C270 CDi rough idle

[HelpThisDude]

Hello all,
My 2001 C270 CDi auto drives lovely, and when 'revved' runs up and down the rev range smoothly. However, when left to idle, the engine revs dip by about 200rpm (with reference to the dash rev counter) every two seconds.
Also When cold, for the first 2 - 5min, the 'dip' can be felt when seated inside the car.
Anyone come across this before? is this a early warning sign?

 

[Cole@MBS]

That could be many things or just one, it could have had cheap injectors fitted at one time, or they are not coded to the ecu correctly, poss a air mass sensor on its way out, something like this really needs to go on a star machine to see what the actual values are of the engine when running!

 

[HelpThisDude]

Cheers S-A-M, hope its not injectors!

 

[HelpThisDude]

I was just reading a related post, they had worn injector nozzles, £115 a nozzle! I see what they mean now about being able to buy a nice car, just not being able to afford run it! Anyway.... I should be getting a fault download later today.... i'll let you know what it shows up.
Cheers again for the reply.

 

[HelpThisDude]

Diagnostics only showed up one fault, a fuel heater fault.
Real time readings high lighted the Exh gas temp before cat reading -50 deg, and fuel inlet temp reading -50 deg. I assume that -50 is a default figure for a failed sensor. But could this just be an additional fault, rather then the cause of the rough idle?

The system was cleared, and I was advised to drive the vehicle and come back another day to do another download. I would be advised on sensor replacements.
I was not charged for the diagnostic.

 

[too_juicey]

You was not charged? Lucky chap! I took mine to MB and they hit me with £55.00. I had a rough idle too, but it turned out to be a dodgy fuel filter and air in the fuel system.

 

[HelpThisDude]

“Too Juicey”, Ipswich Merc wanted £98+VAT. I had a friend use a Autodata computer to read the real-time variables. Not sure if its as comprehensive as the STAR machine.
I have lots of white smoke at idle now too. Smells like a paraffin heater! I really hope its something simple like a blocked fuel filter, rather then injectors bleeding back or dribbling. I will do a service at some point and see if it improves. It’s due in 3k miles anyway. Does anyone know if there is a Fuel temperature sensor on the UK cars? I looked all over the 'fuel heater' for any electrical connections. It seems to 'heat' the fuel using the water from the cooling system.

 

[HelpThisDude]

I changed the fuel filter. When I tried to prime it with the Ign on, no fuel was being delivered to the filter. I got it started by dribbling diesel into the new filter; I assume the main mechanical fuel pump must be doing all the sucking from the tank.
Any thoughts on if this would cause my rough idle? Or just another back ground fault to add to the list on my silver C Class lemon

 

[HelpThisDude]

Just retruned from Derek Wells, got it on the STAR machine there. Injector No2 readings out of spec. Charged me £15 for diagnosis. Cost of MB injector £181.24 + £33 labour.
Ill post back to report to anyone interested if this has solved the issue.

As a couple of note to a new-bee like me following up in the future.
1)You really do need the Mercedes diagnostic software to diagnose problems, the Autodiag whatever was just not good enough. The sensors I mentioned two boxes above that read open circuit, (-50) was because those sensors don't exist on my car.
(Which is what SAM UNIT said in the first place really-"..something like this really needs to go on a star machine.." doh!)
2)There is no electric pump in the rear, hence you can't prime the fuel filter with the Ing on. Dribble fresh diesel into the filter, connect up hoses, pray you have a good battery and then just crank away till it fires.

 

[Cole@MBS]

Sounds like your getting there!!

 

[seanie]

could be the fuel regulator, if it has a leak it will affect the revs like that. look forward to finding out what it is!

 

[meanie]

I was just reading a related post, they had worn injector nozzles, £115 a nozzle! I see what they mean now about being able to buy a nice car, just not being able to afford run it! Anyway.... I should be getting a fault download later today.... i'll let you know what it shows up.
Cheers again for the reply.

Think yourself lucky-£300 each is pretty normal for 'cheaper' stuff like Fords or Renaults!

 

[HelpThisDude]

Not sure about counting my self lucky, my HDI gave me no problems right up to 165k when I sold it. Come to think of it, our JTD is still going strong at 367k with all its original bits ( - service items). Anyway....

Injector No2 replaced in about 20min, 'coded' and all is now well. Getting about 4mpg extra now too (worked out on liters used and miles driven). I wanted to wait till we had some more cold mornings before reporting back. Thanks for all the advice.