P0171 P0174 Check Engine Light CEL

[andrewwynn]

E53 uses millisecond timing of injectors to determine fuel metering.

The math only works correctly if the fuel pressure is 50.0 psi from my understanding.

I think all non diesel e53 use that same baseline pressure.

The e53 doesn't meausure fuel pressure it just assumes it's correct.

You said the pressure measured high but was that just at idle? What's the fuel pressure under load?

Low fuel pressure will give the same results as MAF reporting low air or unmetered air leak. It also may affect one bank before the other since the fuel is supplied on one end.

Did you mention which engine? I missed that important detail.

In my experience the fuel pressure will be rock stable at idle and dip a little under throttle but stay at the slightly lower pressure not shake.

Any flutter in the needle means FPR not regulating correctly.

[racebmwm3]

Originally Posted by JPuma

Do you have a tool to check your air flow. should be around 4.1-4.6g/s around 600 -700rpms.

I do. Keep in mind this is a x5 3.0...

It's around 3.5 @700 RPM

[racebmwm3]

Originally Posted by andrewwynn

E53 uses millisecond timing of injectors to determine fuel metering.

The math only works correctly if the fuel pressure is 50.0 psi from my understanding.

I think all non diesel e53 use that same baseline pressure.

The e53 doesn't meausure fuel pressure it just assumes it's correct.

You said the pressure measured high but was that just at idle? What's the fuel pressure under load?

Low fuel pressure will give the same results as MAF reporting low air or unmetered air leak. It also may affect one bank before the other since the fuel is supplied on one end.

Did you mention which engine? I missed that important detail.

In my experience the fuel pressure will be rock stable at idle and dip a little under throttle but stay at the slightly lower pressure not shake.

Any flutter in the needle means FPR not regulating correctly.

The fuel pressure readings I gave earlier were at idle. Full pressure under load was similar to what you describe.

This is the 3.0 engine.

[andrewwynn]

Theory; MAF reading low:

This should mean that the O₂ sensors will add an equivalent percent of fuel to make up for the error.

STFT ≈20%

3.5 MAF times 1.2*3.4=4.08

Add 7% long term 4.08*1.07=4.366

4.37 is right in line with a baseline error free engine.

Prime suspect: MAF.

You can get a $20 test case off Amazon I would buy a Siemens OE MAF once I was convinced.

(FYI; when my m54 was about 190,000 miles I did just that when my fuel trims were climbing steadily over time)

Not sure if airflow readings are affected but I would def check the condition of the air intake filter.

[racebmwm3]

Originally Posted by andrewwynn

Theory; MAF reading low:

This should mean that the O₂ sensors will add an equivalent percent of fuel to make up for the error.

STFT ≈20%

3.5 MAF times 1.2*3.4=4.08

Add 7% long term 4.08*1.07=4.366

4.37 is right in line with a baseline error free engine.

Prime suspect: MAF.

You can get a $20 test case off Amazon I would buy a Siemens OE MAF once I was convinced.

(FYI; when my m54 was about 190,000 miles I did just that when my fuel trims were climbing steadily over time)

Not sure if airflow readings are affected but I would def check the condition of the air intake filter.

So are you saying the MAF reading should be around 4.37 at idle?

[andrewwynn]

Originally Posted by JPuma

Do you have a tool to check your air flow. should be around 4.1-4.6g/s around 600 -700rpms.

JPuma did the measurement and yep.

4.35 is the center of the range.

[JPuma]

Originally Posted by andrewwynn

JPuma did the measurement and yep.

4.35 is the center of the range.

This was a recent discovery for me. I had a bad maf wire for a long while. tried two MAF's to no avail. my problem was intermittent but didn't notice until I started monitoring airflow. Finally replaced the wire and its been stable since. I may have killed my Cat's because of this.

[andrewwynn]

Cats can recover somewhat if just abused


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[workingonit]

Originally Posted by racebmwm3

this is a x5 3.0...

It's (MAF readings in g/s) around 3.5 @700 RPM....

So are you saying the MAF reading should be around 4.37 at idle?

Originally Posted by andrewwynn

4.35 is the center of the range.

I have a comparison from the day I switched from my stock Siemens MAF, to using the $20 ($23 then) Amazon MAF. First is the Siemens' reading, then the initial reading of the cheap MAF, and a third reading a little later on that day. I didn't zero-out my fuel trims, as I was primarily interested in the MAF readings.


Per JPuma and andrewwynn, the Siemens reading was already pretty good, but the cheap MAF readings were actually a bit high (but that kept the fuel trims from setting lean codes). Since that day, the lowest idle MAF reading I've seen was 4.1 g/s, and the highest was 6.1 g/s. I'm still hunting for a vacuum leak.

[JPuma]

Originally Posted by andrewwynn

Cats can recover somewhat if just abused


Sent from my iPhone using Tapatalk Pro

That sounds promising.