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P0171 bank 1 too lean

Tiny
AMMARSALMAN94
  • MEMBER
  • 2006 MERCURY MILAN
  • 2.3L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 185,000 MILES
So the car was running great no issues, I went away for 2 months where it sat and wasn't driven.

Then I come back, I drive 30 miles on it and no issues. The next day I drive 100 miles on it and then stop to refuel. I shut the car off, refuel, turn back on and the code is there: P0171 System running too lean - Bank 1.

I cleared the code and drove another 100 miles on it. The first 10 miles where very very rough and the car was shaking excessively I had to pull off on the highway. Then it went away.

I finished the 100 miles and reached home, so I turn it off, get some rest, turn back on only to find the code is back.

I used my scan tool to check test results for evap monitors and everything was within the reasonable limits. I suspected a vacuum leak so I had a smoke test performed but no leaks where found.

I suspected the fuel injectors so I put cleaner in the fuel tank right when it had 1-2 gallons left and refueled.

Then I suspected the Mass Air Flow sensor, so I took it off, cleaned it with isopropyl alcohol and put it back. The code went away on its own and I was very happy for it.


An hour later, I took the car for an 8 mile drive and nothing showed up. So I come back a few minutes later, turn it on and go on the road, and as I was waiting on a red light the CEL goes on and the code is back.

I took it to a car shop and they said the fuel pump seems to be working well, they ruled out O2 sensors as well.

It's worth mentioning that the smoke test was performed when the car was warm as there wasn't enough time to wait for it to cool.

However, if it is vacuum leak then why did it only come back when the car was still warm? Just like when the smoke test was performed?

If it isn't a vacuum leak then what could it be? Would a dirty throttle cause this to come back? I can use seafoam spray and clean the intake manifold.


The one thing I haven't fully tested is the fuel pressure. Other than that, what could be causing the issue?
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Tuesday, September 24th, 2019 AT 1:16 PM

9 Replies

Tiny
BMDOUBLE
  • MECHANIC
  • 1,139 POSTS
Fuel pressure should be checked (48-70psi), but a few simple things to check before that is the intake gaskets, smoke tests will not always catch these! Also what was the freeze frame data for the ect/cht temperature when the code was set? If it reads under 120 or so degrees, that also tells me it could be the intake gaskets, even though the light came on when it was warm. You can monitor the O2s11, 21 sensor pids while spraying the intake with brake clean (away from the alternator). The voltage will spike upwards when you hit the leak source. Do this when the engine is cold though.
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Tuesday, September 24th, 2019 AT 2:15 PM
Tiny
AMMARSALMAN94
  • MEMBER
  • 8 POSTS
Thank you very the very fast response. I will do so and will get back to you with the results.
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Tuesday, September 24th, 2019 AT 2:49 PM
Tiny
BMDOUBLE
  • MECHANIC
  • 1,139 POSTS
You are welcome!
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Tuesday, September 24th, 2019 AT 3:15 PM
Tiny
AMMARSALMAN94
  • MEMBER
  • 8 POSTS
Hi again, I got the Freeze Frame data after the code showed up the first time and the last time. I can't make much sense of some of it, however, ect was +185 in both instances.

Here is the first one:
Freeze frame report generated by Torque for Android
=========================================================
Fuel Status = 0 byte
Engine Load = 44.314 %
Engine Coolant Temperature = 185 °F
Fuel Trim Bank 1 Short Term = -2.344 %
Fuel Trim Bank 1 Long Term = 27.344 %
Intake Manifold Pressure = 5.802 psi
Engine RPM = 679.75 rpm
Speed (OBD) = 0 mph
Timing Advance = 4.5 °
Intake Air Temperature = 80.6 °F
Mass Air Flow Rate = 2.91 g/s
Throttle Position(Manifold) = 13.725 %
Air Status = 0 byte
Fuel trim bank 1 sensor 1 = -2.344 %
Run time since engine start = 487 s
EGR Commanded = 0.784 %
Fuel Level (From Engine ECU) = 36.078 %
Distance travelled since codes cleared = 130.488 miles
Evap System Vapour Pressure = -34.5 Pa
Barometric pressure (from vehicle) = 14.214 psi
Catalyst Temperature (Bank 1, Sensor 1) = 926.78 °F
PID: 41 = 0
Voltage (Control Module) = 13.991 V
Engine Load(Absolute) = 21.176 %
Commanded Equivalence Ratio(lambda) = 1
Relative Throttle Position = 1.961 %
Ambient air temp = 68 °F
Absolute Throttle Position B = 14.902 %
Accelerator PedalPosition D = 18.824 %
Accelerator PedalPosition E = 29.412 %
Accelerator PedalPosition F = 18.824 %

End of report.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
=============================================================
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

And the last one:

Freeze frame report generated by Torque for Android
=========================================================
Fuel Status = 0 byte
Engine Load = 38.824 %
Engine Coolant Temperature = 194 °F
Fuel Trim Bank 1 Short Term = 5.469 %
Fuel Trim Bank 1 Long Term = 27.344 %
Intake Manifold Pressure = 5.366 psi
Engine RPM = 687.75 rpm
Speed (OBD) = 0 mph
Timing Advance = 2.5 °
Intake Air Temperature = 86 °F
Mass Air Flow Rate = 2.72 g/s
Throttle Position(Manifold) = 13.725 %
Air Status = 0 byte
Fuel trim bank 1 sensor 1 = 5.469 %
Run time since engine start = 227 s
EGR Commanded = 0.784 %
Fuel Level (From Engine ECU) = 84.314 %
Distance travelled since codes cleared = 29.826 miles
Evap System Vapour Pressure = -17.5 Pa
Barometric pressure (from vehicle) = 14.214 psi
Catalyst Temperature (Bank 1, Sensor 1) = 969.8 °F
PID: 41 = 0
Voltage (Control Module) = 13.508 V
Engine Load(Absolute) = 18.824 %
Commanded Equivalence Ratio(lambda) = 1
Relative Throttle Position = 1.569 %
Ambient air temp = 57.2 °F
Absolute Throttle Position B = 14.902 %
Accelerator PedalPosition D = 18.824 %
Accelerator PedalPosition E = 29.412 %
Accelerator PedalPosition F = 18.824 %
End of report.
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Tuesday, September 24th, 2019 AT 5:05 PM
Tiny
AMMARSALMAN94
  • MEMBER
  • 8 POSTS
After looking further, the Long Term Fuel Trim Bank 1 seems way too high, so the engine is really trying to compensate. While in both cases the Intake Manifold Pressure seems in the good range, no? I will keep an eye for it when the car is cold, if there is an issue then it really points to the gasket.
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Tuesday, September 24th, 2019 AT 5:12 PM
Tiny
AMMARSALMAN94
  • MEMBER
  • 8 POSTS
Just an update. I checked the intake manifold pressure while cold and it showed around 5 PSI, I smell some fuel sometimes, so I guess it's mostly related to fuel lines/tank.
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Thursday, September 26th, 2019 AT 9:40 AM
Tiny
BMDOUBLE
  • MECHANIC
  • 1,139 POSTS
Thank you thank you thank you for the awesome info! I can't say this enough! This is what I look at every day so it has become like second nature to diagnose a vehicle with this data! Your throttle position tells me that you were at a standstill or parked and the engine time running is not long but is close to a cool start which leads me to believe that the intake gaskets are the culprit. The ambient temp also leads me in that direction because every year during this time I do tons of intake gaskets because the rubber in them turns hard as a rock and doesn't provide the sealing action. The freeze frame data can be misleading in that is is not a true "freezeframe" of the exact time that things happened to set the code, instead it is the time that the parameters did not correct to the in spec numbers that the PCM is looking for, in other words when things go lean the PCM will try to run the fuel the opposite way for a certain amount of time, after this amount of calibrated time if things do not change then it will set the code, which could be several minutes after the lean condition occurred and the engine is now warm and the gaskets are all warm and supple. The run time also is an indicator.
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Thursday, September 26th, 2019 AT 10:57 AM
Tiny
AMMARSALMAN94
  • MEMBER
  • 8 POSTS
Thank you again! I will have checked as soon as I get the car back from the shop (unfortunately the radiator and transmission cooler lines need change).
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Thursday, September 26th, 2019 AT 1:15 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,065 POSTS
If I can jump in here. I noticed this:

Engine Load = 44.314 %

At idle, and that you cleaned the MAF? I would replace the MAF with a new unit because cleaning doe not always work and sometimes it can make it worse. Here is a guide to walk you through the repair and the location for your car below:

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.
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Sunday, September 29th, 2019 AT 12:28 PM

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