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Old 04-01-2013, 07:10 PM
buster1015 buster1015 is offline
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Location: SoCal
Join Date: Mar 2013
Posts: 5
Mein Auto: 2001 X5 4.4l
I had bought them from

When my engine is idling the STFTs are about zero varying to +2 and -2. I pulled the MAF connector and the engine revved up to 1100 rpm briefly and quickly resettled at the normal 600 rpm idle. The 15 seconds after pulling the connection the STFTs varied from +3 to +7. Engine idle seemed smooth but the computer continued to adjust the fuel and by the 30 second mark STFTs were -3 to -6. I didn't try to plug the MAF back in while the engine was running but I think there is a good chance that would stall the engine.

I hadn't thought much about your mentioned O2 sensor voltages because I've been working with a wideband O2 while tuning my Porsche lately. The narrow band O2 (type used for fuel trims) is really only good at telling the computer if the engine is running rich, lean, or at stoichiometric. I uploaded a generic O2 output voltage chart. As you can see stoich includes a large range from about +.15 to +.75v. I'm sure the Bosch one is very similar. Your O2 voltages could be due to the sensors going bad OR the sensors are completely fine and you have a different issue (MAF, vacuum leak, clogged fuel filter). At this point we can't tell for sure which cause is giving the lean reading since your voltages are within the typical range.

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There is a way to test the MAF and get an idea if it is working. The basic idea is that the MAF signal varies from 0 to +5v based on the mass air flow rate. Voltage at idle should be low (~1.5v) and close to max (>4v) at wide open throttle. Instead of describing it I found a video that shows the test procedure:

You mentioned high fuel consumption. If the O2 sensors were incorrectly reading lean when there is no real issue then that will prompt the computer to add more fuel (too much) and your fuel consumption will increase but I would guess throttle response and power would still feel okay.

If the MAF sensor is under estimating the airflow (or air leak) then your O2 sensors would be adding fuel to get close to stoich. This was my issue and my fuel consumption went down and I began to notice decreased power at WOT.
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