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Discussion Starter #1 (Edited)
Hey guys,
Case is i have had a engine light for a few months now on my 2000 E46 323i, The code is: P0173 "Fuel Trim, Bank 2 - Malfunction"
I've been trying to figure it out, and think that I've narrowed it down to a vacuum leak. (Did a smoke test, and smoke came out, however I've yet to discover where the leak is, but I'm pretty sure I can find it eventually.)
HOWEVER!
Sometimes, seemingly random, I'll turn on the car, and the engine light will have gone out, as if I had cleared it with a scanner, it returns eventually, but then again at random, it will go out.
Any ideas as to why this might happen? I suspected maybe the leak is so small, that sometimes the car thinks there isn't one, and therefore turns the light off. However from what I've been able to research, the E46 isn't supposed to turn the light off by itself, no matter what.
Any help is much appreciated.
 

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I would honestly invest in a bluetooth OBD Link, that way you can catch it as soon as it displays, there could be some pending codes in there as well, right now my 04 325i has a small misfire at low rpms in the 5th cyl. And it will code then pend and then disappear.

I've seen alot of 02 sensor or Cat relate issues as well in alot of cars not just bmws that have the identical behavior with light coming on then going away

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Depending where the vaccumm leak, it could possibly be a lazy 02 or 02 sensor going out, I've heard bad MAF, and also maybe of the CCV hoses are cracked

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Discussion Starter #5 (Edited by Moderator)
Thanks for the advice, but it's really not what im looking for. i've changed the MAF, went through pretty much every cable to see if it was an electrical problem. And now I've found a vacuum leak - somewhere. It might be the o2 sensors, but im thinking its the vacuum leak still. The thread was more to get a picture of why the liight goes out on its own.

However I appreciate the input, and any other inputs on possible places the leak could be, or another problem it might be are also appreciated. :)
 

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Understand this, OBDII is somewhat self healing. If the problem is just on the threshold of a trigger point and depending on how the car is driving, OBDII codes WILL clear themselfs and the SES/CEL/MIL will turn off on its own if the problem that triggered the Code/Light original is not observed after a specified number of run cycles, typically between 20-40 depending on how the programming is set up.

Understand you have a 20 year old car!!! Guess what, rubber only is reliable for 8-10 years and after that it is just time when there will be a problem.

While the Pre-cat O2 sensors should be replaced every 100k miles on your model, they are working well enough for the moment to detect a fuel mixture problem.

Suggest you read this thread, it pretty much covers everything you need to know about vacuum leaks on these cars - https://forum.e46fanatics.com/showthread.php?t=897616

Another problem area to check - https://forum.e46fanatics.com/showthread.php?t=1057387

Forget the sensors for now, fix the air/vacuum leaks.

Then pay attention to the Fuel Trim values and Freeze Frame data as this is important.
 

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I just wanted to point out that you might have an intermittent vacuum leak.

Ie when the engine warms up it could open or close the leak. Or when it is cold it could trigger the leak.

P0173 is very rare though. You will have to use Google to search here and on Bimmerforums. The “search” function on this site doesn’t work very well.

Edit: also how many miles? And what have you done to fuel system, O2 sensors and vacuum lines if anything?

Thanks. This will help the experts diagnose your problem remotely.


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Agree with jfoj & swordsman.

Changing sensors etc. isn't going to fix your issue. You've got a vacuum leak (saw smoke) and have a fuel trim issue.
Yes they're "self-healing" that's the adaptations part.
Sure you could read fuel trims but you're already past that point.

I.E. intermittent check eng. light, look at fuel trims, then do a smoke test. You've already jumped ahead in line (this time a good thing) and saw smoke.

Dig in and find it.

While this may seem counterintuitive when we are faced with what you have after the repair we DO NOT reset fuel trims/adaptations. Rather we drive the car normally and observe the fuel trims (the longer you drive the better the test methodology) and watch them to see if the DME notices that the once present vacuum leak is now gone and the fuel trims start to return nearer to normal.
That is our validation of actually fixing something. Once we know, then we finally reset the engine light and go for an OBD test loop.

If/when the test loop does all of it's checks (except evap, that takes time) and the OBD system ran it's checks and they're complete, I'm happy and so are my customers.

HTH?
 

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The best way to find intake leaks is with a smoke machine. Find a shop or a friend that has one. The way they work is remove the air flow meter then plug off the opening and then put the machine plugged into the intake. They put out 1psi and if there’s a leak you will see smoke. Makes finding intake leaks a breeze. Good luck with your car.


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Discussion Starter #11 (Edited)
Understand this, OBDII is somewhat self healing. If the problem is just on the threshold of a trigger point and depending on how the car is driving, OBDII codes WILL clear themselfs and the SES/CEL/MIL will turn off on its own if the problem that triggered the Code/Light original is not observed after a specified number of run cycles, typically between 20-40 depending on how the programming is set up.

Understand you have a 20 year old car!!! Guess what, rubber only is reliable for 8-10 years and after that it is just time when there will be a problem.

While the Pre-cat O2 sensors should be replaced every 100k miles on your model, they are working well enough for the moment to detect a fuel mixture problem.

Suggest you read this thread, it pretty much covers everything you need to know about vacuum leaks on these cars - https://forum.e46fanatics.com/showthread.php?t=897616

Another problem area to check - https://forum.e46fanatics.com/showthread.php?t=1057387

Forget the sensors for now, fix the air/vacuum leaks.

Then pay attention to the Fuel Trim values and Freeze Frame data as this is important.
I just wanted to point out that you might have an intermittent vacuum leak.

Ie when the engine warms up it could open or close the leak. Or when it is cold it could trigger the leak.

P0173 is very rare though. You will have to use Google to search here and on Bimmerforums. The "search" function on this site doesn't work very well.

Edit: also how many miles? And what have you done to fuel system, O2 sensors and vacuum lines if anything?

Thanks. This will help the experts diagnose your problem remotely.


Sent from my iPhone using E46Fanatics
Agree with jfoj & swordsman.

Changing sensors etc. isn't going to fix your issue. You've got a vacuum leak (saw smoke) and have a fuel trim issue.
Yes they're "self-healing" that's the adaptations part.
Sure you could read fuel trims but you're already past that point.

I.E. intermittent check eng. light, look at fuel trims, then do a smoke test. You've already jumped ahead in line (this time a good thing) and saw smoke.

Dig in and find it.

While this may seem counterintuitive when we are faced with what you have after the repair we DO NOT reset fuel trims/adaptations. Rather we drive the car normally and observe the fuel trims (the longer you drive the better the test methodology) and watch them to see if the DME notices that the once present vacuum leak is now gone and the fuel trims start to return nearer to normal.
That is our validation of actually fixing something. Once we know, then we finally reset the engine light and go for an OBD test loop.

If/when the test loop does all of it's checks (except evap, that takes time) and the OBD system ran it's checks and they're complete, I'm happy and so are my customers.

HTH?
Thanks for the help guys, especially jfoj, I've read alot of your other posts trying to figure this headache out, and you seem to know your stuff well.

This weekend I'll be handing the car over to my hobby-mechanic friend, and he'll have a go at finding where the smoke comes out.

Before that, I'll take a look at the LTFT's (at idle?) and post the results here.
Hope you guys can find the time to look at the trims.

I'll also check for the brake booster thing you linked Jfoj.

Additionally some time ago, my car started to fail starting, and this came down to no fuel being delivered, however I found a tip online that said to knock ontop of the fuel pump, and voila it worked! However, I never got around to change it for some reason, do you recon this might have anything to do with my current problem?

Also, my car is used, and has 230k'ish miles on it, so theres no doubt a bunch of things are starting to wear. I do not know when the o2 sensors were last changed, or if anything was done to the vacuum lines or the fuel system. I myself, have not.

Another question, if say the vacuum leak is the sole cause of the CEL, would fixing the vacuum leak and delleting the CEL and codes, be enough, or is there something else that needs to be done, take in mind I have droven with this vacuum leak for quite a while.
 

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Thanks for the help guys, especially jfoj, I've read alot of your other posts trying to figure this headache out, and you seem to know your stuff well.

This weekend I'll be handing the car over to my hobby-mechanic friend, and he'll have a go at finding where the smoke comes out.

Before that, I'll take a look at the LTFT's (at idle?) and post the results here.
Hope you guys can find the time to look at the trims.

I'll also check for the brake booster thing you linked Jfoj.

Additionally some time ago, my car started to fail starting, and this came down to no fuel being delivered, however I found a tip online that said to knock ontop of the fuel pump, and voila it worked! However, I never got around to change it for some reason, do you recon this might have anything to do with my current problem?
Yes, of course, a lack of fuel can cause or contribute to a lean condition. Sounds like you are on borrowed time. My fp failed at 107k miles. You also have a vacuum leak. There can always be multiple things wrong.

Also, my car is used, and has 230k'ish miles on it, so theres no doubt a bunch of things are starting to wear. I do not know when the o2 sensors were last changed, or if anything was done to the vacuum lines or the fuel system. I myself, have not.

Another question, if say the vacuum leak is the sole cause of the CEL, would fixing the vacuum leak and delleting the CEL and codes, be enough, or is there something else that needs to be done, take in mind I have droven with this vacuum leak for quite a while.
Yes, absolutely. The cooling system needs preventative maintenance, flush your brake fluid & check pads & rotors, suspension work, numerous oil and fluid leaks, change fuel pump and filter, check power steering system for leaks, and more.

In terms of reliability, cooling system refresh and the fuel pump & filter should be top of the list. The fuel pump can strand you, and if the cooling system fails there is a good chance of wrecking the motor.
 

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Discussion Starter #13
Yes, of course, a lack of fuel can cause or contribute to a lean condition. Sounds like you are on borrowed time. My fp failed at 107k miles. You also have a vacuum leak. There can always be multiple things wrong.



Yes, absolutely. The cooling system needs preventative maintenance, flush your brake fluid & check pads & rotors, suspension work, numerous oil and fluid leaks, change fuel pump and filter, check power steering system for leaks, and more.

In terms of reliability, cooling system refresh and the fuel pump & filter should be top of the list. The fuel pump can strand you, and if the cooling system fails there is a good chance of wrecking the motor.
I think you misunderstood me, I meant if there is more that needs to be done, in regards to the problem I have right now. Like, if there were some adaptions that needed resetting or such.

Also, the fuel pump thing happened atleast 15k kilometers ago, no excuse to not change it, but, I don't think I'm that much on "burrowed time" since there has been absolutely no start-up problems since.

I know there is alot of work that can be done, and probably should, but this car is not my whole life, and I can only enjoy working on it for so long. I change my oil and filters (air filter aswell) every 10-15k kilometers, as a basic maintenance. But I dont have the time or the motivation to go through every possible thing there is to check up on.

I of course check my fluid levels reguarly and fix them if there is a problem, and pay attention to any misbehaviour that might occur, weird sounds, driveability etc.

But as said, I'm probably not as big of a "gas-head" as most people on this forum, I just enjoy trying to fix problems that occur myself.
 

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I think you misunderstood me, I meant if there is more that needs to be done, in regards to the problem I have right now. Like, if there were some adaptions that needed resetting or such.
Nope, nothing special needed. Fix the leak, clear the codes.

Also, the fuel pump thing happened atleast 15k kilometers ago, no excuse to not change it, but, I don't think I'm that much on "burrowed time" since there has been absolutely no start-up problems since.
Ok.

I know there is alot of work that can be done, and probably should, but this car is not my whole life, and I can only enjoy working on it for so long. I change my oil and filters (air filter aswell) every 10-15k kilometers, as a basic maintenance. But I dont have the time or the motivation to go through every possible thing there is to check up on.

I of course check my fluid levels reguarly and fix them if there is a problem, and pay attention to any misbehaviour that might occur, weird sounds, driveability etc.
Understood. That's why I also highlighted the 2 main reliability issues these cars have that are most likely to strand you on the side of the road or brick your engine and cause you real problems. Versus, something like a worn out suspension that might increase your tire wear a little bit and not handle as good but often can still be safe to drive for quite a long time.

But as said, I'm probably not as big of a "gas-head" as most people on this forum, I just enjoy trying to fix problems that occur myself.
:thumbup:

Just be careful, that's how all "gas-heads" (or "gear-heads" as I prefer) start out ...
 
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