Quote (sarge799 @ Apr 18 2014 12:54am)

Check Engine Light on, OBD2 reading indicates bad O2 sensor/system too rich.

Quote (sarge799 @ Apr 19 2014 02:13am)

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)

???

Highly unlikely that both O2 sensors would go bad at the same time. Possible unmetered air issue or fuel delivery issue if the car is running lean. Check for vacuum leaks then check the fuel filter/pump/injectors in that order.

This. Lean on cold start will cause knocking and sputtering. At this point the car is not safe to drive.

That's incorrect. Rear 02s can effect fuel trims as they move the goal voltage to go into "Protect Converter" mode.

From From Star Center News #5, February 2000 -
Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.
When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.
Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.
If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an efforet to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.
If the downstream O2 sensor has a tendancy to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.
Sometimes a service technician will reset the PCM and take it for a test drive. The PCM does a recalibration after the reset, and the probelm may go away until the PCM has completed a relearn of the system and then begins to attempt to correct for the downstream sensor error. If the symptoms reappear after several days of driving after a service is performed, check the downstream O2 sensor, the exhaust system, the wiring, and the catalytic converter. Note: a worn out catalytic converter can produce symptoms similar to a bad downstream O2 sensor.


0171/0174 are never oxygen sensor codes. A stuck sensor will throw a p219X code. 0171/174 paired is always an unmetered air flow issue. Generally a leak (like you found) at the boot between the intake and MAF sensor, or at the pcv system.

Not every OS uses rear O2 data to adjust fuel trims. His might. I'm sure the setpoints and limiters are different from vehicle to vehicle. I've never worked on anything that allowed the rear o2's to effect any noticeable change in fuel trims. The only table in my OS that ties trims and rear o2's together is cat overtemp. He isn't seeing that on a cold start. All it does is richen the mixture above a certain ECT.


I think you are overestimating the amount of fueling fluctuation yoi are describing. If the upstreams are oscillating at all, it is very close to stoich. Very, very close.

This post was edited by FMX_89 on Apr 19 2014 09:14pm

Yeah, it is pretty uncommon, it's just not a "never" thing. I very much doubt they have anything to do with this particular vehicle, but it is worth noting that in the coming years they are getting a much larger role. For 2015( I believe it was 2015, I was really tired in the seminar) Chryslers will give them complete control because the government has decided they must warranty cats for 10 years/ unlimited miles and just one of the converters on a ram with a cummins sells for around $7K. We had several case studies involving converter codes + upstream o2 codes that were actually problems with downstream o2s.




Are you not able to just tell us the P codes? Is it just p0171/174 you are throwing?

Have you cleared the codes since you fixed in the intake leak? It takes many drive cycles for them to clear themselves.

You may have two different problems here. The knock actually sounds like the anti drainback valve in your oil filter may have failed.

It would also help to know what kind of 05 BMW we are talking about.

p0171/0174 means that the pcm is having to positively push the fuel trims above a value (generally 25% LT +ST) to maintain stoic.

Generally on a BMW 171/174 is caused by a bad Mass Air Flow, or a failed crankcase ventilation valve (which is weirder then hell on bmws compared to Domestics or Asian.)

Is the MAF chart you posted with you revving it? If so that MAF has failed (or has a huge leak between it and the intake. you should be seeing around 5g/sec at idle and around 150 at 5K rpm.

You're using an app to figure out your CEL codes?



Just go to any parts store, they usually do scans for free.