Yet another site with info. These things might be avenues. This one points out some more ideas.
tech tips When manning data stream, check the PRNDL section. Shift from park to reverse to neutral to drive. Does the data stream indicate the movement? Some problems that may occur if the PRNDL is not working include no timing advance, set fuel delivery and pinging. If the PCM thinks the car is not driving, it will not advance timing, keep fuel delivery to a minimum and not turn the EGR valve on. Bad PRNDL has also set VSS codes, because the PCM does not believe the car should be moving -- obviously a bad VSS signal!
EGR Problems
..... If a car has an over-active, or non-controllable EGR valve it may develop extremely lean misfire. The EGR reduces oxygen content in the exhaust, and if too much EGR is sensed by the 02 sensor, it thinks the car is running rich (not enough free 02), sends the PCM the message to lean the mixture. Leaning the mixture, on top of too much EGR, and the misfire causes more oxygen to go downstream -- well, you can see what happens to the AF mixture. This type of misfire will actually get worse when propane is added, leading you to the EGR problem.
