I had a customer that had a no-start condition in his 1990 928 GT. All the normal diagnostic checks were completed.
He did find that the engine would start if over one-half of the airflow to the MAF was blocked off (covered by his hand).
The test I recommended was to get the engine warmed up a bit. Remove the connector to the MAF, and start the engine. This forces the LH fuel injection module into "Limp home" mode. In this mode, the LH defaults to the follow pulse width:
RPM < 2000 RPM: 3.5 mS
RPM > 2000 RPM: 6.3 mS
With the limp home mode, you can imagine there are just a few RPM regions where the injection pulse provides the proper fuel mixture. However, unplugging the MAF provides a good test to see if your MAF has failed.
He found the engine did run better with the MAF disconnected. He sent it in and I confirmed that the MAF had drifted so far out of calibration the engine wouldn't run. Just to be sure his LH module was tested and confirmed functional. After recalibration service his 928 runs like new.