# MAF Cleaning (long)



## Steve D (Oct 10, 2002)

Last month my MAF failed for the second time in four years on my 2000 540/6sp. After I installed a replacement I decided to try to take the old one apart and see if I could either fix/clean it or if not at least get a better understanding of how it operated and perhaps how it could fail.

I have had a CAI for the last 4 years and hearing all the rumors about the filter oil harming the MAF I wanted to see for myself how this could happen. When I cleaned my filter I was very careful not to over oil it. After oiling I blew it out with compressed air, let it dry overnight and blew it out again. Then just to be sure I attached it to my shop vac and let the vac draw air though it for a good 15 minutes to get any residual oil out. My MAF still failed a few months later.

When I took the MAF apart there was no evidence of any oily film anywhere and all the parts looked pristine clean. The MAF used in 1999 and newer 540's is a hot film type which incorporates the intake sensor into its structure. The BMW PN is 13 62 1 433 567 for a supposedly "improved" type per SI B 12 09 03. The MAF consists of a plastic housing which contains the thermal sensor on one side and the hot file wire in a channel molded into the housing at the bottom. There is also circuitry encapsulated inside the housing.

I opened the cover to the housing by cutting the molded rubber seal off, but the surface mounted components were sealed in a soft gooey encapsulant and not repairable or even accessible for measurements. The actual element is mounted on an extension of the PC board and supported on a metal backing plate. (See the attached photo).

Notice that the element runs across the top of the board and that in mine about half the element is missing. Probably disintegrated. This is consistent with the 0V MAF output reading I got on my OBD reader (The resistance element forms voltage divider whose output normally varies from 0.6V to 4.5V depending on air flow). Again I saw no evidence of oil or any contaminant on or near the PCB or metal backing. This is obviously not repairable and I still don't know what makes the element fail but it does look very fragile.

The element is buried so deep inside the housing that I don't see how it can be cleaned except by dunking the whole sensor in solvent. Will the solvent damage it? How would you be sure that solvent residue won't remain? I am skeptical what this would accomplish as mine failed but did not seem to have any contamination at all.

The only conclusions I can make are: That cleaning probably won't help and may do more harm than good.
Unless the element is severely over-oiled a CAI should not damage the sensor.
The MAF used on our cars is very fragile and is another of those weak points of an otherwise great car.

<img src ="http://demandred.dyndns.org:3000/540i/albums/540i/aac.jpg">

Steve D


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## humanoid (Mar 31, 2002)

Saw your post on RF. Very good post thanks for the research!


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## Tahoe (Jan 9, 2004)

Great write up and it's good to know that one would likely have to seriously over oil a filter to create a problem. 
My recent experience: I was on a road trip up the Oregon coast last month when my MAF failed. I was passing a motorhome on 101 and the engine just died. I pulled over and restarted but the car wouldn't idle. Ran fine but wouldn't idle at all, just died. I took the car into the BMW dealer in Salem, OR and came back an hour later to be told that there was a BMW bulletin on it and they had replaced it with a new updated model at no charge. No comment at all from them about my AFE cold air intake. I was kind of surprised that they didn't use it as an excuse to not cover it. I guess that since they know it was a failure prone part there was going to be no argument from BMW. Now if it fails again with the updated part I think I'll put the stock air box on before taking it in to any dealer (as long as I'm not on a trip again).


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