LTFT and STFT values are off and don't make sense

Question

Car is a 2015 BMW M5 and it has an aftermarket CAI and a catback (slip on) exhaust. No other performance mods or tunes.

About 6-7 months ago, I bought a Kiwi OBD-II reader, simply out of curiosity as I am a computer guy and a geek and was curious about various ECU numbers and wanted to see how the car was doing. This is when I noticed very strange LTFT and STFT values and have been pretty much going crazy trying to diagnose what might be going on since then.

There really are no fault codes or apparent issues with the car other than a very mild jumpy rpm, "only when during cold start", which I believe is actually normal for the M5 (cannot be sure) where when the car is cold and you start it, RPMs go all the way up to 1800 rpm to warm up the cats and when they drop, there is some up/down movement around 700-900 rpm and it goes on for maybe 1-2 seconds and stops.

It's not a constant rpm surging or car almost stalling. It's more controlled and consistent. I am almost sure that it is standard procedure by ECU because its the same thing day in day out regardless of weather or situation.

Besides this there has been absolutely no other issues with the car. No CEL, no rough idling, no knocking, no misfires, no black smoke from the exhaust.

Let me describe the issues with the fuel trims at hand:

LTFT's

• LTFT's between Bank 1 and Bank 2 are vastly off. When I mean vastly, they can be off by as much as 10%.
• LTFT's are always negative, meaning the car is adjusting for a rich condition? - During cold start, they are about -11% vs -19%.
• Once the car is warm, between idle and 3,500 rpm, they are usually -4% vs -14%, but are never equal, although they do come close to each other a lot such as -7% vs -12%.
• Past 3,500 rpm, either light throttle (so I constantly keep rpms at 4000 rpm, not accelerating) or WOT, both Bank 1 and Bank 2 values reach to -0,5% or maybe +1%, and they become identical and stay consistent all the way to the redline.
• At idle, the LTFT's are a little lower, but still off, around -2.0% vs -8.0%.

STFT's

• STFT's between two banks are almost identical and are almost always positive, meaning ECU is adjusting for a lean condition, which by the way should be normal since I have an aftermarket CAI that is obviously allowing much more air into the engine causing more fuel to be matched (that's my understanding).
• This is where my confusion starts. Not only are the STFT's positive (they are usually around +2.0% to +4.0%), but they are also very close to each other.
• There is "some" variation between the two, but the difference is always around 1% and it can be bank 1 or bank 2 that is off, although bank 1 (which is also the side that is running very negative in LTFT) seems to be richer, but there are many times the same bank runs leaner.
• During idle, STFTs are usually -4.5% (identical both sides, no variation) and the numbers settle at 0.0 to 0.8% with slight variation.
• Under WOT, they go +11-12%, again, identical both banks, and consistent to the redline.
• To me STFT values look normal, they don't vary that much, they are close to 0.0 in low RPMs, and they are usually equal with slight variation.

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Here is what I have done:

• I checked all available values ECU reports, such as lambda values bank 1 and bank 2, O2 sensor voltages (there are so many of them), everything, I mean everything I can see looks almost identical. There is some slight variation, but they are not off 10% like LTFTs are. I cannot detect any pattern whatsoever anywhere else in the car and this is just driving me crazy.

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Based on my limited knowledge this can be one of 3 things:

• MAF sensors are dirty and one side is reading low values causing the rich condition. So I can clean the MAF sensors with CRC cleaner although I was very careful not touching them during my intake installation, I don't see how that is possible. Furthermore, if MAF sensors were dirty, wouldn't this show up in STFT values, not LTFT?

• Injectors are leaking. I have no way of knowing if this is true without some direction.

• O2 sensors are not good. Again, as per ECU data, they are reporting equal values. Now those values might be bad, but they are equal.

I don't know what else this can be. I am just worried that the engine has been running like this for far too long, under way too much load (it's a 600 hp car) and I worry sometime in the future a catastrophic failure may happen, an injector may leak a lot and cause hydrolock or some other issue may happen. Or I am thinking maybe this is just some ECU anomaly. I know my tool works because I plugged it into a whole bunch of cars ranging from 328 to 528 to X5 5.0 and it always reports consistent and more reasonable numbers.

here are some screenshots

Thanks!

Answer 1

I think I have an answer

But bear in mind that this is my assessment with the limited information at my disposal.

I'll walk through my reasoning here. If someone finds a flaw in my logic then I'm all ears.

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Here is a summary of your data

+-------------+-------+-------+-------+-------+
| Condition   | LTFT1 | LTFT2 | STFT1 | STFT2 |
+-------------+-------+-------+-------+-------+
| Cold start  | - 19% | - 11% |  ---  |  ---  |
+-------------+-------+-------+-------+-------+
| Idle        | -  8% | -  2% |    0% |    0% |
+-------------+-------+-------+-------+-------+
| < 3500 RPM  | - 14% | -  4% | ­­≈  0% | ≈  0% |
+-------------+-------+-------+-------+-------+
| > 3500 RPM  | -  1% | +  1% | + 11% | + 12% |
+-------------+-------+-------+-------+-------+

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First, let's clear some misconceptions

1. STFT's between two banks are almost identical and are almost always positive, meaning ECU is adjusting for a lean condition

   Don't take your STFT's to be a measure of system health

   STFT's are not meant to stay positive forever. They are usually designed to compensate for instantaneous changes. At steady load & RPM, the values should go back to zero quite soon.

   Because of this I'm not going to worry too much about the STFT numbers reported.

2. MAF sensors are dirty and one side is reading low values causing the rich condition

   A dirty MAF sensor would under-estimate the air flow

   So the fuel trim would have to be positive to compensate for a lean condition, not a rich one.

3. I worry sometime in the future a catastrophic failure may happen, an injector may leak a lot and cause hydrolock or some other issue may happen

   Running lean is more of a concern than running rich, so impending doom is unlikely.

   Hydrolock is not going to happen due negative fuel trims either.

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Observations

Any proposed theory should be able to explain the following:

1. Negative LTFT's with the engine warm, low RPM

   In other words, if left uncorrected, the engine would be running too rich.

   Common reasons for this include over-reading MAF's, excessive fuel rail pressure and leaky fuel injectors.

   I don't see how a fresh-from-Bavaria M5 would suffer from any of these. Possible, just not probable.

2. LTFT's get less negative with increased load/RPM

   If the LTFT's were positive at low RPM and tapering towards 0 at higher RPM, I'd tell you without skipping a heartbeat that unmetered air is a problem.

   Since the LTFT's are negative at low RPM, you appear to have an issue with air leaking out after being metered by the MAF's.

   Because this is a forced-induction beast, it would be readily explained by a post-turbo, post-MAF leak. However, the relative newness of this M5 makes me wonder if this is even remotely possible. This is also why I asked if anything after the turbos had been touched.

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So what could it be?

Here's my take.

As it turns out, widebands are sensitive to exhaust gas pressure.

I don't mean to bore anyone to tears, but according to this document:

10.6 Pressure dependency of the sensor signal

A pressure change of the measured gas gives a deviation of the sensor output signal of:

Ip(P) = Ip(P0) * P/(k+P) * (k+P0)/P0

So to simulate the two scenarios (for pressures > 1 bar):

• if pressure is lower than expected, the measured current reduces, resulting in a leaner-than-reality reading

• if pressure is higher than expected, the measured current increases, resulting in a richer-than-reality reading

In light of this, I'd say the aftermarket exhaust may be the root cause:

• I know nothing about this exhaust in terms of brand, specifications or cost, but if it has a larger backpressure than the factory setup at idle, it is a plausible explanation for Observation 1.

• As for Observation 2, at higher loads the fuel management is designed to target a richer AFR, so the larger pressure drop may be less of an issue here.

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How to verify this is the root cause

It should be obvious: go back to the factory exhaust and see what happens to the fuel trims! :)

If you see the LTFT's go back to normal at warm idle, the root cause would be confirmed.