The first test you can do is to unplug it and see if the car runs better. This forces the ecu to trip to a default value.
Another test you can do requires a VAGcom
Enter Engine measuring block 003
Start Logging data
In a high gear (4th/5th), give full throttle from 2500 rpm up to 3500rpm.
Open the Log file
Graph MAF actual -vs- rpm in Excel
The MAF is OK if the actual value is at least 850-900 mg/h at 3000 rpm. if the power drops past 3000 rpm and the maf reading drops then it's usually an indicator that the maf is bad. Of course, you need a clean intake, air filter, etc., otherwise these also cause restricitons.
Unplugging it and seeing if the car runs better is the first step. HEre is another thing you can do.
warm up the car, and leave the MAF plugged in. If it can rev in neutral to5100 RPM, it's probably good. If it can't and won't rev to 5100 it's probably bad. Considering the failure rate of the MAFs, this test is pretty reliable.
i was looking into this, if you ahve a pre-2002 tdi, you can get an extended warranty on it for 7 years or 70k miles. granted, if you have a 1998 you probably have over 70k miles, but it cant hurt to ask!
found another way
Pin 5 is the MAF signal. There should be 0-.3 volts key on engine off, .7-1.2 volts at idle and 1.5-2 volts at 3k RPM. I have found that a weak MAF sensor will sometimes pass these tests. If you check the air filter, snow screen and intake and there is no problems you can be fairly sure there is no air restriction causing a low MAF reading.
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