Removing and cleaning the MAF sensor housing on the I6 E39 (Bentley notwithstanding) by bluebee

By diyauto
( 2 )

18 minute read

Removing and cleaning the MAF sensor housing on the I6 E39 (Bentley notwithstanding)

Compliments of bluebee @


As the first step of an intermittent high-altitude P0174 too-lean OBD-II DTC error with fuel cutoff, I followed the Bentleys to remove & clean the "Siemens MS 43.0" MAF sensor housing on a 2002 525i and, in hindsight, I wish I hadn't because a few simple but critical steps are missing from the Bentleys.

This is what the Bentleys, page 130-29, say to do:

- Loosen intake boot clamp

- Disconnect harness connector

- Release clip from air-filter housing

- Remove MAF sensor housing

- Installation is the reverse of removal

In a perfect DIY world, this is what I needed to do on the 2002 I6 E39 (in hindsight):

- Remove 10mm bolt holding air filter housing rigidly in place 

- Loosen intake boot hose clamp with a flathead screwdriver

- Press down on spring-loaded wire on harness connector to pull harness connector off the MAF sensor housing

- Release two clips from the air-filter housing with a screwdriver

- With force, compress intake boot rubber and push away from the MAF sensor housing to disconnect MAF sensor housing from the intake boot

- Lift up on the now loose air-filter housing and wiggle as you pull the MAF sensor housing out of the air-filter housing

- NOTE: Some people recommend pulling the engine air-filter housing away from the headlight assembly (see DIY in later threads below).

- Remove the MAF sensor housing and clean with hexane spray (15 good wet spurts of CRC MAF Sensor Cleaner)

- Allow the MAF sensor to dry thoroughly (it won't take long because the hexane evaporates in a minute or two)

- Installation is the reverse of removal

Following Bently, this is what I actually did on the 2002 I6 E39 (in error):

- Loosened intake boot hose clamp with a flathead screwdriver

- Used a screwdriver to pull up on the spring-loaded wire on the harness connector 

- In a split second, that wire disengaged and ricocheted across the cluttered garage

- Immediately I wished the Bentley had said HOW to remove that spring-loaded harness clamp!

- Spent at least a half hour looking for that missing airborne spring clip, almost giving up on the task

- Released two clips from the air-filter housing with a screwdriver

- With force, tried to compress the strong intake boot rubber to push away from the MAF sensor housing to disconnect MAF sensor housing from the intake boot but succeeded only in distorting the soft hose-clamp steel

- Doublechecked that I was on the Siemens 43.0 MAF section (which is what a 2002 BMW 525i would be according to the chart on page 130-2).

- Read the Bentleys over and over, wondering what was missing (finally realizing they're discussing & picturing a slightly DIFFERENT setup!)

- On my own now, I loosened both spring clamps further downstream on the intake boot intending to remove intake boot, but also failed as it's wedged in there tightly

- Finally decided to remove the 10mm bolt holding air filter housing rigidly in place (so that I could wiggle the air cleaner housing a bit)

- Lifted up on the now loose air-filter housing and wiggled and pulled the MAF sensor housing out of the air-filter housing

- Held the MAF sensor housing in one hand and sprayed 15 times with hexane spray (good wet 5-second spurts of CRC MAF Sensor Cleaner) making sure to clean both protection screens and the all-important sensor in the center of the MAF housing.

- Allowed the MAF sensor housing to dry (it only took a minute and I let it sit for 10 minutes just in case while I fuddled with the harness clip trying to put it back together after it went airborne.

- Installation is the reverse of removal

crc_maf_mds.pdf (49.7 KB, 679 views)

Originally Posted by sidneyj 
I ended not even using the Bentley as it is just not accurate.

I'd still recommend the Bentleys; but lately it hasn't been helping me as much as Bimmerfest DIYs have.

Unfortunately, all the DIYs I found on bimmerfest for the MAF were for other models, and, IIRC, none bothered with the particular detail of 'how' to remove the MAF harness connector nor the fact you really have to remove the 10mm bolt from the air-filter housing (NOTE: DIYs on other forums pointed out to me confirmed the Bentleys are wrong for the I6 and that you should remove not only the top 10mm bolt but also another bolt under the engine-air-filter housing and remove the housing instead of crushing the air-intake hose aft of the MAF sensor housing. Lesson learned!).

Originally Posted by Fudman 
Did this solve your problem?

Time will tell because the P0174 too-lean condition, with fuel shutoff (cured by restarting the car) and SES light only occur at altitude. The last time this happened, the code cleared back at sea level before I had a chance to borrow the Actron scanner. This time I borrowed it right away back at sea level to find the DTC P0174 (rear bank too lean condition).

Googling, this DTC P0174 appears to be a maladjusted 14:1 air:fuel ratio in the rear half of the engine.

The main culprits for too-lean conditions appear to be:

- maladjusted MAF (air ratio)

- clogged fuel filters or injectors (fuel ratio)

- leaky vacuum hoses (I'm not sure how they affect the air:fuel ratio)

- oxygen sensors (I guess they measure the amount wrongly)

I'm not for miracles in a can, but, I figured I should clean the MAF anyway so I bought the CRC hexane spray (hoping for a miracle!). 

This is the second (or third?) time the light lit on a long trip almost always at about 3,500 or 4,500 feet of altitude (not at 2,000 which I often travel over with the Santa Cruz mountains between San Jose & Santa Cruz.

So, whatever is causing the too-lean condition is happening at altitude; and it's happening on only bank #2.

Two questions:

Q1: How often do YOU guys clean your MAF sensors?

Q2: Are there TWO sensors in that housing (one for each bank)?

Originally Posted by repcapale 
Just push down on it and pull the sensor off.

I know that now. I put in the DIY up in post 1 how to remove the MAF connector by pushing down on the metal spring and pulling on the connector.

But I didn't know that then ... and no DIY (IIRC) told me ... and the Bentleys weren't even close to being right on the steps needed.


Originally Posted by Jason5driver 
MAF Removal and Cleaning (2000 540i and 2001 525i)

Oh my! Where were you when I needed you! 

That V8 and I6 MAF writeup is BETTER than mine (and better than the Bentleys, which show the wrong pictures for the I6).

They went even further than I did, by removing the engine air-filter housing from the headlight assembly. 

And, to do so, they MUST have removed the 10mm bolt holding down the air-filter housing (but they don't mention that in their writeup for the 2001 I6).

BMW V8 and I6 MAF Removal and Cleaning.pdf (736.7 KB, 871 views)

Originally Posted by doru 
check this here.

Nice post for an I6 E39 after 2001 (like mine).

Unfortunately, I tried to view the pictures but you must log in and they don't allow a yahoo email address so I wasn't able to view the pictures.

You recognized that the V8 is different than the I6 and for the I6 I see you not only take off the 10mm bolt, but, you even remove another bolt and lift out the engine air-filter housing.

That's good to know. I didn't remove the engine-air-filter housing but I did wiggle it a LOT to get the MAF out. I also basically crushed the hose after the MAF housing.

I think your method of more completely removing the air-cleaner housing is better than mine of crushing the hose aft of the MAF housing.

Originally Posted by andyffer 
I think after getting 31.5 on the highway from Tucson to Phoenix

Oh my! I get nothing near that, I assume. 

I never checked my gas mileage.

Besides, the only real way to test gas mileage is to weigh the fuel (because it expands and contracts greatly on temperature changes and we can't see how much we filled up the tank ... I'll bet we're off by more than a half gallon each time). 

The only other "good" way to test gas mileage is to average our results over long periods of time ... maybe I'll start since I doubt I get anywhere near 25 mpg, let alone 31! 

I "think" I get about 400 miles out of an 18-gallon tankful ... which works out ten mpg less than what you appear to get!

Originally Posted by cn90 
* More air flow ---> hot wire cooling down ---> ECU injects more fuel to match the ratio.

So is this the logic at altitude?

a. Piston goes down on the "pull" part of pull-push-pow-puey.

b. A piston-full volume of air is sucked past the MAF sensor hot wire.

c. Since we're at 4500 feet, LESS air is actually going by that hot wire.

d. So (the argument goes), the wire isn't cooled down as much as it should.

e. The ECU, in effect, is tricked into thinking the air is moving slower than it is.

f. So, less fuel is injected into the 2nd bank of cylinders (not sure how??)

g. Hence, a too-lean condition is sensed by the oxygen sensors

This is my best-guess hypothesis ... but step e is really where I lose the logic of what is happening at altitude.

BTW, if the MAF were bad, would/could it affect only the second bank of cylinders?

Originally Posted by Mack 
2. Spray MAF generously with CRC MAF cleaner ONLY!!

Two questions:

Q1: Do you spray just the sensors in the middle of the MAF or the entire plastic and metal screens on each side?

Interestingly, the Material Data Sheet (MDS) I posted for the CRC MAF cleaner I purchased show it's almost entirely hexane.

Q2: I wonder if we can find a cheaper hexane spray for other purposes (like office cleaner or whatever).

BTW, after reading this safety warning about the normal straight-chain hexane (aka n-hexane) spray, next time I clean my MAF, I'm wearing gloves! Also, stock up 'cuz these guys are trying to get hexane banned!


For cross reference, here's another material data sheet for MAF cleaner, specifically posted in a thread where a guy from Sweden was having trouble locating it (it's hexane spray, pure and simple).

- o2 sensor fault code, and missfire without setting code for missfire?! 532i ´97


This E46 thread today made me feel better than I'm not the only one who accidentally pulled off the spring clip off both the MAF and the large cooling system radiator hoses! 

Need PUSH-CLIPS for Upper Radiator Hose Bmw 323i


For the record, there is a discussion of whether cleaning the MAF will hurt it over here today:

E39 (1997 - 2003) > e39 mass air flow sensor


Yes it can be cleaned. I'm considering doing this myself but I've heard it only shortens the life of the MAF.


Originally Posted by JimLev 
Where did you hear that? I've been using CRC every year, since my original MAF died, that was back in ~2006, I now have 134K on the car and probably 70K on this MAF.
Spray it and then let it dry before you put it back in the car.

EDIT: See also:

  • 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)


For reference, this MAF thread was opened today:


Originally Posted by aspensilver540 
OK, I am new to engine work so forgive me if I am making some simple or obvious mistake.
My car: 3/97 540i 116K miles

I looked at James' 2000 540i MAF cleaning DIY but obviously it is different:
There aren't 2 T-20 screws on mine so I don't see how to pull the sensor out of the housing as he shows. I am confused how then I should do the cleaning! 
My MAF, what screws do I have:
New bare MAF part:
The inside looks like this:
Bentley MAF removal procedure:
I can do this easily, and I have CRC MAF cleaner ready. Should I try cleaning by spraying into the housing with the part out, or is that a bad idea and I just need to figure out what screwdriver to use?
Originally Posted by Fudman 
Did this solve your problem?


Originally Posted by bluebee 
Time will tell because the P0174 too-lean condition, with fuel shutoff (cured by restarting the car) and SES light only occur at altitude.

I belatedly realized I never updated this thread with the true cause of the P0174. 

Over a period of about a year or two, the P0174 progressively got worse and worse - until - I failed inspection.

The real problem turned out to be a crack in the CCV lower vent hose and another crack in the corrugated thumb of the boot on the ICV & TCV.

Details here:

- Does the order of the misfire OBDII DTCs diagnostic trouble codes actually matter (1


This novel cleaning approach was posted today ...

E39 (1997 - 2003) > 1999 540i MAF Cleaning


Originally Posted by GSXRYDER 
Well it seems to have worked. 

I tried 'traditional cleaning by taking it out and spraying cleaner in but due to sensor location inside the MAF itself the cleaning did NOT work. I ordered a 'cheapie' off Ebay and the car stumbled and no power 'ran like garbage' . Was getting ready to order a new one and did some reading and found the sensors 'rarely go bad and they are 'very robust' from Bosch. At this point didn't have much to lose, so figured why not?

Watched a few videos on youtube, read some posts and decided to take the 'factory' MAF apart. I got a razor knife small pick and small screwdriver and proceeded to disassemble the MAF. I have to admit it did 'take some doing' but I stuck with it until I got the cover 'off'. When I did I could see the sensor. Sprayed the sensor with CRC MAF cleaner, bused it with a small hobbyist paint brush, wiped with tissue, resprayed with MAF cleaner. put the cover back on with some epoxy. Waited one hour for the Epoxy to fully cure, reassembled it and wow, like magic the cars power is back, starts idles runs great...

Put together a short Video of how I did it...not sure how long it will last, but the fact is it is WAY better now than it has been...Will report back on longevity...


This question came up again today ...

E39 (1997 - 2003) > how to properly clean a MAF sensor


Originally Posted by Roger1962 
how does one go about cleaning a MAF sensor with out killing it..