 3,496
From: Maryland
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Your symptoms can be attributed to a vacuum leak. You need to get a proper scan tool and scan for codes.
Thread Starter
Joined: May 2025
Posts: 4
Likes: 0
w204 c200
i have gone through 2 sets of different coils and changing the positions and it still says cylinder 1 that has an issues. and the coils are from a trusted mercedes benz shop
Thread Starter
Joined: May 2025
Posts: 4
Likes: 0
w204 c200
Hi, welcome to the forum!
What year is your c200?
Why was the coil changed in first place and how do you know the new coil is good? These days new doesn't necessarily mean good. It could be weaker than the OEM coil. Have you tried to unplug coils while the engine is running to see which one makes no difference?
I have no idea what the voltage test is either, sounds very generic, could be from coil ground to car ground, which would indicate a bad ground, but that's just a wild guess. Maybe ask your mechanic to explain what was measured.
What year is your c200?
Why was the coil changed in first place and how do you know the new coil is good? These days new doesn't necessarily mean good. It could be weaker than the OEM coil. Have you tried to unplug coils while the engine is running to see which one makes no difference?
I have no idea what the voltage test is either, sounds very generic, could be from coil ground to car ground, which would indicate a bad ground, but that's just a wild guess. Maybe ask your mechanic to explain what was measured.
the coils sre from a trusted mercedes benz shop and i have gone through 2 sets of coils and the problem is still detected at cylinder 1, even though ive switched up the position of the coils. my c200 is a 2012 with the m271 evo
Thread Starter
Joined: May 2025
Posts: 4
Likes: 0
w204 c200
is a vacuum leak also called low compression? because if it is then yes i do have a leak on cylinder 3. but the rough idle is detected from cylinder 1. i scanned using a program called xentry. i dont really know how good it is but it was able to test for compression in the pistons.
