M278 #5 misfire before and after engine rebuild.
Thread Starter
Newbie
Joined: May 2026
Posts: 2
Likes: 0
CLS W218 4.7
M278 #5 misfire before and after engine rebuild.
Hey,
I have an issue with cyl #5 which extremely misfires, has rough idle.
Engine had wall scoring so I rebuilt it. New cylinder sleeves, pistons, rings, both heads refreshed, conrod and crankshaft bearings, timing etc. It's ALMOST brand new
Checked resistance to all four cam solenoids to ECU - OL.
I checked resistance from the coil and fuel injector connectors to the ECU, which was the same as #6.
I also checked resistance from the O2 sensor before cat - same as the other side.
I ALSO checked ECU as I have another one programmed to the car.
Swapped injectors and coils.
Right now I'm facing cam position sensor fault. Checked and confirmed it is the sensor (swapped from intake to exhaust and code also transfered).
But I don't think that cam sensor should only turn off cyl. #5 - that would be weird.
Before rebuild it was also misfiring at #5 throwing injector code at that time if I remember correctly.
There was some oil at the solenoids connectors (before rebuild)
No visible smoke from exhaust.
As on video.
Before start I cleared all the faults. It tries to find a "good" spot to idle but fluctuates from 850 to 1000RPM and then throws CEL.
Fault codes:
P034800
P030585
P030500
P030085
P030000
And any ideas why FAN is at full speed?
Still on investigation but would love to hear some advice.
Thanks!
I have an issue with cyl #5 which extremely misfires, has rough idle.
Engine had wall scoring so I rebuilt it. New cylinder sleeves, pistons, rings, both heads refreshed, conrod and crankshaft bearings, timing etc. It's ALMOST brand new
Checked resistance to all four cam solenoids to ECU - OL.
I checked resistance from the coil and fuel injector connectors to the ECU, which was the same as #6.
I also checked resistance from the O2 sensor before cat - same as the other side.
I ALSO checked ECU as I have another one programmed to the car.
Swapped injectors and coils.
Right now I'm facing cam position sensor fault. Checked and confirmed it is the sensor (swapped from intake to exhaust and code also transfered).
But I don't think that cam sensor should only turn off cyl. #5 - that would be weird.
Before rebuild it was also misfiring at #5 throwing injector code at that time if I remember correctly.
There was some oil at the solenoids connectors (before rebuild)
No visible smoke from exhaust.
Before start I cleared all the faults. It tries to find a "good" spot to idle but fluctuates from 850 to 1000RPM and then throws CEL.
Fault codes:
P034800
P030585
P030500
P030085
P030000
And any ideas why FAN is at full speed?
Still on investigation but would love to hear some advice.
Thanks!
Last edited by lookw218; May 11, 2026 at 04:32 PM. Reason: Forgot to mention codes,
Junior Member
Joined: May 2026
Posts: 19
Likes: 7
From: Germany Munich
CLS 350 CDI 4 Matic
Wow, massive respect for tackling a full M278 engine rebuild! It sounds like you've been incredibly thorough with your troubleshooting so far.Regarding the radiator fan running at full speed: That is almost certainly the car putting itself into a failsafe/limp mode. Whenever the ME (ECU) detects a critical sensor fault or loses a specific signal, its default safety protocol is to command the fan to 100% to prevent any possibility of overheating. Fixing the sensor faults will likely make the fan go back to normal.Since the cam sensor fault followed the sensor when you swapped it to the exhaust side, definitely replace that one first. Even if it seems weird that it would only isolate cylinder #5, these engines can do very strange things when the timing data is inconsistent.For the persistent #5 misfire: Since you have already ruled out the coil, injector, and the ECU itself... and you mentioned there was oil at the solenoid connectors before the rebuild. Could it be a compromised wire in the engine harness going specifically to #5? Oil wicking through the harness is a known issue, and a slightly degraded wire/pin could cause a bad signal. Also, just to be 100% sure the new sleeves and rings are perfectly seated, have you done a quick compression/leak-down test on #5 since putting it back together?Good luck tracking this down, keep us updated! You are so close to having a brand new engine.
