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Category: BMW
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Experience:  ASE certified tech ADVANCED LEVEL SPECIALIST
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2004 325i: i have a p0174 and p0171 diagnostic code come..lean

Resolved Question:

i have a p0174 and p0171 diagnostic code come up on my 2004 325i lean air

what should i do
Submitted: 6 years ago.
Category: BMW
Expert:  AMEDEE replied 6 years ago.
Hello and Welcome to Just Answer. My name is XXXXX XXXXX I will try my best to assist you and your 100% satisfaction is my goal. Please realize that all my answers are based solely on the info that you provide me with....
I don't know if you are a tech or a just working on your own vehicle so we may have to fine tune the answer. I only know as much about your problem as you have told me in your post, so my answers will be based on what you tell me so please add all the details you can.

You could have a bad mass air flow sensor. Or the snorkel from the mass air flow sensor to the throttle body could be ripped. Both are very common and can cause you vehicle to loose power along with many other drive ability issues. First look for cracks or rips or anywhere air can enter the engine that is not accounted for by the mass air flow sensor. Next comes the Mass Air Flow (MAF) sensor. This is a very important sensor input to the computer. It uses a hot wire sensing element to measure the amount of air entering the engine. The MAF sensor then outputs an analog voltage signal to the PCM proportional to the intake air mass. The PCM calculates the required fuel injector pulse width in order to provide the desired air/fuel ratio. If the sensor is bad than this air/fuel ratio will be off causing severe performance issues. In extreme cases, I have seen these two problems cause a no start condition. This input can also be used in determining transmission Electronic Pressure Control, shift and torque converter clutch scheduling. The check engine light may even pop on if the sensor is bad, the snorkel is ripped or a tube has fallen off. This will result in a lean condition.

Also, You might have an air or a vacuum leak. This is very common! Open the hood and listen for a hissing sound when the engine is running.
If it is hard to pinpoint take some brake cleaner or starting fluid around the intake manifold and vacuum lines and see if the engine stumbles or if the idle is affected. Be extremely careful when doing this!
Expert:  AMEDEE replied 6 years ago.
Pay close attention to the Air Intake Bellows, PCV hose and even the fuel pressure. IF the fuel pressure is low, it will also cause this code.

You can check the Positive Crankcase Ventilation (PCV) system by removing the dip stick. It should have a slight vacuum of about a 1/4 inch of vacuum or 8 to 15 millibar at idle speed.

The operating fuel pressure. It should be about 50 PSI at idle speed.
Customer: replied 6 years ago.

hey thanks

i do have some questions the snorkel may be ripped as you said, how much does a new one cost and how long does it take to repair. what if after repalcing the air snorkel the codes still appear? i assume check out the MAF sensor. what is entailed in checking to see if it is working or not working? if not working how much does a new one cost and how much time does it take to replace? thanks

Expert:  AMEDEE replied 6 years ago.
The air snorkel should not be too expensive. You will have to call a local BMW dealership to get a price on it. IF this is the problem, it does not take too long to replace. You might even be able to do it yourself.

If this is the problem, the light will eventually go off after this is replaced. Or the codes can be cleared with a scanner.

A tech can look at the live data with a scanner and see the grams per second to see if the mass air flow sensor is reading correctly.

The mass air flow sensor is a bit more expensive than the boot. HOwever, again, you will just have to call a parts house to get a price.
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