From my review of the service information, it appears that lamda equipped M103s do not have the intake air temperature sensor on the inlet duct. Being as how the lamda system fine tunes the mixture an intake air temp is not needed.

Even if your O2 sensor was dead or the lamda system was otherwise malfunctioning, I would not expect fuel economy to drop to the levels you reported.

If you have a multimeter with a duty cycle function you can check the "duty cycle", which is a test of overall lamda system function including the 02 sensor. The following web page probably gives better instructions for running this test than the service CD.

www.landiss.com/mixture.htm

The duty cycle should fluctuate at idle and 2500 and the approximate eyeball average duty cycle at the two speeds should be within ten percent of each other.

You can also remove the RH front carpet section and do a static O2 sensor test. With the engine running remove the single wire plug (which should illuminate the check engine light) and measure O2 sensor voltage. It should be over 0.45V. You can also disconnect the two wire connector - check the O2 sensor pigtail for heater continiuty and check that the harness end has system voltage.

It's best to remove the strut to replace the upper mount. Chances are the dust boot is toast and maybe the jounce bumper. It's a fairly easy task to R&I the struts. Jacking up the front of the car the car up and lowering it down on jackstands under the control arms will keep the springs compressed with front end weight while you remove the strut to replace the mount.

Duke