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Steve, BMW Expert
Category: BMW
Satisfied Customers: 22846
Experience:  BMW Expert
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BMW 740i: emission test..P0171..P0174 (System too lean, Bank 1 and 2)

Resolved Question:

I have a BMW 740i E38 2001.
Because I have to renew my registration I had an emission test. That test failed and I got the Codes P0171 and P0174 (System too lean, Bank 1 and 2). I checked already the air mass meter. I sprayed it with pure alcohol to clean it and it seems to be ok.
What options do I have? Can I go to BMW that they check the onboard computer for more details? Can I do the repair (if I can) by myself and let the emission test do at the same gas station - or do I have to do the repair at a regular shop?
Thanks in advance
Submitted: 5 years ago.
Category: BMW
Expert:  Steve replied 5 years ago.

Hello and welcome.


It sounds very much like you have an intake vacuum leak.


Unmetered air leaks allow excess air into the engine and cause the fuel/air mixture to go out of alignment and set the fault codes you have. This is the most common cause of those fault codes.


To check for vacuum leaks, visually inspect the intake area for any cracked hoses/pipes. If you can't see anything, you can try this trick: spray a can of carb cleaner around the intake with the engine idling. If the idle changes, this indicates that the cleaner is entering the engine and you have localized the area of the leak. You can then inspect further.


Most common source of air leaks is from the rubber intake tubes between the airbox, and the throttle.


Another common issue that causes this problem is a failed crankcase vent valve. This is located under the intake, in front of the throttle body. This valve regulates a vacuum on the crankcase, when they fail, a high vacuum is put on the crankcase - giving an unmeterd air leak. You can check for this by removing the oil cap with the engine running, if the engine stalls, or idle dips a lot, and there is a high vacuum holding the oil cap down, then this valve has failed.


If you are able to do the repair on your own you can, but I would recommend leaving this to a professional - dealer or non-dealer.


I hope that is helpful, please let me know if you have further questions, or if I can clarify anything for you.


It has been my pleasure to help today and I wish you the best of luck with the vehicle.




Many thanks.



Customer: replied 5 years ago.
Hi Steve,
I also wanted to know if the BMW dealer can give more detailed information when he connects his computer to my car. In other words: can he diagnose exactly the problem?

I just checked the engine with the oil-ca-test. The engine keeps running well.
Actually it is running fine all the time...
Expert:  Steve replied 5 years ago.

Hello again,


Yes, it is possible that the dealer can get more information using sophisticated BMW specific diagnostic equipment. A dealer diagnosis will be the quickest and easiest way to resolve this.


Please let me know if you have further questions.




Many thanks.



Steve and 3 other BMW Specialists are ready to help you
Customer: replied 5 years ago.
Hi Steve, I tried to check for a vacuum leak with carb cleaner - no success. That means I didn't find a leak.
What about the air mass flow meter? When I pull the connector to it while the engine is running the engine shuts down immediately. Is that the "normal" behavior of the air mass flow meter?
Thanks Frank