I purchased this car a few months ago because the transmission wouldn't shift. I got that fixed but I cannot figure out why it idles so poorly. I have replaced both fuel filters and O-rings along with the clear fuel lines. It starts great without any smoke but quickly starts idling so poorly I am surprised it stays running. I also ran 2 cans of diesel purge through it. I am getting a P0300 code (random misfire). I took it on a 500mi round trip this weekend and ran smooth on the highway getting over 30mpg. I am going to have the injectors cleaned. Does anyone have any ideas before I starting throwing money at it?

I would first hold a flashlight behind each of the fuel lines while the engine is running to make sure there's no air in the lines. Beyond that, the delivery valves can be in need of a 're-torquing.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

^^^ What he said. Mine idled very poorly for the first few minutes after sitting unused for about a month. I was afraid I would have to do fuel lines and O rings but after but I didn't see any air bubbles in the fuel lines. Eventually the problem went away as the car warmed up.

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CENSORED due to not family friendly words

Mine has the opposite problem. It starts great in the morning and idles fine for the first few minutes, then develops a "miss". No smoke. It is harder to start when warm. I'm trying to develop a plan to attack the problem. Does this order make sense?
1) check fuel lines for air leaks
2) rebuild fuel injectors (?)
3) delivery valves seals (?)
4) ip (?)

Check fuel lines for air leaks
check the pre-filter for air leaks and proper installation with new o-ring
replace o-ring on back of fuel control valve
delivery valve seals

Crack the fuel line open at each injector, one at a time, to see if you can isolate the problem to one cylinder.

I would be surprised if it is injectors or IP (other than DV seals)

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Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

I don't know what the history of the car is regarding what has been done to it by the prior owners. Another poster mentioned the delivery valves. If they were done in the past, and the proper procedure was not used, it can cause the issues you are having with the idle. When I did the delivery valves on my car, some members on the board indicated that the copper crush washers could be reused. I did the job following those instructions, using the old crush washers. Once I started the car, the idle was horrible, with the engine shaking all over the place. Prior to doing the delivery valves, the engine was as smooth as silk. I redid the delivery valves again, using new seals and new crush washers, torqued them per the instructions found on this forum, and the engine was once again idling as smooth as silk. Its a cheap fix, but it does require a close attention to detail. Good luck!!!

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96 E300d

The o-ring on the back of the shutoff valve gets missed when replacing the fuel line o-rings, and if that introduces air, your car will idle poorly but the fuel supply lines will be clear.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I just performed an R&R on the cylinder head to remove seized glow plugs. It ran fine at first but I had to remove the camshaft cover to retorque a couple of prechambers. When I put it all back together I have a very rough idle and it is smoking terribly (white smoke) upon initial accelleration but the smoke stops when it shifts into fourth gear. I don't know if all this is related but I thought I would throw it out there for the experts.....