The question is WHY ??? The question is WHY ???

BTW.. you are using the OEM Spec spark plugs Correct.. if not, stop and go the them. No fancy four-prong cross-fire platinium crap.. go get the OEM spec BOSCH only plugs. They may cost a little more, but they are the ones that work.

So, start here. (btw.. the A/C just puts a load on the engine and is most likely not the culprit)

so far, you have done all the expensive stuff (If the parts were new or rebuilt..) so try the cheap and easy stuff (BTW.. have you ever changed the fuel filter? )

Before you start anything realize these cars are FAMOUS for vacuum leaks, particularly vac leaks associated with the HVAC system.

Check the vac at idle by taking a reading straight off one of thenipples on the bottom of the airintake plenum. It should be 16-18inhg. If low, turn the HVAC off. It should go up. BTW.. make sure all nipples on the bottom of the plenum have hoses attached.

If you have a 1-wire o2 sensor and it has more than 40k miles, replace it. if a 3 or 4 wire and it has more than 70k miles, replace it (btw a dying o2 sensor has a lower voltage output and will make you go rich...high HC )

Next attach a DVOM to the o2 sensor lead with the sensor lead still connected to the motronic. It should be fluxing from about .4vdc to about .6vdc. (both at idle and at all driving speeds.

If not in those ranges, pull the vac line off the fuel pres reg on the injector rail. Look for a momentary change on the DVOM. Reconnect the vac line.

As a point of reference, and to see just how much correction the motronic and o2 sensor are putting into fuel delivery, go 'open-loop' by opening the connection between the motronic and the o2 sensor. keep the DVOM connected to the o2 sensor lead. I suspect the engine will go VERY lean, maybe even die

If no change, check the fuel pressure to the rail by putting a press gauge in-line where the fuel line meets theback of the fuel rail (the end opposite the FPR (youwant to read the line pressure betweenthefuel pump and the fuel rail. It should be about 28psi with the vac line attached and about 40 when the vac line is not connected to the FPR. (check the Bentleys e28 manual for the exact press range based on having a 2.5 or 3.0 bar FPR.)

anyway, try these things and let us know what you find..

Good Luck .. JS

Regards

John Stein

• Engine Ping - beanie 2008-12-02 23:03:37 (59 views)
  • if pinging .. it is most likely lean... - johnhstein, 85-M6CSi, 85 745i 2008-12-03 15:52:42 (67 views)
    • Also, check the oil pan bolts and spray carb cleaner - Vern 2008-12-05 15:08:14 (31 views)
  • do you have a vacuum reading and a - sfdon 2008-12-03 14:58:43 (45 views)