Long Term Fuel Trim is not correct since SRI install

JJMICROPRO

MEMBER
2016 MAZDA 6
2.5L
4 CYL
2WD
AUTOMATIC
95,000 MILES

I installed a Short ram air intake recently, and it kept causing a CEL, the P0101 MAF code, and weird performance stuff. I made some adjustments and cleared the code; it came back a few times after 2 cold starts and around 20 miles each time. I made some more adjustments and then cleared it again and it hasn't come back in like 50 miles or so and 3 cold starts. It feels mostly normal, but the long-term fuel trim is weird. At idle it is around -11% and then it somehow goes to like -14% or -19% under some load. It eventually got up to around 3 or 5% under WOT, bouncing off the redline when I was trying to figure out what happened. Short fuel trim is pretty normal, around 3% idle and up to like 30% when maxed out. Is this something to be concerned about? I don't know why a code isn't popping up or even pending (I checked a few times) because I thought the threshold was like 8% for 2 cold starts or so. Thanks!

7 Replies

JACOBANDNICKOLAS

MECHANIC
109,645 POSTS

Hi,

During a cold start, the engine is run in what is called an open loop. Basically, in this state, the fuel mixture is primarily controlled by the map/ MAF. Once the engine reaches a specific temp, the oxygen sensors take over and the vehicle is then run in a closed loop. Basically, the PCM uses info from the o2 sensors to determine air/fuel mixture.

In a perfect world, fuel trims would be 0%. Most vehicles usually stay between +10% and -10%. What you indicated is the system indicated there is too much fuel and the computer is trying to adjust for it.

Now, I need to better understand exactly what you installed. If it is a replacement for the factory air filter/air inlet tube, confirm there is nothing restricting airflow.

Also, I attached the diagnostics for the P0101 below. Take a look through them and let me know if you are comfortable performing them.

Let me know.

Joe

See pics below.

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Tuesday, January 18th, 2022 AT 7:31 PM

JJMICROPRO

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4 POSTS

Hi Joe, thanks so much for the quick response! I installed an aftermarket air intake which connects from the throttle body through a tube to the MAF to a cone air filter. Replaces the stock airbox, filter, resonator, air inlet tube, everything. It has an adapter for the MAF and a plug in for the engine breather hose. Theoretically the air flow should be higher, and usually (based on my reading of a bunch of random forums) issues with these aftermarket intakes usually have to do with lean condition and vacuum leaks, not rich A/F and low LTFT, that's why I'm confused. Is it normal also for long trim to get farther from zero under a small load compared to idle? And then close to zero at max load? Could it be a dirty MAF even though that usually causes lean? Those diagnostics don't look too bad, I'm pretty sure I could do that. Although for some reason the code is gone and hasn't come back, not even pending.

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Tuesday, January 18th, 2022 AT 7:53 PM

JACOBANDNICKOLAS

MECHANIC
109,645 POSTS

Hi,

I agree with you as far as trims. Also, anytime you load the engine, the computer will compensate with a richer mixture. I don't think it should be instant for the LTFT.

If there is any concern with the MAF, it wouldn't hurt to clean it. Here is a link showing how it's done:

https://www.2carpros.com/articles/mass-air-flow-service

Let me know if it helps.

Joe

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Tuesday, January 18th, 2022 AT 8:00 PM

JJMICROPRO

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4 POSTS

Great, thanks for the info! I'll clean the MAF and see if that works. Thanks!

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Tuesday, January 18th, 2022 AT 8:02 PM

JACOBANDNICKOLAS

MECHANIC
109,645 POSTS

Hi,

You are very welcome. Let me know if it helps. I'm interested in knowing.

Take care,

Joe

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Wednesday, January 19th, 2022 AT 1:11 PM

JJMICROPRO

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Hi Joe, I cleaned the MAF and reset the ECU again and it hasn't shown any difference in fuel trim or air flow. At idle, the MAF reading fluctuates a lot between 1.9 and 2.3 g/s, which I've heard is low for a 2.5L engine. LTFT is still around -8% at idle, goes to -14% or so when I hit the gas, and sometimes gets to around -5% when cruising. STFT is around 3% at idle, and then spikes up to 30% when accelerating when the LTFT goes down to -14%. When this spike happens, I can definitely hear the fuel injection gurgle and the STFT graph oscillates like crazy. Also, I have been noticing sudden power losses when accelerating out of turns. All this stuff leads me to believe there could be some kind of air flow restriction in the way that the intake is installed. Because the hesitation mostly happens out of turns. And now today after a long time the P0101 is back, I saw it pending a few times but it worked itself out until today it came back and lit up on the dash. I don't really know why it's doing all this because it all works just fine with the stock intake setup, no weird fuel trims. The car is driveable just fine except for the hesitation out of turns which is for a solid half a second or so where the RPMs drop like 1k and then go back up when I come out of the turn and it gets some more air. Any ideas?

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Sunday, January 23rd, 2022 AT 5:22 PM

JACOBANDNICKOLAS

MECHANIC
109,645 POSTS

I attached the diagnostics for the MAF. Take a look at them. Since the 101 is back, something is happening. That code could result from a number of things. What I suggest is to perform the test. If it passes, I suspect the ultimate cause is highlighted in pic 3 or there is an air leak after the MAF.

Let me know.

Joe

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