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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 12-29-2009, 11:17 AM
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POof540i POof540i is offline
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Exclamation Diagnosing a bad MAF

Hi everybody and thanks for reading my thread.

My car has a problem. In cold weather my car will have rough starts. As soon as I start the car, with the aid of the smog pump, it'll idle fine, but as soon as the pump turns off, the car begins to idle erratically. It idles like it wants to turn off, but doesn't and when I accelerate, it sounds and feels like it's misfiring. So I'm trying to find what's wrong with my baby.

I'll tell you what I've done so far in the last 20k miles. I've changed the plugs with OEM plugs, the pre-cat O2 sensors, one CPS, VCG, CCV, fuel filter, belts and tensioners, and multiple vacuum hoses. So what remains that could affect the idle of my car are the intake manifold gaskets, coils and the MAF. I checked the coil boots and they all look good. The SES light is not on and I haven't scanned it for stored codes yet.

I kind of suspect it could be the MAF, so I put it to the test. I removed the MAF assembly while the car was on and it died. Then I tried a different method. I unplugged only the plug to the assembly before starting the car, fired her up and to my surprise, it idled and accelerated very very smoothly. The best idle and acceleration its had in months. So then I did the opposite thinking I was on to something. I kept the MAF plugged in, then started the car and lo and behold, it began idling rough again and backfired mildly after accelerating and taking my foot off the gas.


Which begs the question, Is my MAF to blame? My common sense tells me yes, but my common sense has been wrong before. Correct me if I'm wrong, but Bosch makes the OEM MAF right? Any help will be appreciated, thanks for looking.
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  #2  
Old 12-29-2009, 12:55 PM
edjack edjack is offline
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That seems to be the correct diagnostic for the MAF sensor, as reported on at least a couple of forums. Might try cleaning it before replacement.

If you're going to futz with this car, get a scan tool (not a Peake) and a Bentley's manual. You'll save the cost of these by avoiding just one dealer visit.
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  #3  
Old 12-30-2009, 03:25 PM
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POof540i POof540i is offline
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Thanks for the help. I've been meaning to buy a Peake scanner and a Bentley manual, but I'm cheap and do most of my troubleshooting from this other sites like it. Still no codes, but I'm going to try and clean it today. I'll post my results.
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  #4  
Old 12-30-2009, 04:03 PM
KeithS KeithS is offline
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When my MAF was on it's way out it did not cause idle problems except wanted to stall if you bliped the throttle. . It did set the SES light and the car was surging under light acceleration. You can try cleaning using CRC MAF sensor cleaner. Each time I cleaned it lasted less time than previously before setting the SES. The last time I tried it took only a day and I finally bit the bullet and changed it. Problem solved, car felt like it had another 50 hp.

Got mine here, great price and the BOSCH generic right on the home page was identical in every way to the factory unit including every code printed on it, except it did not say BMW on the box.

http://www.getcoolparts.com/
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  #5  
Old 01-01-2010, 07:42 PM
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POof540i POof540i is offline
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So I cleaned the MAF today using this procedure: http://www.louv.tv/cars/m5/MAF/ and got the best highway gas mileage I've ever gotten since I bought my baby. The best mileage I've achieved is 25 mpg driving at an average speed of 80 mph. Today, after cleaning the MAF, I averaged 29 mpg at an average speed of 80 mph. Awesome awesome awesome. My car accelerates much more smoothly now and feels more powerful. Before today, my car felt and sounded like it was misfiring, but now it feels like its running on all cylinders. Btw, I did not have the SES light on nor was there any pending codes. FYI, my car has 135k miles and I think it was still using the original MAF.

I'll post an update of the effects on city mpg, if any, in the next few days and if I did more harm than good by cleaning the MAF. I currently get 16-17 mpg in city driving.
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  #6  
Old 01-02-2010, 08:34 AM
KeithS KeithS is offline
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Quote:
Originally Posted by POof540i View Post
I'll post an update of the effects on city mpg, if any, in the next few days and if I did more harm than good by cleaning the MAF. I currently get 16-17 mpg in city driving.
As long as it's fully dry before energized, it's difficult to do more harm than good by cleaning. Just remember, if yours was like mine, the symptoms will return. If they do return in a short amount of time (a few months), I would then just replace it.
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  #7  
Old 01-02-2010, 09:07 AM
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POof540i POof540i is offline
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Quote:
Originally Posted by KeithS View Post
As long as it's fully dry before energized, it's difficult to do more harm than good by cleaning. Just remember, if yours was like mine, the symptoms will return. If they do return in a short amount of time (a few months), I would then just replace it.
Thanks for the reassurance and I believe your right, I think the symptoms will return, but I'm glad its fixed for now. I let it dry for about 4-5 hours in the sun so I hope that was enough time to dry it completely. The hardest part was removing the MAF sensor. It uses a 5-point tamper-proof bit which is hard to find. I went to autozone, kragen, pep boys, sears, harbor freight and nothing! So I used pliers. The ham-fisted method but it worked.
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  #8  
Old 01-02-2010, 09:32 AM
KeithS KeithS is offline
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The cleaner evaporates quickly, 10 minutes would probably have been enough. I also had trouble finding the security torx tools. I did get them at Autozone where they were a set of L shaped (like allen wrenches) tools. One end was regular, the other end was security.
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  #9  
Old 01-02-2010, 10:23 AM
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540 M-Sport 540 M-Sport is offline
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If anyone needs a new MAF for a later 540 (I think 1998 on) I have a brand new in box, Bosch VW maf that will work. You simply replace the maf sensor from the VW maf, and install in your maf housing. You need the proper 5 point and 6 point torx security bits though. (5 point for the VW and 6 point on the BMW maf). I will sell for $75 with free shipping to your door.
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  #10  
Old 01-02-2010, 10:08 PM
hockenheim hockenheim is offline
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Quote:
Originally Posted by edjack View Post
That seems to be the correct diagnostic for the MAF sensor, as reported on at least a couple of forums. Might try cleaning it before replacement.

If you're going to futz with this car, get a scan tool (not a Peake) and a Bentley's manual. You'll save the cost of these by avoiding just one dealer visit.
OP, sorry for the thread hijack, but I was just about to pull the trigger on a Peake so the above comment makes me curious - why NOT a Peake, and what scan tool do you recommend?

TIA
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  #11  
Old 01-03-2010, 09:41 AM
KeithS KeithS is offline
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Quote:
Originally Posted by hockenheim View Post
OP, sorry for the thread hijack, but I was just about to pull the trigger on a Peake so the above comment makes me curious - why NOT a Peake, and what scan tool do you recommend?

TIA
The peak only can read the basic engine diagnostic codes. There are other systems (much more expensive), usually requires the use of a laptop computer, that can read parameters from many of the other subsystems as well as capturing live data and things like trim parameters. These more expensive systems get closer (but still do not equal) the BMW factory diagnostic equipment.

Now with that said I only have the Peake code reader myself. Very easy to use, pretty close to fool proof. In my 16 years of total BMW car ownership (10 actual years, 3 different vehicles) I've never had a problem that I could not figure out by just using the Peake reader. It does take some interpretation, as all the readers do. For example the bad MAF came up with codes like "off idle fuel trim error", both banks.
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  #12  
Old 05-20-2014, 04:59 PM
FutureFuzz 26 FutureFuzz 26 is offline
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i see this thread is ancient, however I am having exact symptoms as the OP. with NO MAF related codes.

any update with this thread? did the symptoms ever come back?
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  #13  
Old 05-21-2014, 02:16 AM
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bluebee bluebee is offline
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I realize this is an old thread, so I'll just reference this, found by typing /maf in the bestlinks, so that others benefit:
- DIY for replacing (1) (2) (3) & cleaning your mass air flow (MAF) sensor (1) (2) (3) (4) (5) and how 'not' to clean your MAF (1) & buying a replacement MAF cheaper than the BMW MAF (RangeRover MAF) (VW MAF) (Hyundai MAF) & MAF resistance testing (1)
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Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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  #14  
Old 05-21-2014, 05:32 AM
Chisum Chisum is offline
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I just replaced the MAF in my sedan. I started getting lean coads on both banks. I swaped the MAF from my tourning and the coads followed the MAF. I tried to save some money and bought the MTC brand MAF from ECS Tunning. Big mistake. I was getting more codes with the new MTC then the failling MAF. I'm returning the MTC and getting the VDO brand which seems to work well with our cars.

Chisum
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