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Code P0171

Tiny
JDMARTIN15
  • MEMBER
  • 2008 AUDI A4
  • 2.0L
  • 4 CYL
  • TURBO
  • 4WD
  • AUTOMATIC
  • 125,100 MILES
I have an issue finding the problem with my car listed above. I'm getting a P0171 code. Symptoms include hesitation and loss of power at acceleration from a stop and bad fuel mileage. Following are the repairs that I've tried: changing the fuel filter, PCV valve or (crankcase vent valve), EVAP purge valve, cam follower, new spark plugs, and MAF sensor. I've had this issue for over a year. Audi mechanics are scammers. Any help would be appreciated. Thanks in advance.
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Sunday, January 23rd, 2022 AT 10:45 PM

12 Replies

Tiny
BORIS K
  • MECHANIC
  • 798 POSTS
Hello,

P0171 can be set for many reasons and is hard to diagnose.

You have replaced a lot of obvious parts already, all of which can cause P0171 to set.

The first step would be to smoke test the intake and fuel tank EVAP system for leaks.
Smoke testing is a quick way to check if any of the intake or vacuum pipes have a potential air leak.
Small air leaks in either the intake or EVAP system can cause this code to set
Item like the EVAP lines from the fuel tank, activated charcoal filter or even the fuel filler cap can leak and could cause this code to set

Smoke testers are freely available online or even Amazon.
See picture below

How to check vacuum leaks using smoke and other methods.
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Leaks in the crank case ventilation system can cause this code to set.
You already have replaced the PCV valve. With a smoke tester you could induce smoke into the crank case system and check if the oil filler cap or even the valve cover gasket are possibly at fault.

A good guide on possible issues is on the below link.
https://youtu.be/xeoalEoygGg

How to smoke test.
https://youtu.be/kZsqEfP-5zM

Please let us know how you get along.

Cheers, Boris
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Monday, January 24th, 2022 AT 7:11 AM
Tiny
JDMARTIN15
  • MEMBER
  • 42 POSTS
Thank you, Boris. I will give this a try. I made a DIY smoker, and I don't know if I ran the hose in the proper location and performed the test correctly. Sadly, I threw the DIY smoker away.
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Monday, January 24th, 2022 AT 3:34 PM
Tiny
JDMARTIN15
  • MEMBER
  • 42 POSTS
I did a test with the carburetor cleaner and couldn't muster anything, but when I removed the oil cap while the engine was running. I couldn't pull it completely out without the engine wanting to stall out, but I didn't feel vacuum when I removed the cap. Is that good or bad regarding the intake manifold gasket or the manifold entirely?
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Monday, January 24th, 2022 AT 5:16 PM
Tiny
BORIS K
  • MECHANIC
  • 798 POSTS
Hello,

Removing the oil filler cap while the engine is running will cause this erratic idle behavior.
This is because you are introducing a huge amount of outside air into the crank case ventilation system which in turn will lean out the mixture in the intake system.

The only way to check if there is any crankcase vacuum is to use a vacuum tester and attach it to the crankcase ventilation system. I find the easiest place is at the dipstick tube. You could also attach a smoke tester to the dipstick tube to check for possible crankcase ventilation leaks.
With the vacuum gauge attached we should have a vacuum of around 15-40 mbar at idle.

A handheld vacuum pump is the best way of diagnosing a vacuum system as you can read vacuum and also induce a vacuum to test lines, see picture below.

Another couple of info links
https://youtu.be/67TjF-sXxkA
https://youtu.be/JF3O6ChxBv0
https://youtu.be/hQEciqR1ST8
https://youtu.be/cKbsBnKlZLQ

Please let us know.
Cheers, Boris
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Tuesday, January 25th, 2022 AT 9:17 AM
Tiny
JDMARTIN15
  • MEMBER
  • 42 POSTS
So, I used a smoker a friend made. I didn't see anything through the intake manifold or crankcase vent valve, but I ran the hose through the intake manifold and smoke came out through the side of the airbox (as seen on the video attached). Also found a rogue disconnected hose by the brake booster (see images attached). It was tucked in and hidden probably by the previous owner. Any clue on where it can possibly connect to or what the part is called? Thank you.
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Tuesday, January 25th, 2022 AT 1:43 PM
Tiny
JDMARTIN15
  • MEMBER
  • 42 POSTS
I searched everywhere online and couldn't find anything about what this plastic hose part is called and where it connects. The hose leads to the driver's side door. Some images I found online (see images attached) seem to show that it should connect under to the side of the master cylinder and booster or somewhere around there, but I can't see for sure. Help is appreciated.
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Tuesday, January 25th, 2022 AT 10:18 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Just to jump in on this, if you are seeing smoke coming from under what appears to be exhaust, then that is most likely not the cause of the lean code.

Here is a guide that talks about lean codes in more general info but if you find a vacuum hose then that is most likely the source:

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

What I would suggest is we cap that line if you can run the engine and feel it pulling a vacuum through the hose.

Then you can clear the code and run the engine and make sure that code does not come back.

If we confirm the code is gone then we can use the documents below to try and locate where the line goes.

However, can you trace the other side of it and see if it leads to the intake or a vacuum pump?
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Saturday, January 29th, 2022 AT 9:38 AM
Tiny
JDMARTIN15
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  • 42 POSTS
Thank you for the response, Kenny K. I've done some research and inspection and figured out that the disconnected hose connects to the battery. It is a vent for the battery that releases the battery gases under the fender. I thought I was on to something. Back to the grind. Thank you for clarifying the smoke coming from the exhaust. I was confused. I cannot find a leak using smoke anywhere in the engine. Maybe it's a small leak. I've tried inducing smoke through the PCV valve, intake manifold, dipstick. Are there other areas I should run smoke through to inspect for leaks? Thanks again
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Saturday, January 29th, 2022 AT 11:50 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
There are not many other areas that you would be able to use smoke however, I would not use this method past the intake just because most vacuum leaks happen when the engine gets warm, so we need to use this method to check for it and see if we have anything. Basically, you need to listen for an increase RPM when you spray around the engine.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Thanks
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Saturday, January 29th, 2022 AT 2:04 PM
Tiny
JDMARTIN15
  • MEMBER
  • 42 POSTS
It's been a while since I updated. I found out the issue, which was the diverter valve. I installed a spacer and an updated new diverter. I removed the spacer because the connector kept coming off due to being a tight fit and it solved the issue. The connector on the diverter valve was loose. Thank you for the help, it was a long process for a simple issue.
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Monday, May 30th, 2022 AT 3:58 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
That is great info. Thank you so much for coming back and updating us with what happened and what solved this.

This info is surely going to help others in the future. Thanks again for returning and letting us know what happened.
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Monday, May 30th, 2022 AT 4:48 PM
Tiny
JDMARTIN15
  • MEMBER
  • 42 POSTS
Thank you for the help.
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Monday, May 30th, 2022 AT 4:49 PM

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