E34 CEL, O2 sensor, control and MAF codes

[Heaviside]

I've been chasing an O2 sensor check engine light for the last few months and as I'm running out of ideas, I figured I'd ask around here. I've done a few searches and can't find anything in the same vein as my issue, but if I missed something, I'm more than happy to be pointed to the relevant thread(s).

I have a '94 525i M50 with just over 200k on it. A few months back I got code 1221: O2 sensor fault code (and the car started to run very rich). I naively assumed this was the exact problem and replaced the sensor. The light came back on, so I replaced it again, figuring the odds of a faulty replacement weren't zero.

After some further inspection (and rough idle +loss of power), I found the ICV hose was cracked and had that replaced, light still came back.
Getting desperate, I replaced the fuel filter and had the engine smoke tested (no leaks) and saw no change.

Now I also get the MAF code, but the engine definitely responds when the MAF is unplugged and I can't hear anything from the 'tap test' that I've seen people use.

I have two main questions:
Are there other issues that could be causing these fault codes and the car's bad performance or is it most likely a faulty MAF sensor?

What's the deal with the super cheap ($30-60) MAF sensors on ebay? I.e. is it worth $30 to check if my MAF is faulty before committing to the $200+ Bosch or are these as shoddy as they seem?

[shogun]

Super cheap MAF from Ebay:
My recommendation is to buy a quality Brand, or at least a remanufactured quality one by the roginal maker.
On MAF's there is basically nothing to repair by a DIYer.
They are usually sent back to Bosch as core. Just some months ago I was told by an E31 850 owner from Germany who works for Bosch the following:

The MAF's are refurbished in our factory in Goettingen, Germany. In most cases only/just the hot wire is damaged. But in any case after such a repair they have to be calibrated again.
I recommend not to buy the cheap copies from the internet for $60 or similar. Often we get such socalled "repro-parts" into our quality department for testing. Sometimes they even have the Bosch logo and part number (=copies), but only the looks from outside is same as the original Bosch.
In most cases the nominal values/curves tested are far apart from the original Bosch MAF. In best case you buy in such a case just a higher fuel consumption, worst case scenario is a damage to the engine due to too lean nominal curves for fuel/air ratio.

You may be lucky with such a cheap no brand MAF, but maybe not.

try a MAF tap test
with the engine at idle speed, gently tap the MAF sensor with the fingers of an open hand, if the engine stumbles or stalls, the MAF sensor is defective.

http://www.fourwheelstampeders.ca/wiki/index.php/MAF_Sensor
http://www.carlton24v.co.uk/afmmaf.htm
http://www.bba-reman.com/content.aspx?content=air_mass_meters_audi_ford_sea t_skoda_vw_vauxhall


An engine with a bad MAF sensor may be hard to start or stall after starting. It may hesitate under load, surge, idle rough or run excessively rich or lean. The engine may also hiccup when the throttle suddenly changes position.
If you suspect a MAF sensor problem, scan for any fault codes. A MAF problem should (but does not always) set a fault code.

Of course, don’t overlook the basics, such as low engine compression, low vacuum, low fuel pressure, leaky or dirty injectors, ignition misfire, excessive backpressure (plugged converter), etc., since problems in any of these areas can produce similar driveability symptoms.

MAF sensors can be tested either on or off the vehicle in a variety of ways. You can use a MAF Sensor Tester and tachometer to check the sensor’s response. If testing on the vehicle, unplug the wiring harness connector from the sensor and connect the tester and tachometer. Start the engine and watch the readings. They should change as the throttle is opened and closed. No change would indicate a bad sensor. The same hookup can be used to test the MAF sensor off the vehicle. When you blow through the sensor, the readings should change if the sensor is detecting the change in air flow.Another check is to read the sensor’s voltage or frequency output on the vehicle. With Bosch hot-wire MAF sensors, the output voltage can be read directly with a digital voltmeter by backprobing the brown-andwhite output wire to terminal B6 on the PCM. The voltage reading should be around 2.5 volts. If out of range, or if the sensor’s voltage output fails to increase when the throttle is opened with the engine running, the sensor may be defective. Check the orange and black feed wire for 12 volts, and the black wire for a good ground. Power to the MAF sensor is provided through a pair of relays (one for power, one for the burn-off cleaning cycle), so check the relays too, if the MAF sensor appears to be dead or sluggish. If the sensor works but is slow to respond to changes in air flow, the problem may be a contaminated sensing element caused by a failure in the self-cleaning circuit or relay. With GM Delco MAF sensors, attach a digital voltmeter to the appropriate MAF sensor output terminal. With the engine idling, the sensor should output a steady 2.5 volts. Tap lightly on the sensor and note the meter reading. A good sensor should show no change. If the meter reading jumps and/or the engine momentarily misfires, the sensor is bad and needs to be replaced. You can also check for heat-related problems by heating the sensor with a hair dryer and repeating the test. This same test can also be done using a meter that reads frequency. The older AC Delco MAF sensors (like a 2.8L V6) should show a steady reading of 32 Hz at idle to about 75 Hz at 3,500 rpm. The later model units (like those on a 3800 V6 with the Hitachi MAF sensor) should read about 2.9 kHz at idle and 5.0 kHz at 3,500 rpm. If tapping on the MAF sensor produces a sudden change in the frequency signal, it’s time for a new sensor.

If the readings jump all over the place, the computer won’t be able to deliver the right air/fuel mixture and driveability and emissions will suffer. So you should also check the sensor’s output at various speeds to see that its output changes appropriately. Another way to observe the sensor’s output is to look at its waveform on an oscilloscope. The waveform should be square and show a gradual increase in frequency as engine speed and load increase. Any skips
or sudden jumps or excessive noise in the pattern would tell you the sensor needs to be replaced. Yet another way to check the MAF sensor is to see what effect it has on injector timing. Using an oscilloscope or multimeter that reads milliseconds, connect the test probe to any injector ground terminal (one injector terminal is the supply voltage and the other is the ground circuit to the computer that controls injector timing). Then look at the duration of the injector pulses at idle (or while cranking the engine if the engine won’t start). Injector timing varies depending on the application, but if the mass air flow sensor is not producing a signal, injector timing will be about four times longer than normal (possibly making the fuel mixture too rich to start). You can also use millisecond readings to confirm fuel enrichment when the throttle is opened during acceleration, fuel leaning during light load cruising and injector shut-down during deceleration.

[whiskychaser]

In short, forget cheap MAFs. Get hold of a genuine working replacement from a friend or don't bother. If you have an air leak, you may find that the engine actually dies when you plug the MAF back in. What did you use for the smoke test? I have a relative in the entertainment business and used a smoke machine worthy of a rock concert. It was great fun but didn't really highlight the problem. Simply remove the oil filler cap and the engine revs should die a little. If they don't and the filler cap seal is in good condition you may well have found your problem - you have a leak

[Heaviside]

Thanks for the quick and thorough responses.

The engine changes rev a bit but doesn't die when the MAF is plugged back in. As for the smoke test, I had a shop do it with their proper smoke machine; we filled it until it came out the air box and saw no other leaks. I can try the filler cap test just to make sure.

It sounds like my best bet is to test the MAF with the digital readout and tap test and/or replace it with a bosch and see if my problem goes away.

[632 Regal]

You have reset the computer after doing each of these changes?

Try this link if you haven't:

http://www.bimmernut.com/forum/search.php?searchid=52535