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1999 E46 318i se fault code p0171.

30K views 120 replies 15 participants last post by  VJPo2 
#1 ·
Hi guys,

I thought i would post this to try and help out other people with similar fault.
First off last year I got fault code p0171 with management light on,so i went searching and found egr start fan had exploded,so I changed that,cleared code and it seemed o.k for 2 days but then it came back on:mad
so checked it was the same code,yes it was so went looking again and found the crankcase breather hose had turned into Plasticine,so i changed that cleared codes again spot on the light stayed off,until last week mil back on (same code again) so after more searching and checking of hoses i found the disa valve diaphragm was split (you have to look carefully to see it) through where the bar goes in but it was there,i will try to get some photos on soon.
If this helps just one person its worthwhile.
 
#4 ·
it seemed to be after a run,but not always,it came on most often after acelerating to join the motorway (but not when accelerating)but then yesterday i was just coasting down a hill and on it came.so it got me on one then and i took it to bits.

after the light came on it was like i had four big lads in the back,the had very poor pull.
 
#6 ·
bloody light back on

well guys just an update for you all,after a week of no light,car going very very well,i went out tonight for a drive and.......its back!
i did drive it really hard, then when i chilled out ping on came the light???
so i went home plugged it in and got code p0170 this time,(had to be day before i go back to work) i have checked all the hoses and pipes,new disa,all thats left is maf?? or o2 sensors??
i think its time to send this bloody car to a new owner,bring on my 330 convertable.
when i locate whats causing the light i will post back,or if anyone has any ideas im all ears.

cheer's
John.
 
#11 ·
I have this code come up too on carsoft, 171 fuel supply system - along with 141 lamda regulation... I recently changed my MAF which brought back the power... 1 day after clearing all codes i still get the 141 error but that doesn't turn the light on by itself. Something else is going on that brought up the 171 code previously. Call me a pessimist but I'm expecting the mil to come back on within the week and the 171 code to return... reason being that I get problems with my idle... my next move was going to be the O2 sensors... I will post up if it comes back! If by any luck it stays off then in my case i was a faulty MAF contributing to the 171 code.
 
#12 ·
another bottle of sdp injection cleaner, and another visit to the garage left with garage who had several runs. put once more on the computer and garage found nothing all appears normal,have had car back after one more bottle of injection cleaner gave car goon run 300 miles round trip car did seem to run ok whilst on holiday still in my view lacking slight power but didn't notice any hesitation untill i next trip out when put foot down from cold in first gear the next day back from holiday and hesitated again...
was told by indi if something is wrong fault will develop and should be easier to find the cause as nothing showing and no fault codes......
 
#15 ·
Guys, I've the same code p0171, comes up at random every few weeks after it's been cleared. Been off a while last night turned heating on driving around 20 MPH in 3rd and light pops up.

Any suggestions please how to fix

It has come up 4 times, each time cleared then pops back up after a few weeks. Not sure how to fix....
 
#16 ·
It's not like me to leave a problem unanswered, if memory serves me correct my light stayed off once i replaced the MAF with a brand new genuine bosch replacement. The light has not come on since... I still struggle with plugging all the air leaks and achieving a consistent smooth idle but that hasn't brought the light on since then. Cleaning the MAF didn't work for me and don't be tempted to buy cheaper alternatives than original.... hope this helps
 
#17 ·
Thanks, that doers help. Car used to idle rough, slightly smoother strangely after the code came up and was cleared.

Local garage say's its the Lambda sensor? I am unsure as this code relates closely to the MAF.

Another option would be to wait till the code comes up again, Am I right in saying the fault code can be broken down to pin point fault diagnosis when it lights up?

I wouldn't like to spend money on something when it could be another cause to the problem, I.e MAF or Lambda?

What is your suggestion I do to save incurring further costs and to rectify the route problem.

Cheers
 
#18 ·
Lamda etc were also on my list of possible solutions... What i did before spending the money was disconnect the MAF and drive the car. The difference in power was huge! I felt better about risking the money after doing this, I didn't feel like i was working blind anymore.
 
#20 ·
I bought mine from a guy selling original parts on the net it was about £30/£40 cheaper than BMW, which I think was around £170... but you have to check its been a while now since i bought mine... cheap ones are all over ebay... just do a search and you'll see.
 
#24 ·
Use full info

OBD II Fault Code

* OBD II P0171
* OBD II P0174

Fault Code Definition

* OBD II P0171 Fuel System Too Lean (Bank 1)
* OBD II P0174 Fuel System Too Lean (Bank 2)

Symptoms

* Check Engine Light will illuminate
* In some cases, no adverse conditions may be noticed by the driver
* In other cases, there may be performance problems, such as a lack of power on acceleration and some "coughing" or misfiring
* The vehicle may have trouble idling, especially when warm or when sitting at a stoplight

Common Problems That Trigger the P0171 and P0174 Code

* PCM software needs to be updated
* Vacuum leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses, etc.)
* Mass Airflow Sensor (MAF)
* Plugged Fuel Filter or weak Fuel Pump
* Plugged or dirty Fuel Injectors

Common Misdiagnosis

* Oxygen Sensors

Polluting Gases Expelled

* NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog
* HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog

The Basics
Combustion engines operate by burning an air/fuel mixture of about 14.7 to 1-14.7 parts air to 1 part fuel. When the air ratio goes below 14.7 parts, this is called a "rich" mixture. If the air rises above 14.7 parts, it is called a "lean" mixture.

Rich Mixture = Too much fuel, not enough air
Lean Mixture = Too much air, not enough fuel

To keep the engine running properly, the Engine Control Module measures the oxygen content in the exhaust with oxygen sensors and makes adjustments to the mixture by injecting more or less fuel.

The control module operates within specific parameters and under normal conditions, it will make minor adjustments to the air/fuel mixture. When these adjustments become too large, a fault code is set. When the P0171/P0174 code sets, the oxygen sensors are detecting too little oxygen in the exhaust and the control module is adding more fuel than normal to sustain the proper air/fuel mixture.

P0171/P0174 Diagnostic Theory for Shops and Technicians
When a vehicle has the fault code P0171 and or P0174, it means that computer can no longer automatically adjust the mixture between air and fuel. Code P0171 applies to Bank 1 and P0174 applies to Bank 2. Code P0174 mainly applies to V6 or V8 engines because 4-cylinder engines generally have only one bank, though there are a few exceptions. Some of the high performance 4-cylinder and straight 6-cylinder engines (such as those in BMW and Lexus) split the cylinders into groups of two or three and call them separate banks.

When the code says that the Fuel System is "too lean," it means that the computer has been adding more and more fuel, which is called Long Term Fuel Trim. Ideally, the Long Term Fuel Trim should be close to 1 to 2 percent. When a code P0171 is set, it means that the Fuel Trim is anywhere from 15 percent to as high as 35 percent compensated. When this happens, the computer knows that there is an improper condition in the control of the Fuel System.

The first step in the diagnosis of a code P0171 and/or P0174 is to look at a minimum of three ranges of the Long Term Fuel Trim numbers on a scanner. Check the idle reading-3000 RPM unloaded and 3000 RPM with at least 50 percent load. Then check the freeze frame information for the code to see which range(s) failed and what the operating conditions were.

Before we get into the main causes of P0171, let's explore why this code matters.

Why Does a P0171/P0174 Code and Running "Too Lean" Matter?
"Lean" running cars and light trucks are highly polluting vehicles. Most of the NOx pollution, which is poisonous and can cause asthma, is caused by vehicles that are running too lean. A lean running car can also misfire, which puts raw fuel (HCs) into the atmosphere. When you are behind a car or truck that smells bad and makes your eyes burn, it is misfiring and running too lean. In comparison, a "rich" running engine has no smell (CO is odorless) or you may detect a rotten egg smell, which is caused by the Catalytic Converter consuming too much sulfur, like when you are behind a big SUV at full throttle going up a long, steep hill.

P0171 is not an Oxygen Sensor problem. Before a P0171 code is possible, the computer first ran a series of tests to validate the readings from the oxygen sensors. Since the oxygen sensors passed their readiness tests and didn't set any codes, the computer then looked to the Fuel Trim adjustment. When the computer determined the air-to-fuel mixture to be too lean, it then set the P0171 code.

What Are Some Common Causes of Code P0171/P0174?
Always check to make sure that there is not a PCM software update due or available. Often, as the vehicle's engine wears, the PCM's Fuel Map software inaccurately compensates for this condition. The fuel mixture grows lean and eventually, the code sets.

A vacuum leak is very common. It could be a torn PCV hose, a torn Intake Air Boot, or even a broken seal on the dipstick (the dipstick is a part of the PCV system and if it does not seal, too much unmetered air will enter the engine). Don't rule out a sticking/leaking EGR Valve or leaking EGR or Intake Manifold Gasket. If it is a V6 or V8 engine and the code is only on one side/bank, it could be a defective Intake Manifold Gasket or cracked/leaking manifold.

What if There Is No Vacuum Leak or Both Codes (P0171 and P0174) Set?
An "under reporting" Mass Air Flow Sensor can be a common cause of a code P0171 and P0174. Essentially, this means that the Air Flow Sensor is telling the computer that much less air is entering the engine than actually is.

Since the oxygen sensors are telling the computer that more fuel is needed, this causes confusion in the computer because the Mass Air Flow Sensor is still saying there is too little air and the Oxygen Sensor is reporting that the mixture is still too lean. The computer tried to compensate, but since resolution is impossible, it sets the code. It is important to restate that the Oxygen Sensors are accurate-the fuel mixture is too lean. In this case, the Air Flow Meter or Sensor is inaccurately reporting the real amount of air entering the engine.

How Do I Know if the Problem Is the Mass Air Flow Sensor?
There is a very effective "truth test" for any Mass Air Flow Sensor. Start the engine, let it idle, and then check the Barometric Pressure reading on the scan tool data. If the reading is about 26.5 Hg and you are close to sea level, you know that you have a defective Air Flow Meter because it is telling you that you are at about 4500 feet above sea level. (These conversion tables will help.) When the Mass Air Flow Sensor sees this Barometric reading, it adjusts its Air Density table and then "under reports" the actual amount of air entering the engine. It does this because the Barometric Pressure Sensor is actually part of the Mass Air Flow Sensor.

Sometimes the Air Flow Sensor and the sensing wire get covered with dirt, dust, or oil residue, which can also set a P0171. Cleaning the sensor might hold off problems for a while, but eventually, the MAF sensor should be replaced. Always make sure the Air Filter and its enclosure are dirt-, dust-, and oil-free. If you clean and replace the filter and its enclosure as needed, you will prevent the new MAF from failing.

Additional Causes of Code P0171/P0174

* A plugged Fuel Filter or poorly functioning Fuel Pump can set the P0171 code. The computer hears (accurately) from the Oxygen Sensor that the Fuel Mixture is too lean so the computer keeps increasing the amount of fuel being delivered into the combustion chambers. But in this case, the Fuel System can't increase the amount of fuel.
* If you still can't find the problem, be sure to check and verify that fuel pressure and delivery are at spec. If fuel pressure and volume check out okay, scope the injectors and perform injector drop and/or flow tests to see if they are capable of delivering enough fuel. Dirty/contaminated gas can definitely plug injectors and trigger these lean codes.
 
#31 ·
Mate the best advice i can give you at this stage is: if you have the time but not the money to keep throwing at a mechanic you have to try somethings yourself to set your mind at rest (well thats what works for me). It sounds like you trust your mechanic, thats cool but there are somethings you should be trying first to eliminate areas... Do you have your own diagnostics equipment at home ? Are you able to wipe the codes?

The reason why i asked you to disconnect the MAF and drive your car is that you will know immediately if it feels different. My old MAF didn't come up as faulty but it was and since then that code has not come back nor has the light come on. But when i drove without it i new that's where i had to start. If memory serves me correct once i unplugged it another light came on but that didn't stop me from driving...
 
#32 ·
Thanks dude, I know what your saying. I don't have a machine to check anything. I know its a good car though. This morning it felt like when I bought it at 62k. smooth and fast, after work slight judder and light back on. I'll ask a friend local to me to have a look over on Sat, then local Garage to sort it once pin pointed. MAF's are expensive though i'm aware of.

So far this is what i've had done on my car in the last few months

Jul-12 Front Ball Joint Arm replaced £222
Jul-12 Wheel Alignment £20
Jul-12 Pully & Fan Belt £108
Aug-12 Coil Pack x1 £50
 
#34 ·
If your maf is faulty, when you run the car without it plugged in, the car will run better without power issues because it reverts back to a factory setting of air to fuel ratio.unplug the maf and take the car for a run and see if performance improves. If it improves, replace the maf.Make sure the maf is a genuine Bosch/BMW part because the cheap ones are absolute rubbish and won't solve anything!!
 
#35 ·
Thanks, I shall try to get a BMW friend of a friend to diagnosis on computer. If it can be fixed roadside or at his great, if not local garage. the thing is, what ever the local garage did improved performance as it felt so much better then suddenly judder and engine light. I shall ask the BMW dude about the MAF and to test. At first the local garage thought it would be lambda sensor, running a calibration after the coil pack fitted he found it to be lazy - could the sensor cause this issue in your mind? rather then MAF - if that makes sense?
 
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