alovan

Dear All,

I would like to solve a starting issue with injectors in my E350 CDI. I'm interested in the details of the problem and I'd like to understand the reasons behind them.

Vehicle details:

• Mercedes-Benz E350CDI 4MATIC BlueEfficiency (S212 Estate)
• 2011 model year
• OM642.858 engine
• 270'000km (~168'000miles)

Symptoms:

• Clearly audible diesel knocking sound when engine is cold
• Barely audible diesel knocking sound when engine is warm between 1500-2000rpm
• No misfire/judder, performance or consumption issue (Avg: 6.5L/100km)

Performed tests:

• Compression test: TBD (I understand it's required to give corrections any sense, so I'll post the updates)
• Quantity correction values of smooth running control
  • 
    
• Visual inspection
  • No sign of residue/leak around injectors
• Mechanical inspection
  • Oil and filters has been changed recently and no metal chips were visible

Hypothesis:

• My general understanding on diesel injector issues
  • Misfire/judder: caused by lack of fuel --> stuck/dirty injector --> injector is under-fueling
    • If there are no mechanical issues with the pistons, valves
  • Knocking: caused by bad injection timing --> worn/leaky injector --> injector is over-fueling
    • If there are no mechanical issues with the bearings, rods
• My general understanding on quantity correction (Xentry)
  • The calculated quantity of additional fuel required to maintain smooth running (to get a proportionally equal torque delivery from the cylinders)
  • As we are talking about injectors in the same system, one injector might be balancing out torque delivery issues caused by other ones
    • Positive values: more fuel (torque) is required --> injector is under-fueling
    • Negative values: less fuel (torque) is required --> injector is over-fueling
• Based on my understandings and measurements
  • High positive values --> more fuel required --> under-fueling injector --> stuck/dirty injector --> misfire/judder
    • Cylinder 6, but there is no misfire/judder
  • High negative values --> less fuel required --> over-fueling injector --> worn/leaky injector --> knocking sound
    • Cylinder 3

Actions I would make:

• Change injector on Cylinder 3

Questions:

• Can someone suggest whether my hypothesis is valid or not?
• Can someone suggest whether changing injector on Cylinder 3 is an appropriate action to solve the issue?

Thank you very much in advance!

kajtek1

Do a compression test before final conclusions.
I never had to go that far, but injector leak test seems to be pretty popular.
270k km is not high mileage for injectors, but since you are having trouble, I would consider replacing all of them.
Having roots in Poland, I can get rebuild, certified injector for $110, so it would not be a big deal for me.

Plutoe

I appreciate the time and effort you took to collect data and deduce a hypothesis-------I would suggest that spend an equal amount of time researching "injector quantity correction and ohms tests" then you may want to revise your data and hypothesis

alovan

Thank you for your suggestions!

• Regarding changing all injectors
  • If I cannot figure out what's the issue, I will change them all. But currently I'm interested in finding out the cause.
• Regarding resistance measurement
  • I'll definitely do that. I remember on VAG vehicles VCDS can even display capacitance which is useful for piezzo injector fault finding.
• Unfortunately I did not have the time to do the compression test so that's still among the todos.
• Meanwhile I checked after a cold start where the knocking is coming and it seems to be on the left side (facing front) which includes cylinder number 1-2-3 so it's a usefuly info
• I also checked rolling out the car from 2000rpm to 1000rpm and the knocking is not audible so the sound seems to be audible during injection