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Bad cold running FIXED! 700 1989

posted by  someone claiming to be Herb Goltz on  Mon Dec 30 04:19 CST 2002 [RELATED]

Hello All:

My '89 760GLE w/ B280F v6 had been running a little rich and ran very poorly on start-up when cold. It would start after somewhat extended cranking and then would run as if only 3 or 4 cylinders were firing for up to a few minutes. Eventually all cylinders would catch and it would run fairly normally afterward.

Once it was warm I tried to take some measurements. I wasn't sure whether the bad cold running and the rich fuel condition were related. The O2 sensor voltages were very low (~0.0xV) and didn't seem to be fluctuating the way I expected them to.

I hypothesized that the cold running problem might not be directly related to this apparently bad O2 sensor, since at start-up most of the sensors would not be giving inputs to the ECU. My best guess for the cold running problem was inappropriate fuel pressure caused by bad fuel pressure regulator. I ordered one from FCP Groton (only $36 for genuine Bosch!). I installed the fuel pressure regulator (easy job-- 20 minutes including clean-up) and 6 new NGK plugs, which fixed the cold running problem.

I observed that the exhaust smelled less rich. I got my multimeter out and took measurements at the O2 sensor after the car was warm, and lo and behold the 02 sensor was giving readings from about 0.2-0.85V again.

Thanks to Steve Ringlee and the great 700/900 FAQ for helping out with the diagnostics, and be sure to check your FPR if your 02 sensor checks out bad!

Herb Goltz, London, ON, Canada '89 760GLE w/170K mi






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