Code P0171 running lean

DANIEL HOWARD

MEMBER
2003 CHEVROLET MALIBU
3.1L
6 CYL
2WD
AUTOMATIC
139,000 MILES

I got a P0171 running lean code a couple weeks ago. I upgraded my obd2 scanner to one that can read fuel trims and it consistently reads around 14% long term through all throttle positions. Short term stays in the low single digits. I tested the fuel pressure today and when I turned the key to the “on” position with the engine still off it read 14 psi. With the engine on it read 36 psi and didn’t change at all when adding throttle. When I unplugged the vacuum to the fuel pressure regulator the pressure jumped to 44psi. The pressure remained consistent after the engine was turned off as well.

Is it normal for the primed fuel pressure to be low(14) but then shoot up when the engine starts(36)?Any suggestions as to why it is running lean?

3 Replies

ASEMASTER6371

MECHANIC
52,797 POSTS

Good afternoon,

No, it is not. You have very low pressure and that is why you have the code. There is not enough fuel at the rail and the ECM is leaving the injectors on longer trying to give more fuel.

https://www.2carpros.com/articles/how-to-replace-front-wheel-bearings-and-seals

I would replace the fuel pump and the regulator as well.

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

Roy

Pressurized Pressure
Engine On .................... 59 psi

Stabilized Pressure
Engine Off .................... 52 psi

Average PSI beteen Injectors
Pressure Drop .................... 1.5 psi

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Sunday, June 28th, 2020 AT 4:34 PM

DANIEL HOWARD

MEMBER
2 POSTS

Thank you so much. I just ordered the repair manual. I replaced the fpr about 5 months ago because the car would occasionally have difficulty starting and I smelled gas in the fpr vacuum line. I felt like it started better after the part change but I don’t have before and after fuel trim or pressure data. Is there a chance that this part is defective? Should I just replace the pump?

I test drove today with my scanner on and noticed long term fuel trim was 9% at idle, jumped to 17% when I floored it to get on the parkway, and was around 14% normal driving.

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Monday, June 29th, 2020 AT 11:30 AM

ASEMASTER6371

MECHANIC
52,797 POSTS

Those readings are from low fuel pressure. There is not enough fuel in the rail. The ECM is trying to compensate by keeping the injectors on longer but it cannot get the fuel mapping correct.

When you replace the pump, re check the pressure to verify the pressure is in specs.

You are good with the regulator. Replace just the pump.

Roy

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Monday, June 29th, 2020 AT 11:34 AM