Rough IDle (sometimes)
Thread Starter
Newbie
Joined: Sep 2025
Posts: 8
Likes: 2
2015 E400 4MATIC
Rough IDle (sometimes)
One more request for help.
06 S430 4MATIC. 175,000 miles on the clock. Still runs well, most of the time.
The car seemed to lack power. As general maintenance, the plugs were replaced with NGK Rutheniums, a new Bosch MAF from FCP, air filters, oil and filter, etc. Still low power. Fuel pressure builds to 55 psi instantly with the key on and stays at 48-55 psi under throttle actuation.
The car also stumbles/hesitates when coming to a stop and when accelerating from a stop.
Replaced fuel filter. Lots of black nastiness came out of the inlet side. New filter installed, and the power is much better, sometimes.
Initially, it drove like a racecar comparatively, and idled fine. After about a month, it is slow off the line, but then kicks in power at about 2k+ rpm. When slowing to a stop, the engine starts to run rough/chug as you near a stop, then stumbles when pulling away. Sometimes it's worse than other times. Occasionally, it doesn't do it all. When the car is still "cold" (<40C), it doesn't do this at all.
DAS shows 02 sensors operating properly, secondary air pump tests fine, as does the EGR. DAS shows P0171 and P0174 codes for the ECU. I have checked all the vacuum lines and seals around the airbox, and they are all in good condition/recenlty replaced.
I am running out of thoughts. Could the new fuel filter already be plugged up?
Thanks for any thoughts.
Z
06 S430 4MATIC. 175,000 miles on the clock. Still runs well, most of the time.
The car seemed to lack power. As general maintenance, the plugs were replaced with NGK Rutheniums, a new Bosch MAF from FCP, air filters, oil and filter, etc. Still low power. Fuel pressure builds to 55 psi instantly with the key on and stays at 48-55 psi under throttle actuation.
The car also stumbles/hesitates when coming to a stop and when accelerating from a stop.
Replaced fuel filter. Lots of black nastiness came out of the inlet side. New filter installed, and the power is much better, sometimes.
Initially, it drove like a racecar comparatively, and idled fine. After about a month, it is slow off the line, but then kicks in power at about 2k+ rpm. When slowing to a stop, the engine starts to run rough/chug as you near a stop, then stumbles when pulling away. Sometimes it's worse than other times. Occasionally, it doesn't do it all. When the car is still "cold" (<40C), it doesn't do this at all.
DAS shows 02 sensors operating properly, secondary air pump tests fine, as does the EGR. DAS shows P0171 and P0174 codes for the ECU. I have checked all the vacuum lines and seals around the airbox, and they are all in good condition/recenlty replaced.
I am running out of thoughts. Could the new fuel filter already be plugged up?
Thanks for any thoughts.
Z
Last edited by ffzeus; Apr 26, 2026 at 02:12 PM.
Thread Starter
Newbie
Joined: Sep 2025
Posts: 8
Likes: 2
2015 E400 4MATIC
Solved
The driver 's-side crankcase vent hose had become disconnected from its connection. Apparently, on the first smoke test, it was pressed up against the fitting enough to prevent the smoke from escaping. After a second test, it had backed off enough to let the smoke out. Reconnected the hose, and all is good now.
Z
Z
Newbie
Joined: May 2026
Posts: 10
Likes: 6
W220 S500 Mopf
Thank you so much for sharing the fix.
And yes, my 'from the hip' guesses would have been either something with the PCV, or a faulty MAP sensor.
You never really know whether these faults are caused by the air box, or the manifold side.
The way I test this is by buying a cheap cigar (despite I'm a non smoker) and blowing smoke into the disconnected brake servo pipe.
You can then see very quickly where the leak is.
And yes, my 'from the hip' guesses would have been either something with the PCV, or a faulty MAP sensor.
You never really know whether these faults are caused by the air box, or the manifold side.
The way I test this is by buying a cheap cigar (despite I'm a non smoker) and blowing smoke into the disconnected brake servo pipe.
You can then see very quickly where the leak is.
Thread Starter
Newbie
Joined: Sep 2025
Posts: 8
Likes: 2
2015 E400 4MATIC
Smoking
Hi,
Yes. I had smoked it once before; however, the hose was pushed against the fitting tightly enough, and no smoke escaped. I guess after driving it a bit, and having the whole thing shudder and stumble, it caused the hose to misalign enough to leak smoke the second time. As it is, the other components are peace of mind, if nothing else.
Z
Yes. I had smoked it once before; however, the hose was pushed against the fitting tightly enough, and no smoke escaped. I guess after driving it a bit, and having the whole thing shudder and stumble, it caused the hose to misalign enough to leak smoke the second time. As it is, the other components are peace of mind, if nothing else.
Z
Last edited by ffzeus; May 2, 2026 at 06:57 PM.
