BimmerFest BMW Forum banner
1 - 20 of 37 Posts

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #1 · (Edited)
P0174 = "System Too Lean (Bank 2)" on a 2002 525i at high altitude (3,500 to 4,500')

2002 BMW 525i P0174 OBD-II trouble code compendium:
P0174 = "System Too Lean (Bank 2)".

Note: Some references indicate "too rich" instead of too lean (this is confusing).

Yesterday, for the second time at high altitude (3,500 to 4,500 feet at Lake Tahoe) I encountered a service-engine-soon (SES) solid yellow warning accompanied by slight engine stumbling which disappeared when I pulled over and removed the key from the ignition and restarted the car. While the stumbling went away when I restarted the car (probably due to fuel cutoff), the SES light remained. Back at sea level, I borrowed Actron OBD II scanner revealed only one stored diagnostic trouble code (DTC), P0174 (this is the second time this happened but I didn't get the codes last time before the SES cleared itself).

Apparently that DTC (and a similar P0171 which I didn't get) means the oxygen sensor detected "too much oxygen in the exhaust" of that bank. This doesn't seem to happen below 2,500 feet where I often travel from San Jose at about sea level over the 2,500' Santa Cruz mountains to the beach back at sea level.

Based on research on the net:
- The MAF sensor may be dirty (or the silicone "potting" material may have leaked)
- A vacuum leak may be downstream of the MAF sensor (e.g., cracks in the MAF housing elbows)
- The "intake boot" (whatever that is) to the ICV may be bad
- There may be a MAF sensor electrical connector problem
- The fuel filter may be dirty (or low fuel pressure at the fuel pressure test port)
- Bad crank case ventilation system oil separator (ccv)
- Bad Intake Manifold Actuator (whatever that is)
- Bad DISA (differential air intake control) whatever that is
- Loose oil filler cap
- Blocked fuel injectors
- (anything else)???

Possible solutions:
- Clean the MAF sensor
- Replaced any cracked MAF housing components
- Replace the "intake boot"
- Test ccv by plastic bagging oil-filler hole
- Replace the "ccv"
- Replace the fuel filter
- Replace the fuel pump
- Reseat the oil filler cap

I'll also need to better understand how to test the fuel pressure test port.

References:
- E39 SES
- E39 Service Engine Soon OBDII P0174
- BMW OBDII Trouble Codes
- E46 P0174, P0171 Codes
- E46
New Disa - Car Died Again
- E46 Gas Flow Issue
- E46 CCV DIY
- E46 SES light
- E39 stumped P0174
- Battling vacuum leaks
- Cleaning the MAF sensor
- Mixture too lean, P0171, P0174, P0313

 

Attachments

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #2 · (Edited)
Related questions ....

Some related questions:

Q: What is "bank 2" in an I6?
Q: What/where is the silicone "potting" material?
Q: What/where specifically is the "intake boot"?
Q: Where is the "fuel pressure test port"?
Q: Does the E39 have a "DISA"?
Q: What is a "lambda control"?
 

·
Registered
Joined
·
6,925 Posts
Some related questions:

Q: What is "bank 2" in an I6?
Q: What/where is the silicone "potting" material?
Q: What/where specifically is the "intake boot"?
Q: Where is the "fuel pressure test port"?
Q: Does the E39 have a "DISA"?
Q: What is a "lambda control"?
Here are some answers:
A1: Cylinders 4-6
A3: Find the air filter box, the MAF is next in line. The rubber boot with raises box pattern after the MAF and before the intake manifod is the intake boot.
A5: DISA is the differentiated intake system and changes the length of your intake.
A6: Lambda control is your catalytic converter/oxygen sensor emission control system.

How many miles on your car? This code is typically associated with your oxygen sensors. 100K is the recommended replacement interval for O2 sensors. There are two sets (pre-cat and post-cat). Only one set (I can never remember which one) needs to be replaced)
 

·
Registered
Joined
·
4,976 Posts
What you have in most cases would be a vacuum leak. Since it is giving a code for bank 2 that helps you in
regards to what area of the engine to begin the investagation. Bank 2 is the back half of the engine. I would look in the following areas. The back of the intake are a number of vacuum hose's that can and will fail --
you will need to remove the cabin filter on the drivers side of the car and you will need a flashlight and a
small mirror to see around back there. On the top of the engine under the engine cover, drivers side, is whats
called the distribution unit. There are 4 screws that hold it to the intake. There are 2 hose's that hook to the ccv valve from that unit. There are 6, 0 rings on that distribution unit. When the 0 rings fail, it has been noticed that the back 0 rings go first. I found mine failing by using a medicine dropper and filling the wells around the posts going into the intake with small amounts of water. The cranked the car, where the water was still sitting I knew that the 0 rings were OK. Where the water was pulled into the engine, I could tell that the 0 rings had failed.
I need to ask this while here, What is a intake manifold actuator
 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #5 ·
Here are some answers
Thanks for those answers.

How many miles on your car?
About 88K miles.

This code is typically associated with your oxygen sensors.
It seems, from the research, that the implications are whatever affects the air:fuel ratio of the back half cylinders.

To that end, the MAF sensor, vacuum leaks, fuel filter, & fuel injectors seem to be implicated most.

Since it's easiest, I picked up a can of CRC MAF cleaner and will look up how to use it & report back (as a first step).
 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #6 · (Edited)
Bank 2 is the back half of the engine.
That would indicate something asymetric. For example, the MAF sensor, I would think, if dirty, would be symmetric. Do you agree?

On the top of the engine under the engine cover, drivers side, is whats called the distribution unit. ... Where the water was pulled into the engine, I could tell that the 0 rings had failed.
Very interesting diagnosis technique! thanks.

What is a intake manifold actuator
It's listed as a culprit here ...

 

Attachments

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #7 · (Edited)
I cleaned my MAF sensor and learned a few gotchas! :(

1. Leave the wire clip on! Do not use a screwdriver to pry off the MAF sensor electrical connector. It took longer to find the clip that flew into the air than the rest of the job combined.

2. Unbolt the air cleaner (one 10mm hold-down bolt at the top, another bolt at the bottom I think) to give yourself some wiggle room. I removed the MAF sensor housing w/o doing that and realized after bending the hose clamps and crushing the hose aft of the MAF housing that it would have been a LOT easier had I just removed the bolts holding down the air cleaner (this is NOT in the Bentleys which are plain wrong for the I6!).

3. CRC MAF cleaner is simply an 8-dollar can of hexane (see MDS attached). I wonder out loud whether we can get a can of non-branded hexane spray cheaper?

 

Attachments

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #8 · (Edited)
To help others not follow the Bentleys (which show the wrong picture and the wrong procedure), I wrote up a separate short DIY for and R&R of the MAF sensor housing on a 2002 I6.

I think instead of using a screwdriver or pliars to remove the spring clip, I could have just pressed down on the spring-clip harness connector! :(

Why you guys intuitively know this stuff and I don't is beyond me ...

 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #9 ·
Does anyone have an idea why this too-lean-in-the-rear-cylinders DTC P0174 code only seems to occur when I travel over high California mountains (3,500' to about 4,500') but it does not occur below about 2,500 feet?
 

·
Registered
Joined
·
4,976 Posts
Air is denser when closer to sea level than at higher altitudes, thus it's harder for humans to breath at
those higher levels also. Thats the reason I think you have a vac leak near the back of the engine.
Have you checked all the spots yet?
 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #12 · (Edited)
Air is denser when closer to sea level than at higher altitudes ... Have you checked all the spots yet?
I appreciate the advice; I did do a visual look (but did not take apart anything yet as the leak is now gone due to the fact I'm back at sea level).

I think I understand why you suggest a possible vacuum leak in the hoses under the drivers-side cabin-air filter ... but ...

I fundamentally don't understand why there would be more (or less) of a vacuum leak at 3,500 feet than at sea level.

Hypothetically, if there were a "slight" vacuum leak ... why would it be greater at 4,500 feet than at sea level?

 

Attachments

·
Premium Member
Joined
·
8,008 Posts

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #16 ·
I have nothing against replacing things that need to be replaced on schedule anyway ... so plugs are due (I think) at 100K miles (I need to check).

The O2 sensors, I think, are also due at 100K miles. At 88K, I'm close enough that it might be worth considering ... but plugs couldn't be the actual culprit because the code is for a "too lean" condition which has nothing to do with them since it's before the fact.

As for the sensor, I wonder. I think there are two sensors in front of the catalytic converter so it's believable that one handles the first three cylinders while the second handles the second bank (P0174 is a too lean condition on the second bank).

But, most things I read pointed to the MAF (I still don't know if the MAF has a separate sensor for each bank) or to vacuum lines (does the second bank have its own set of vacuum lines?) or to fuel delivery (an injector is about the only thing I can think of for fuel that is specific to a bank).

NAGGING QUESTIONS:
Q1: The MAF ... does it have a separate sensor for each cylinder bank?
Q2: The vacuum lines ... which lines are specific for the second bank?
Q3: Where is the O2 sensor for the second bank located anyway?
 

·
Registered
Joined
·
4,927 Posts
I have nothing against replacing things that need to be replaced on schedule anyway ... so plugs are due (I think) at 100K miles (I need to check).

The O2 sensors, I think, are also due at 100K miles. At 88K, I'm close enough that it might be worth considering ... but plugs couldn't be the actual culprit because the code is for a "too lean" condition which has nothing to do with them since it's before the fact.

As for the sensor, I wonder. I think there are two sensors in front of the catalytic converter so it's believable that one handles the first three cylinders while the second handles the second bank (P0174 is a too lean condition on the second bank).

But, most things I read pointed to the MAF (I still don't know if the MAF has a separate sensor for each bank) or to vacuum lines (does the second bank have its own set of vacuum lines?) or to fuel delivery (an injector is about the only thing I can think of for fuel that is specific to a bank).

NAGGING QUESTIONS:
Q1: The MAF ... does it have a separate sensor for each cylinder bank?
Q2: The vacuum lines ... which lines are specific for the second bank?
Q3: Where is the O2 sensor for the second bank located anyway?
Just trust me on this...

Replace the pre-cat O2's and your spark plugs.
 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter · #18 ·
Replace the pre-cat O2's and your spark plugs.
I agree it's probably a good idea to replace both. I don't know if I have all the right tools before I do that job.

I assume basic tools will work to replace the spark plugs ... but for the pre-cat oxygen sensors ... on the 2002 525 I6 ... are they in the engine bay as shown in the V8 pic below or elsewhere?

Do I need this special tool to remove them?

 

Attachments

·
Registered
Joined
·
4,976 Posts
The 02 sensors are in the very spot on your engine as in the pic you provided. You will need the special
02 wrench that is also in your pic. I use the more widly used 02 socket that is available at the local
auto parts stores. Runs around 20 bucks. The 02 sensors generally run about 65 each at Auto Zone
and are Bosch which is the OEM part from BMW. Dealer will want a couple of hundred for each of them.
 

·
Registered
Joined
·
4,927 Posts
I agree it's probably a good idea to replace both. I don't know if I have all the right tools before I do that job.

I assume basic tools will work to replace the spark plugs ... but for the pre-cat oxygen sensors ... on the 2002 525 I6 ... are they in the engine bay as shown in the V8 pic below or elsewhere?

Do I need this special tool to remove them?

1. Yes, you will need a O2 socket, what you have pictured.
It can be bought at Auto Zone.

2. Yes, your picture shows the correct location for the pre-cat O2's.

www.eactuning.com/parts/product_details/11781742050

http://www.autozone.com/autozone/ca...=&searchText=oxygensensors&_requestid=1170474
 
1 - 20 of 37 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top