Diagnosing rough idle and general poor running
Discussion
Thought i'd get some opinions for an annoying problem
Picked up a very nice Laguna recently - £650 for a low mileage 56 plate car, which is in the main fine - except for a couple of small issues.
Idle is rough - not rough as in stalling at junctions or revving it's nuts off - more uneven and making the car shake (so sitting in at lights it becomes annoying)
Car is also down on power, on a fast run it feels like it's holding back and occasionally stuttering.
I've immediately gone for the obvious and swapped out the coil packs regardless, despite my suspicion that it's more likely a fueling issue.
Anyway, popping an OBD2 reader on the car i get the following
First thing i noticed (apart from the obvious unsteady idle) is that the ST fuel trims are all over the place, presumably as the PCM tries to keep the mixture in check. The output from the O2 sensor also being all over the place.
Contrast this with the same kit plugged into the Mrs car
Lovely even idle. nice triangular waveform on the fuel trim (just as you'd expect) and an even response from the O2 sensor.
My suspicion is that the O2 sensor in my car is either struggling to respond in time or providing false readings. LT fuel trims are positive, suggesting that the car is running predominately lean and likely causing the issues i'm seeing.
Wondered if anyone else had any suspicions as to what might cause this - inlet / water temps are normal as is the output from the MAP sensor.
Picked up a very nice Laguna recently - £650 for a low mileage 56 plate car, which is in the main fine - except for a couple of small issues.
Idle is rough - not rough as in stalling at junctions or revving it's nuts off - more uneven and making the car shake (so sitting in at lights it becomes annoying)
Car is also down on power, on a fast run it feels like it's holding back and occasionally stuttering.
I've immediately gone for the obvious and swapped out the coil packs regardless, despite my suspicion that it's more likely a fueling issue.
Anyway, popping an OBD2 reader on the car i get the following
First thing i noticed (apart from the obvious unsteady idle) is that the ST fuel trims are all over the place, presumably as the PCM tries to keep the mixture in check. The output from the O2 sensor also being all over the place.
Contrast this with the same kit plugged into the Mrs car
Lovely even idle. nice triangular waveform on the fuel trim (just as you'd expect) and an even response from the O2 sensor.
My suspicion is that the O2 sensor in my car is either struggling to respond in time or providing false readings. LT fuel trims are positive, suggesting that the car is running predominately lean and likely causing the issues i'm seeing.
Wondered if anyone else had any suspicions as to what might cause this - inlet / water temps are normal as is the output from the MAP sensor.
kambites said:
If it has a positive crankcase ventilation system, check all the valves are working properly. I had a similar set of symptoms on the Octavia and it turned out to be a failed PCV valve diaphragm.
TaHas a weird breather arrangement, the rocker cover has a port that connects directly to the inlet manifold
Any just throw an o2 sensor at it anyway!
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