Hi,

I have a pre-ownd 2007 GL320 CDI at 150000 Km, bought approx. 2 years ago. Since around one year ago it started to develop some heavy noise during acceleration, that matches a loss in low-end power. It started to do it from time to time and it happened more often. Nowadays it is pretty hard to hit a "good" day.

The "bad" days:
- heavy injector ticking noise when accelerating
- engine struggles to reach 2000rpm (from a cruising 1400)
- engine drops in revs when touching the throttle and then it slowly catches up
- a heavy, low frequency tapping noise (~10-15Hz) coming from the front of the car
- whenever the torque needs are higher (e.g. climbing a ramp) all the above symptoms become louder
- the "bad" mood gets worse as engine gets warmer (e.g. after driving 200km).

The "good" days:
- smooth engine sound when accelerating (almost like a gas one)
- engine reaches 2000rpm with ease (without downshifting, or I don't feel the downshift)
- no injector noise, no tapping noise
- when I stop for 1-2 hours and start again it acts "good" for 20-30 minutes, as I need to be very careful with the throttle when driving in the city as it wants to jump over the car in front.

Regardless of the "good" and "bad" mood, when i floor it, it launches like crazy. The issue happens on partial load only (before 2500rpm)

The loss/no loss of power thing might be subjective because in the "bad" days it struggles to reach 1800rpm where the torque should be high and in the "good" days it jumps quickly to 2000rpm.

Extra information:

- nov 2011 software update for the engine and transmission (it doesn't do 2-1 downshift while moving when braking to a stop)
- TC lock-up clutch adaptation
- 2-3 upshift adaptation
- one injector reached >5 correction at the 4th consecutive test (was looking good if a single test has been performed). Now all the 6 injectors have corrections < +/- 0.5
- checked air lines and they look good
- transmission oil&filter change, engine oil&filter changes, fuel filter changes, air filters changes. Basically, every service is done properly and there is no change in behaviour
- the DPF empties itself regularly at ~1000km
- there never have been any fault codes (that's the main issue actually, as there is not a single starting point too look after)


After reading hundreds of Mercedes and non-Mercedes symptoms, I have reached the following suspects:
- turbocharger vanes
- quantity adjustment valve in high-pressure rail
- MAF(s?)
- high-pressure fuel sensor
- O2 sensors

Does anybody have any insights or starting point ideas?

 

i'd also look into the ignition transformer. i had some similar symptoms and when brought to the dealer it ran perfectly and had no codes. they found an intermittent short in the transformer.

 

It is a diesel engine.