Air Filter Casuing A Lean Condition…

Keith Owner/Technician Wyoming Posted   Latest   Edited  
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Driveability
2002 Toyota Tacoma 3.4L (N 5VZ-FE) 5-spd (R150F)
P0171 — System Too Lean Bank 1

CEL is on, pulled codes and found p0171, run VE test found low cleaned and it came up, replaced and higher, see attachments, removed aftermarket air filter nd found fuel trims drastically reverse back to normal. i have never seen a filter take an engine lean, what am i missing

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Mark Diagnostician
California
Mark Default
   

When I read the e-mail I thought for sure it would be a Nissan. Is the MAF near the air filter? I think you probably nailed it. Irregular air flow from aftermarket air filter causing the MAF to read low. I do not have a physics degree or understanding of air flow. I would assume the air filter is creating a situation where there is little air flow in the space the MAF is sampling.

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Vincent Diagnostician
Texas
Vincent Default
 

Interesting. was the aftermarket air filter very soft?

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Randy Diagnostician
California
Randy Default
 

Was it a k&n . I have seen this a lot with that filter

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Paul Mechanic
Illinois
Paul Default
 

Turbulence. , The factory plumbing with air filter smooths air and is channeled over the sensors hot wire. Had two domestics that one was missing clamp to keep the two halves together- Windstar. The other was a G series gm that was missing the air filter element, housing was intact. Installed element, fuel trims normal. My opinion anyway given my experiences.

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Albin Diagnostician
Washington
Albin Default
 

If the filter design changes the air flow pattern that goes up to the MAF, it can easily skew the trims. In my lifetime, I have only seen one or two filters do that. If you get in the habit of test driving cars while using a FRTD, you will pick out a lot of issues like this. The issues might not be causing a CEL, or driveability problems, but they are still there. You never know what you will…

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Keith Owner/Technician
Wyoming
Keith Default
 

we found an original toyota intake set up and installed, replaced the ebay cold air intake set up, numbers are normal now, no more cel either. First time we have seen a filter cause a lean, seen a lot of rich conditions

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Geoff Diagnostician
Hawaii
Geoff Default
 

You are not missing anything. You replaced the WRONG PARTS, with the right parts, and then the car was fixed. Amazing how that works. :-)

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Jamey Owner/Technician
California
Jamey Default
 

But... but... what about my chrome pipe!!!

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Geoff Diagnostician
Hawaii
Geoff Default
 

Put the pipe down, sir. Say "no" to drugs. :-)

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Jamey Owner/Technician
California
Jamey Default
 

This is exactly why in California an aftermarket cold air intake needs and Executive Order. Changing the way the air enters the throttle body or from the throttle body to the intake runners will have an affect on trims. Your vehicle had enough turbulence to cause an MIL. Most do not cause a code, but can skew trims. For every action, there is a reaction. For better or worse. Rarely is it the same.

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Randy Diagnostician
California
Randy Default
 

Engineers has calibrated for air intake to be at certenanted amount of air volume if you change that ecm does not what to do except add our extract . Keep stock

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Gord Owner/Technician
Saskatchewan
Gord Default
 

You are right on Randy, we see this a lot with guys installing performance air intakes. 90% of the time it will skew the AFR. We always suggest a recalibration for the MAF transfer function, most guys look at me like I have three heads (LOL) because its a bolt on and no one else suggests it.

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Jamey Owner/Technician
California
Jamey Default
 

Gord, I do not get into a lot of aftermarket performance parts, so forgive my ignorance. How do you recalibrate the MAF curve when another filter/housing is installed? Do you do it with tuning software?

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Gord Owner/Technician
Saskatchewan
Gord Default
 

Jamey, that is correct we are using HP Tuners.

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Scott Owner/Technician
California
Scott Default
   

Hi Keith, Good find, FWIW, the air flow sensor and its environment has a calibration transfer function that the ECM uses to infer cylinder air charge. If that known environment is changed, then all bets are off. A week or so ago we had a vehicle in with a terrible idle and what we found was that the air filter had become distorted and was causing a disruption of the airflow around the sensing…

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Anthony Technical Support Specialist
Pennsylvania
Anthony Default
 

Hi Scott: At least the engine is sealing. :) Guido

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Scott Owner/Technician
California
Scott Default
 

That filter brings into question the order of: suck, squeeze, bang, blow! 🧐

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Richard Technician
Manitoba
Richard Default
 

Like many of the other respondents mentioned I think it comes down to turbulence created by the filter. I seen this happen with OE filters when partially restricted by snow.

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Elias Owner/Technician
California
Elias Default
 

Have seen before way back on a relatively new porsche 996...was working at dealer at the time, they voided his warranty, shut the hatch and said have a nice day! I couldn't believe it.

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