Running too lean, both banks. P0171 & P0174

[740e38]

Hey guys, here's what's going on. On my way to do my emission test, the CEL come's on. Just my luck.. I pull over and read the codes, I got P0174 and P0171. I looked under the hood and saw that my air box had a clip come off, so I put it back on and reset the light. Here's where things got worse.

The car ran 100% fine before, idle was very smooth and low. When I reset the light, the car idle'd SO rough for the first two or three starts. Then it get's better, but the CEL will come on after about 20-30km..

Doing some research, I understand that this is probably a vacuum leak, so more air is getting in than it should. I looked around and all hoses and clamps are good, and my MAF is new so no issue there..

Is there anything that explains the really rough idle's after clearing the codes? I just find the all so strange.. Needless to say I didn't go to my emission test. :(

Any help?

[Mayorchuck]

Is it a new Bosch MAF?

[740e38]

Yea it is, OEM. It was put in over a month ago, this issue just appeared yesterday.

[dseven]

I had these codes and post cat oxygen sensor,changed sensor and cleaned maf,and they gone,vacuum leak also could cause that,spray some brake cleaner around,there is a video posted somewhere on how to do it and what to look for

[maddoggyusa]

well the issue couldn't have been your air box... because that is upstream of the MAF. so it was something different that all of a sudden reared it's ugly head. resetting the check engine light resets your fuel trim which must cause it to run rougher. post cat o2 sensors shouldn't cause this? I thougth they were mostly for checking cat efficiency. the pre-cat o2 sensors are what the engine uses to control the engine (i.e. knows when you're running lean/rich). o2 sensors will eventually go bad and read lean.... rec'd to replace every 100k or so.I had similar codes... new pre-cat o2 sensors did not help... after replacing all intake gaskets codes went away. but o2 sensors is way easier and seemed worth a shot!if you want pre cat o2 sensors... these were about the cheapest OEM ones I could find...http://www.amazon.com/gp/product/B000GZEK00/ref=oh_details_o06_s00_i00?ie=UTF8&psc=1

[Actros617]

Visual inspection cant really tell whether you have air leaks, especially around the intake manifold gaskets and the throttle body which are prone to leaks, as stated above you have to use quick evaporating aerosol ie brake cleaner, with the car running spray around the intake intake manifold and throttle body if idle increases or smooths you have an air leak, check the plastic intake boot for crack and hole and check the OSV located in the rear theirs a rubber cap that is plugging a accessory hose check that for cracks.

[740e38]

well the issue couldn't have been your air box... because that is upstream of the MAF. so it was something different that all of a sudden reared it's ugly head. resetting the check engine light resets your fuel trim which must cause it to run rougher. post cat o2 sensors shouldn't cause this? I thougth they were mostly for checking cat efficiency. the pre-cat o2 sensors are what the engine uses to control the engine (i.e. knows when you're running lean/rich). o2 sensors will eventually go bad and read lean.... rec'd to replace every 100k or so.I had similar codes... new pre-cat o2 sensors did not help... after replacing all intake gaskets codes went away. but o2 sensors is way easier and seemed worth a shot!if you want pre cat o2 sensors... these were about the cheapest OEM ones I could find...http://www.amazon.com/gp/product/B000GZEK00/ref=oh_details_o06_s00_i00?ie=UTF8&psc=1

Thanks for the product find. My car has 300k and I doubt those things have ever been changed, so I will get to that. I hope it helps, at least a bit. It makes sense that it resets my fuel trim, that does explain a rough idle. It will be a good start to check those.

Edit: Thinking about it a bit more.. the check engine light came on for both bank 1 and 2 at the same time. Isn't it unlikely that BOTH the 02 sensors would fail at the same time?

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Visual inspection cant really tell whether you have air leaks, especially around the intake manifold gaskets and the throttle body which are prone to leaks, as stated above you have to use quick evaporating aerosol ie brake cleaner, with the car running spray around the intake intake manifold and throttle body if idle increases or smooths you have an air leak, check the plastic intake boot for crack and hole and check the OSV located in the rear theirs a rubber cap that is plugging a accessory hose check that for cracks.


Do you by any chance have a picture with what exactly I should spray to check for leaks? I am not sure what is and isn't safe to spray with brake clean. I haven't done this before... :D Thanks.

[Actros617]

Any brake cleaner will do,be sure you spray where the intake manifold contacts the intake cylinder head's.

here someone using the same method http://www.youtube.com/watch?v=s1fTSP8VHvA

[RareAngelPrince]

Is it a new Bosch MAF?

I have a bosh Maf and my car has the issue and today I doesn’t want to start

[acspecialists]

On the MAF, it must come from a BMW box and cost $400. I almost swapped tranny's because of aftermarket MAF. I had lean codes as well. Next is a hose clamp inspection, pull on the intake piping and put a nut driver on all of the clamps. It sounds like your intake boot is cracked. If you just put in a MAF some has been messing around in that area? Lastly so a smoke test, there are some great you tube videos for a home made tester.

I have been through this and speak from experience. If you really want to get into it fire up INPA and read Timm's diagnostic page.