P0171 fixed, many parts changed, but 7-8% LTFT remains

Hi all,

I have a 2014 Mazda 3 2.0L with 95000 miles on it.

It triggered a P0171 a couple of months back (20-28% fuel trims at idle, 10%-15% at higher RPMs). It was a bad MAF sensor and changing it fixed the issue. In the course of the diagnosis I also ended up replacing my upstream O2 sensor, PCV valve, intake manifold gaskets, fixing an exhaust leak around the flex pipe, and spark plugs (the O2 sensor was forced on me by the dealer, the others were cheap or good maintenance practices anyway).

Now the P0171 is fixed, but the car shows 7%-8% fuel trims across the RPM range. (LTFT and STFT are 7% and 0% at idle, 10% and -3%-ish at higher RPMs). The trim is being set by the upstream sensor according to the OBD data.

Given that a whole bunch of possible culprits have been fixed, what could possibly be the reasons for this? Since the behavior is fairly stable across RPMs, I feel like I can rule out vac leaks or residual exhaust leaks? I've checked the reported fuel pressure values for high pressure and low pressure fuel pumps and they're within spec.

Does this point to clogged fuel injectors and fuel filter as possible culprits? I'm not sure if this helps but when removing the intake manifold I cleaned my throttle body and intake valves, and they were in surprisingly decent condition.

Thank you!

To be honest ankush.jain, I don't think I'd worry about it! :silly:

7-8% trim correction isn't really a red flag for me. Really, I've found it to be a pretty normal amount of trim for a vehicle with any miles on it. That's exactly why fuel trims are there - to allow the vehicle to correct for variables outside the control of the engineers. Fuel quality, changes in mechanical condition over time, manufacturing tolerances of fuel control components, like that.

That's why we like the +/- 10% rule-of-thumb. Within 10% of zero, we let the engine controller do its job. Beyond 10%, we might start asking questions depending on fault codes and symptoms.

Could the trims be improved further? Sure! But you might be approaching the point of diminishing returns. As in, you could replace the injectors ($$ labor + $$ parts), but only gain a few percentage points in fuel trim. A fuel filter issue seems unlikely, as you've reported that the fuel pressures are within spec.

Tyler wrote:

To be honest ankush.jain, I don't think I'd worry about it! :silly:

.

+1!

Tyler wrote:

To be honest ankush.jain, I don't think I'd worry about it! :silly:

juergen.scholl wrote:

+1!

Thank you again!

I was a bit uncomfortable initially because the numbers would touch 10% occasionally, and I was worried that some residual problems might still be there.

But after driving the car around for a week or so with the new MAF, they have settled further, and are now in a 4-5%ish range, which is something I'm very comfortable with. I apologize for the premature hysteria!

ankush.jain wrote:

I apologize for the premature hysteria!

No apologies needed. Those kinds of questions are why the forum is here.