The MAF is there just to tell the computer how much air is being flowed through the intake system. It uses either a hot wire or a hot film that heat up and the air that flow through it will cool the hot wire or hot film. From the amount of time it needs to cool the hot wire or hot film the computer can determine how much air is flowing. The computer uses the air flow to adjust the injection pulse to establish the correct air-fuel ratio.

When you disconnect the MAF the computer is aware that the MAF has been disconnected and will a used a set of pre-determine fuel map to run the engine.

You should get a scanner that can provide real-time data such as short and long term fuel trim, as well as air flow (g/s) from the MAF. Throwing part at it is not the correct way to diagnose the problem.

But oldskewel,
Running the vehicle while in "limp mode" is bad too, isn't it?
The fact that it "runs" better would point to the MAF, wouldnt it?
It "drives" worse without it plugged in, due to the tranny issue, but it "runs" better with the computer estimating the Air Flow without the MAF Sensor.

About 3 years ago, I had the same exact thing happen in my 3.0i 2001 X5. I bought a cheap MAF, and got all kinds of other problems. The minute I replaced it with the Siemens MAF it all went away. I know, its expensive but it looks like it's the only one that works in this SUV.

This was already pointed out in this post.

https://xoutpost.com/1140715-post9.html

My basic comment / thinking is to try to help you to consider the problem in a simpler way to help you solve it.

"limp mode" - does that mean "TRANS. FAILSAFE PROG"?
If it does, from what I could tell from watching my car idle and drive, the engine ran perfectly. I doubt it was harming anything. Like I said, surely it was less efficient, and less powerful if I were to floor it. But the ECU is apparently capable enough to estimate airflow based on all the other sensors it has to make things run well enough. There was a noticeable clunkiness in the AT shifting, and I would be concerned about driving it with more power in that mode - worried about the AT getting damaged.

And I am absolutely NOT suggesting you don't have a problem with your MAF. You might have a crap MAF as upallnight and others are suggesting. I am not disagreeing with that at all. If I had one of those crap MAF's in my car, it would run like crap. Then I'd unplug it and it would run perfectly except for the AT failsafe trigger. How easy would that diagnosis be?

By unplugging the MAF, you take it out of the problem. Once unplugged, unless it is physically blocking airflow, the car should run fine - as I've explained and tested for you on my own car. So if you still have engine problems with the MAF unplugged, you've got problems other than the MAF. And if you find and fix those problems to make the car run well again (yes, again, this should be possible), at that point you can plug your MAF back in. Maybe it will run fine, or maybe like crap. At that point, diagnosing the MAF should be pretty simple.

So the debugging concept will have taken one complex situation, and reduced into two simpler ones just by unplugging a connector.

If the MAF is fine and working, then it may be able to mask some of the problems with the rest of your engine - again, making it harder to find them.

And I really don't follow your questions quoted above. Are you talking about my car, your car, or in general?

For example, separating "drive" and "run". It's easy for me to feel changes in this, since my car basically does everything perfectly right now (just finished engine rebuild). But in my explanations, I tried to separate how the AT shifting feels (would lump that into "drive") vs. how the engine rev, smoothness, power feels (would lump those into "run"). Separating variables might help your diagnosis.

Limp mode means the engine computer has shut down one or more cylinders and you have very limited power.

Transmission failsafe is different: the transmission will lock into 3rd or 4th gear until the problem is resolved.

With limp mode the SES/MIL will be on with trans failsafe the gear icon will be lit.

It's important to use the correct lingo.

Yes, very important. Thanks for clarifying. As I mentioned, my comments relate to the TRANS. FAILSAFE PROG, so if the OP really meant something other than that, then it's important to note the difference. :thumbup: