MAF, transmission light
 

2001 BMW X5 E53 3.0 160K miles
Got codes for MAF replacement.
Also, transmission light came on after changing bad alternator.
Disconnected battery & transmission light went off.
Past few days, car runns fine but transmission light came back and downshifts hard..
Replaced MAF & disconnected battery again and transmission light went off (shifts fine now) but truck runs like crap.
Bad acceleration, feels like misfiring

Scratching my head!

You need to scan for BMW specific codes. If you get a bad misfire the computer will shut down that cylinder and you'll get terrible performance. that will set off a ses light though so I'm thinking check fuel supply.

What brand MAF did you buy? I have seen a lot of cheap units cause huge issues. An OE manufacturer Siemens/VDO on sale at FCP Euro for $143 right now and has high marks as a replacement.

If it helps in the troubleshooting, your engine should idle just fine with the MAF disconnected. I've got a 2001 3.0i as well, and have done this many times. You'll get a warning message (I forget what), but it resets itself when the MAF is reconnected.

So if you can't idle well with the MAF disconnected, you've got other problems, and it may be easier to find and fix those problems before bringing the MAF back into the system.

I was going to mention: unplug maf to eliminate that variable


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I had AZ clear the codes in the system.
With MAF unplugged, truck still runs like crap, rough.
Transmission light came back on.
I know if I clear the light (disconnect neg terminal) the light will go off but still runs rough, even with MAF unplugged.
But as soon as I plug it back in, tranny light will reappear.|
Been through this a few times, same result.

I have to get the new codes read now.

got the codes read.....

now it shows cylinders 2,4 & 6 are misfiring w/fuel cut-off
random misfire detected
System too rich Banks 1,2




WTF, that wouldn't go bad all of a sudden.
ran good before MAF was replaced

very strange

- UPDATE -
Replaced MAF
Replaced plugs & coil packs
Had a leak so replaced radiator while i was at it

NOW - Started it up and it seemed to idle & rev great in park.
As soon as it went into reverse, it started to stutter.
The SES & transmission light came on AGAIN
I drove it around the block and it ran like crap.
I disconnected the battery to clear the transmission light and started it up again.
It still ran crappy
I disconnected the MAF and it bogged down a little & the transmission light came on immediately after.
If it was a bad MAF, what would it do?
Why would the Transmission light come on after disconnecting the MAF?
Wouldn't that mean the MAF is good?

A genuine Siemens MAF or some cheap ass crap bought on eBay or Amazon? If it's not a genuine Siemens MAF there's your problem.

I've got a 2001 3.0i just like you.

Just now, as a test for you, I went out there (had driven the car earlier, so it was warmed up), unplugged the MAF and drove it.

After starting it up, no problems at all. As soon as I shifted into R (from P), the TRANS. FAILSAFE PROG warning came on. I went on a little drive. The shifting was definitely affected, much clunkier shifts, which makes sense, since the TFP warning is that the system thinks something is wrong and is using the AT in a safe mode to prevent damage. But other than the clunky shifting, the car drove fine. As in the engine seemed perfect as it always is, smooth, powerful, no missing, no initial problems, no high RPM problems. Just fine. Now, I expect there is a loss in efficiency and probably high end power, but nothing I could tell.

And if your MAF is unplugged, it does not matter how bad it is (unless it is obstructing airflow, in which case you could just remove the whole thing). It could be completely dead and broken and if it's not plugged in, it won't be making anything worse, and the rest of the system can run just fine without it, as my test above shows.

So as I mentioned a few posts up, if you unplug the MAF and the engine won't idle and (as I now have confirmed) drive just fine, you've got problems beyond the MAF. You may have problems with the MAF too, but it only makes things tougher to solve multiple things at the same time. So I'd focus on fixing whatever is broken outside the MAF first.

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: