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P0171 Diagnosis
Ok, so since it looks like no one knows how to diagnose this, here is what I learned:
1. The p0171 is a fuel mixture too lean. This means that the various sensors in the car intake and exhaust systems is reading a fuel mixture that is less than the ideal 14.7:1 ratio.
2. The fuel mixture is automatically adjusted by the ECU.
3. There are 2 types of adjustments (called trims), the short term and long term. The STFT is an immediate adjustment to get the mixture back to 14.7:1. The LTFT samples the STFT and makes a long term change to the trim to maintain the 14.7:1 ratio.
4. If the LTFT is greater than 20%, a Bmw will throw a MIL.
Causes of P0171
1. The p0171 code is usually caused by 2 things, Vacuum Leak and Fuel Starvation
Diagnosis
By using the LTFT, STFT and the Engine Speed, you get an idea of which cause you should look at.
You will need an OBD tool that will allow you to read data in real time.
Hook up the tool and set it up to display STFT, LTFT and Engine Speed
Record the LTFT at idle, this will be your baseline
Next, rev the engine and record whether the LTFT goes up or down
Hold the engine speed at about 1500 rpm and record the LTFT
Findings
If your LTFT increases as the rpms increase, you have a fuel starvation issue
If your LTFT decreases as the rpms increase, you have a vacuum leak
In my case, I had an increase, then a decrease. In this case, i had both vacuum and fuel issues. I fixed the vacuum issues first (they are cheaper and easier). Now, I am working the fuel starvation. Fuel filter is first on the list.
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