I'm trying to help a friend out with a CEL on his 2009 Merc GLK 280.
Hooked it up to an OBD2 reader and retrieved a single code:
P0300 - 'Random/Multiple Cylinder Misfire Detected', via the 'OBD Fusion' app on my iPad.
Here is the
P0300 freeze frame:
+-----------------------------------------+-------------+ | Fuel System 1 status | 1 | | Fuel System 2 status | 1 | | Fuel System 1 status | 1 | | Fuel System 2 status | 1 | | Calculated load value | 23.14% | | Engine coolant temperature | 49 C | | Short term fuel % trim - Bank 1 | 0% | | Long term fuel % trim - Bank 1 | 11.72% | | Short term fuel % trim - Bank 2 | 0% | | Long term fuel % trim - Bank 2 | 7.03% | | Intake manifold absolute pressure | 28 kPa | | Engine RPM | 1293.75 RPM | | Vehicle speed | 17 km/h | | Ignition timing advance for #1 cylinder | 42.5 deg | | Intake air temperature | 46 C | | Mass air flow rate | 5.63 g/s | | Absolute throttle position | 13.33% | | Time since engine start | 26 sec | | Fuel rail pressure | 380 kPa | | Commanded evaporative purge | 0% | | Fuel level input | 45.49% | | Barometric pressure | 99 kPa | | Control module voltage | 13.29 V | | Absolute load value | 17.25% | | Fuel/Air commanded equivalence ratio | 1.54 | | Relative throttle position | 1.96% | | Ambient air temperature | 36 C | | Absolute throttle position B | 12.94% | | Accelerator pedal postion D | 6.27% | | Accelerator pedal postion E | 6.27% | | Commanded throttle actuator control | 2.75% | +-----------------------------------------+-------------+
I collected further information via the scan tool:
The front sensors on both banks return 1.00 V. which indicates that they're toast (assuming they're narrowbands, normal output range should be 0.2 - 0.8 V).
The rear sensors read around 0.63 V.
Both readings taken with the engine warm and at idle. Blipping the throttle registers a change in the voltage output of the rears only; the output of the fronts doesn't change.
Long-term fuel trims are +7% for one bank, +11% for the other.
Short-term fuel trims are 0% at hot idle.
Mass Air Flow, Fuel Flow
At hot idle:
Mass air flow = 3.66 g/s Fuel flow = 1.27 l/h, Fuel rail pressure = 380 kPa
This indicates to me that AFR is around 14.88, close to stoich.
I'd like to say based off my calculation that the front lambdas are to blame, but a couple of things are puzzling me:
If the front lambda sensor voltage is indicating rich, why is the fuel trim positive (indicating that the injector pulse-width is increased because the AFR is deemed too lean by the sensor)?
The misfire happens after a cold start with minimal throttle (note the time since engine start in the freeze frame). I'm not sure I completely understand the interaction here with the faulty lambda sensor, since cold start enrichment is playing a role here. If the system is running slightly richer than stoich, it would it not be less of a reason for the falsely-sensed rich condition to cause a misfire?
The owner is complaining of poor fuel economy as well. If the front lambda is sensing rich, I would expect the AFR to be leaner than reality, so I would expect fuel economy to be slightly better.