It's a heater wire sensor that, as flows across it, the engine determines how much more or less fuel is needed based on the airflow. So if you've got a highflowing cold air intake, it'll notice, or if your filter is due for replacement from being clogged it'll also notice. It's also helpful when doing heavy elevation changes where the air is getting thicker or thinner as you go.

There is also an air temp sensor that ties into the MAF (seperate unit) that measures how hot or cold the air is for density purposes. Early E39 M5's had the temp sensor located too close to the radiator, so it would get false high temp reading. The aftermarket caught onto this and sold relocation kits, but since then BMW figured it out and relocated them also, solving the problem.

Some ways you can tell if the MAF is failing is by using the OBC and watching the fuel consumption, but I don't remember where the breakdown tables on based on our engine size. Meaning that if your using plenty of fuel, but the car doesn't feel as fast as it could, that maybe the MAF is slowing loosing it's sensitivity to flow changes and your just wasting gas, in turn ruining your catalytic converters (common problem in earlier M62's). They're kind of like 02 sensors, after time of repeated heating and cooling cycles, they don't adjust to changes as quickly. If you're having starting problems, rough idle in the morning, flashing SES and backfiring through the intake (the worst to have), then try unpluggin the MAF and see if that fixes things. If it does, then you've found your culprit. One thing to note though, if you've got these issues, especially the flashing and backfiring, you'll be worse off if you reset the codes as opposed to just driving w/ the MAF unplugged until you get your replacement. Reseting the codes erases everything the computer learned when the MAF was functioning normally, so if you leave your bad MAF plugged in, then try to drive around on it w/ it giving bad readings, you'll definately be worse off that if you had left it w/ at least some amount of good learned settings.[/QUOTE]


Thanks, I sort of figured it had to do with fuel mixtures but this explanations really helps. I don't think my MAF is out of wack because I do not experience any of the symptoms you mention above. Once when I started her, there was a puff of bluish smoke out of the exhaust and a nasty smell emitted but it cleared up in a few seconds and never done it again.

My fuel consumption on the 4.6 runs about 18 to 19 mpg with straight interstate type driving. Just went on a long trip about 4k miles and averaged 18.1 for the whole trip. Running around town I am getting about 14 - 15 unless I get on it and it drops monumentally. The wifes 4.4 gets about 17 round town and as much as 23 on the highway. So long story short, I am getting about what BMW states on their sticker. I figured the difference in mpg between the two is the fact the at 4.6 has a little lower gear ratio 390 vs 360 for the 4.4 (and it is kinda fun to punch the 4.6 now and again). My old 840 had 282 gears (I think) and it would get over 25 on the highway at 85 mph (about 2500 rpm). The 840 would run 100 mph and the tack would not even hit 3K.

Oh I am getting off topic but thanks for the explantion. I took the beast by the indy shop today and they wanted $125 minimum to run a diagnostic. They said they would find the codes and then diagnose the problem. That is more than the stealership charges. We have an O'Rielys and an Autozone, so I will check and see if they can give me the trouble code(s).
Thanks again.