Chasing code P0171

[tractoman]

My kids 2008 A4 Quattro keeps setting the CEL about every 500-1000 miles with P0171, lean bank 1.
I have checked PCV, hoses, breather pipe, N80, gas cap seal, fuel filter, dipstick seal and most every other obvious cause for this code. I even drained the intercooler pipes and very little oil came out.
I have looked but not too extensively for vacuum leaks. I can find nothing obvious. I don't have a way to smoke test the intake and hoses so is the LP the best way to try to find a vacuum leak? I am getting irritated and since it only happens randomly it is hard to track down the cause. He can go from LA to San Francisco with no light which is about 400 miles but eventually it will set the CEL. What are the common failure items for this code that I am missing?
He is back in LA now so will be awhile before I can give it another look.
Maybe the MAF?
I am at wits end.

[Theiceman]

Have you actually replaced anything ?
PCV is number one cause of this and is high fail on this platform

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[tractoman]

Have you actually replaced anything ?
PCV is number one cause of this and is high fail on this platform

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New Audi brand pcv and pcv hoses. All of the plastic latching ends of previous pcv hoses were a little suspect. When I replaced them, the hose connecting to the oil filter housing/block side under the intake just kind of pulled out.

[blakefife]

Try replacing the diverter valve or upgrading it to machined piston style its like $130 at ECS tuning. My car throws the same code rarely and it was only after I messed around with the DV valve and added a spacer for blow off, then removed the spacer again because the idle sucked and now I still have high idle issues and lean codes every now and then. smh

[Older_not_Wiser]

The metal breather tube on the turbo side of the head has a check valve in it that is part of the PCV system. On earlier years, that check valve is in the actual PCV. T hat check valve leaking can create that code.

[tractoman]

The metal breather tube on the turbo side of the head has a check valve in it that is part of the PCV system. On earlier years, that check valve is in the actual PCV. T hat check valve leaking can create that code.

Just replaced along with PCV.

[SlvrArrw]

Does the car idle normally?

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[tractoman]

Does the car idle normally?

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Yes, seems to. Cold start idles a little over 1000 rpm and then after a minute slows to about 850 or therabouts.

[Jeevan]

I know this sounds crazy but go back to basics once again and do these simple checks:

while the car is idling, check if you can remove the Oil cap at all. If you cant, you most like have a bad pcv valve or a wrong version of PCV.

you need to do a leak test on all the intake hoses after the MAF, if you dont have an air compressor. Find someone to do it.

After that you need to look at the data of MAF and o2 sensor. Even a cheap scanner can do it. View some youtube videos on how to look at graphs those graphs should be easy to judge on any brand of car.





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[tractoman]

I know this sounds crazy but go back to basics once again and do these simple checks:

while the car is idling, check if you can remove the Oil cap at all. If you cant, you most like have a bad pcv valve or a wrong version of PCV.

you need to do a leak test on all the intake hoses after the MAF, if you dont have an air compressor. Find someone to do it.

After that you need to look at the data of MAF and o2 sensor. Even a cheap scanner can do it. View some youtube videos on how to look at graphs those graphs should be easy to judge on any brand of car.





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I do have a compressor. How do I leak test intake hoses this way? PCV is new and oem.

[Jeevan]

dude, you keep saying it new. There is always a possibility that it could be bad out of the box. It doesnt hurt to verify.



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[tractoman]

dude, you keep saying it new. There is always a possibility that it could be bad out of the box. It doesnt hurt to verify.



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Of course I will do the oil cap test. It seems difficult to remove all the time to me. I know it runs like crap when the cap is off.

[Brillo]

Of course I will do the oil cap test. It seems difficult to remove all the time to me. I know it runs like crap when the cap is off.

Yes, the cap will be somewhat difficult to remove when idling, and run rough when it is removed. This is one indication that the PCV valve is running properly. Since you're a 2008 there will be a check valve at the top of the front right breather tube going down to the pre-turbo port. If you want to check it, make sure absolutely no flow is possible towards the valve cover.

[tractoman]

Yes, the cap will be somewhat difficult to remove when idling, and run rough when it is removed. This is one indication that the PCV valve is running properly. Since you're a 2008 there will be a check valve at the top of the front right breather tube going down to the pre-turbo port. If you want to check it, make sure absolutely no flow is possible towards the valve cover.

The breather pipe is new. I just installed it and check valve is correctly functioning.

[Cyrik]

If you've taken ALL of the necessary steps to confirm it isn't a PCV issue or a small boost leak from your charge pipes, next thing I would do is take off the intake/TB and reseal it with fresh gaskets. I know it's not a common issue on these cars but with it being a ~15 year old plastic intake I wouldn't be surprised if it's leaking slightly.

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[Cyrik]

Try replacing the diverter valve or upgrading it to machined piston style its like $130 at ECS tuning. My car throws the same code rarely and it was only after I messed around with the DV valve and added a spacer for blow off, then removed the spacer again because the idle sucked and now I still have high idle issues and lean codes every now and then. smh

Sorry, but diverter valves don't cause lean codes often, if at all. If anything you'd be running rich.

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[Jeevan]

If you've taken ALL of the necessary steps to confirm it isn't a PCV issue or a small boost leak from your charge pipes, next thing I would do is take off the intake/TB and reseal it with fresh gaskets. I know it's not a common issue on these cars but with it being a ~15 year old plastic intake I wouldn't be surprised if it's leaking slightly.

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thats what I was about to say

[esandes]

My kids 2008 A4 Quattro keeps setting the CEL about every 500-100 miles with P0171, lean bank 1.
I have checked PCV, hoses, breather pipe, N80, gas cap seal, fuel filter, dipstick seal and most every other obvious cause for this code. I even drained the intercooler pipes and very little oil came out.
I have looked but not too extensively for vacuum leaks. I can find nothing obvious. I don't have a way to smoke test the intake and hoses so is the LP the best way to try to find a vacuum leak? I am getting irritated and since it only happens randomly it is hard to track down the cause. He can go from LA to San Francisco with no light which is about 400 miles but eventually it will set the CEL. What are the common failure items for this code that I am missing?
He is back in LA now so will be awhile before I can give it another look.
Maybe the MAF?
I am at wits end.

Yes, clean out the MAF meter. Start with the easy stuff.

[EvolutionArmory]

The metal breather tube on the turbo side of the head has a check valve in it that is part of the PCV system. On earlier years, that check valve is in the actual PCV. T hat check valve leaking can create that code.

This is backwards. Early cars have a blank breather tube. Later cars have the check valve in the tube.

[EvolutionArmory]

If you have VCDS you can find vacuum leaks pretty easy.

Go to block 032 and look at fuel trim. Write down the values. Clear your codes. This resets fuel trim also. Start spraying carb cleaner around on hoses and parts of the intake with the car at idle. If fuel trim spikes when you spray a part, you’ll know the vacuum leak is in that area. Fix the leak, clear codes again and drive it for like 10-20 miles up and down the highway and check fuel trim again. If your trim is lower than what you wrote down before you cleared your codes and drove, you fixed a leak. If it’s about the same, now you check MAF.

What is your MAF reading at idle? It should be about 2.5 GS at warm idle. If it’s higher or lower, clean it with carb cleaner, clear codes and do the same test drive. If your fuel trim is better, your MAF was dirty. If it stays the same, maybe replace the MAF.

Another cause could be a front O2 sensor but those are harder to diagnose unless you have a O2 sensor fault.

Another cause which is also overlooked a lot is the rear main seal. It can cause a lean code because believe it or not, it can allow unmetered air into the engine if it’s leaking.

Also, the oil dipstick tube seals can also cause leak codes and is often overlooked.

[EvolutionArmory]

I’m not saying you did this but a lot of people put the wrong PCV valve in their cars. Did you buy yours from the dealer?

A 2008 should have the part number ending with a P and have a breather tube with a valve in it.

2005.5 and early 2006 has the R part number and a blank breather tube.

Confirm what part number you PCV is.

[tractoman]

I’m not saying you did this but a lot of people put the wrong PCV valve in their cars. Did you buy yours from the dealer?

A 2008 should have the part number ending with a P and have a breather tube with a valve in it.

2005.5 and early 2006 has the R part number and a blank breather tube.

Confirm what part number you PCV is.

It is the breather tube with check valve and pcv ends in p. Both are oem with 4 rings on them.
Car is away at school with my kid but will pick up on this when he is home again.
Thanks for the tips so far.

[EvolutionArmory]

Then you absolutely have the right combo.


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[Older_not_Wiser]

This is backwards. Early cars have a blank breather tube. Later cars have the check valve in the tube.

Pretty much what I said... 'On earlier years, that check valve is in the actual PCV.' Meaning it is not in the breather tube.

[EvolutionArmory]

Pretty much what I said... 'On earlier years, that check valve is in the actual PCV.' Meaning it is not in the breather tube.

Sorry dude. You’re absolutely right. I mixed up your post in my head real bad now that I went back and read it again.


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[esandes]

Could be an o2 sensor going bad too.