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Hi aim going pretty much crazy with my 2002 BMW e39 525i with 78k miles i have been getting these codes p0171 , p0174 system too lean bank 1 bank 2.
I have done pretty much everything and spent hundreds on dealers and repair shops any help would be highly appreciated i have done the fallowing repairs >

1.replaced all 4 oxygen sensors Bosch brand.
2.replaced air cleaner box.
3.replaced both air boots.
4.replaced ccv valve.
5.clean maf sensor.
6.used seafoam cleaner
7.car seems to run fine engine idles good not rough i always us highest gas so any ideas on what to do would be highly appriciated thanks

p.s

I already changed both air intake elbows and i don't hear any engine noise and all the air hoses seem to be fine i think because i sprayed the air intake and most hoses with carb cleaner and didn't noticed any difference on the idle any help and suggestions would be thank full
 

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the #1 thing you should have done before resorting to all that you described:
should have been to:

1) check fuel pump to see it gives out correct fuel pressure;
2) if pressure before fuel filter is within specs, check fuel filter(or change)
3) clear the codes and re-test

**in the unusual event that it still comes on, then the fuel pressure regulator should be next in line to check.

LOGIC: when you have system lean, both banks, that indicates "not enough gas". Therefore, check gas supply first. ;)

TIP: when you have codes showing both symptoms for both banks 1 & 2; you should move further upstream and see which engine components are "central"/shared by both banks 1 & 2

;-)
 

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As I said before not a good mac

Rely on mac in the trade thats what i've been told feel like quite a retard
feul filter needs changeing but have not found one for this car yet it has 4 connection and the one that show in the book are only two thanks again for all your help
 

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My 98 540i was throwing the same codes. It was my throttle body gasket. About $10. Spray some throttle body cleaner around the gasket area while the car is idling. If it starts to speed up then you know that's the problem. Mine wanted to die instantly. The icv (ignition control valve, only on 97 & 98's) was completely gummed up. Changed gaskets and cleaned out the icv w/ WD40 and it runs perfect now. I'm going to do this to my 98 740iL next week as preventative maintenance (206K miles).
Good luck, hope this helps!
 

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well thank you all for your help found what was wrong it was blocked vacum pipes and connector that was causeing all the trouble
 

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Well done. The problem is gone now, but what was blocking the line?
 
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