uneven idol

My dads 635 csi has an uneven idol, dropping down to about 500 rpm, then jumping to about 1000. Once then engine warms up though it stays at 750. Ive been told it's probably the oxygen sensor from a friend that had the same problem with his bimmer, but not positive. Any ideas on how much one of these would cost, or on whether or not the oxygen sensor is the problem?

It's possible that it's your oxygen sensor, but I'm not entirely positive. My jeep was surging really bad last week and I suspected it was the oxygen sensor, so I replaced it. Turns out it hadn't been changed *ever* so it was good I had done it, and it fixed the surging. Other symptoms of an oxygen sensor that is either failing or not functioning (besides the uneven idling/surging) is poor emissions and poor gas mileage.

I checked Autozone.com for an oxygen sensor on a '86 BMW 635CSI and it listed a Bosch (OE type) one for $124.99.

Napaonline.com lists three different ones:
Beck Arnley (OE type) $209.00
Beck Arnley (replacement type) $94.99
Echlin Ignition $134.00

THANKYOU VERY MUCH!!! that is a lot of help. i guess its going to be a fairly hefty blow to the wallet, but hey for a 15 year old car, what do you expect?

It won't be your o2 sensor. The sensor is out of the loop during warmup. Once you reach operating temp it is back in the circuit. Since it runs ok when warm, check the cold start injector, temp sender and thermal time switch.

chappy 87 - 635