Lean code PO 171

TOMBKLYN11222

MEMBER
1997 DODGE NEON
2.0L
4 CYL
2WD
AUTOMATIC
150,690 MILES

I have a constant check engine code 171 lean fuel system code. Which in turns makes my car use a lot of gas and makes the exhaust smell very rich.
after driving the car with a warmed up engine and in idle at 843 RPM, my scanner shows LT FTRM1 at -5.5 %, the ST FTRM at 0.8% the coolant temperature at 221F, and the 02S1 voltage 0.745, 02S2 at 0.060 volts.
But when I rev the engine up to 2202 RPM my LT FTRM1 goes to 24.2 %, the ST FTRM goes to 12.5%, while the 02S1 voltage is 0.960 and the 02S2 shows 0.000 voltage. I had the car smoked for vacuum leaks and nothing was found, what should I be looking at to solve this issue?
Thank you,
Tom s.

5 Replies

SCGRANTURISMO

MECHANIC
4,897 POSTS

Hello,

You might want to check the exhaust manifold. If it gets loose or you have an leak before your upstream O2 sensor it will cause a lean condition and make the Power-train Control Module(PCM) AKA, "computer" lengthen it's duty cycle to the injectors causing both long term and short term fuel trims to increase.
If you let your engine idle what does the voltage on your upstream O2S1 do? Please get back to me with what you find out.

Thanks,
Alex
2CarPros

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Friday, March 15th, 2019 AT 11:52 PM

TOMBKLYN11222

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3 POSTS

Thanks Alex,

I will look into that exhaust manifold possibility.
With the engine warmed up after driving and in park for about ten minutes, the idle RPM is 843 and my oxygen sensor 1 shows 0.745 volts while the oxygen sensor 2 shows 0.060 volts.

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Sunday, March 17th, 2019 AT 11:35 PM

SCGRANTURISMO

MECHANIC
4,897 POSTS

Hello again,

What I was looking for was to see if your upstream or O2S1 do? It is trying to keep your vehicle's Air:Fuel Ratio(AFR) at stoichmetric or 14.7:1. It does this by constantly running the AFR from rich to lean and back again. So as you are watching the voltage on O2S1 it should be switching very rapidly from somewhere around 0.1V to 0.9V to 0.1V to 0.9V and so on. It constantly goes rich, lean, rich, lean and in the middle is stoichmetric or 14.7:1. Get back to us with what your O2S1 sensor is doing.

Thanks,
Alex
2CarPros

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Tuesday, March 19th, 2019 AT 3:40 AM

TOMBKLYN11222

MEMBER
3 POSTS

Since my reader does not show a graph I only am able to see the voltage reading.
The 02s11at idle on a hot motor is 0.745, if I rev the motor to 2,202 RPM that voltage goes to 0.960, and while driving at 53 mph, 40 mph, 45 mph the voltage on that same sensor stays at the 0.960. At one point while driving at 48 mph the voltage dropped to 0.880 volts. The only issue is that I don't remember if my foot was on the gas pedal or off the pedal when these readings were taken. The 02S12 was respectively at 0.060 at idle, 0.000 at 2202 rpm, 0.040 at 53 mph, 0.000 at 40 mph, 0.000 at 45 mph, 0.080 at 48 mph.

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Wednesday, March 20th, 2019 AT 12:07 AM

SCGRANTURISMO

MECHANIC
4,897 POSTS

Okay,

From what you have told me it sounds to me like your upstream O2 sensor, that's O2S1 is biased rich and has failed. When this happens the O2 sensor might be switching but it will be switching from like 0.6V - 0.9V, instead of 0.1V - 0.9V. Therefore as the PCM is always getting a signal of your engine running rich, it is trying to lean out the AFR and that is what is setting DTC P0171 Feul System to Lean. Replace O2S1 and get back to us with what you find out.

Thanks,
Alex
2CarPros