Check engine light: O2 sensor or vacuum leak?

[Bruno]

I am working on a '89 535ia.
Sometimes while driving the check engine light will come on, and come off if you release the accelerator.

The code says: O2 sensor.
But when I check the vacuum, I have a very low reading (10in) and if I accelerate, the car wants to stall then the RPM picks up and the vacuum reading reads 0....

Spraying carburator cleaner all over the engine didn't do a thing...

The car just warm up when the engine light comes on...
The car runs pretty rough when you just start it.

ARG.

[Jazzman]

Bruno,
A similar thing happened to my 740iL. I got a code 1222 for the O2 sensor with the engine running rough. I can't even accelerate out of drive without the engine running to almost stalling. So I limped home. I thought it was vacuum leak also but finding no leaks I suspected the O2 sensor. I replaced the two sensors and when I started it, it ran great for a while then the same symptoms came back. This time the engine stalled and I could not restart it. To make a long story short I ended up replacing the fuel pump because it was pumping but not that much and it was ready to give up. This weak pump caused the lean condition which the O2 sensor picked up and sent the signal to the ECM which triggered the CE light.
I would seriously look at the fuel pump 'cause your symptoms look exactly like mine. Good luck!

[BMWfreak]

Hi !
In my m20 525i I noticed a slight decrease in performance, sort of hesitation when trying to rev fast and hard. Also some days, but not everytime when sitting on lights my CE light would come on. When I even slightly touch the gas pedal it comes off ! Again, it happens sort of randomly !!! Other than that my car works pretty damn good for 173k miles on everything original. I just recently changed timing belt,V-belts, water pump and thermostat but I don't think it should matter.
Any help would be appreciated
Daniel

[Jazzman]

One other thing to check is the MAF sensor. If this is ready to fail it will exhibit the same symptom because it will either send more/less vacuum signal to the ECM. Bentley should give you steps to check the sensor. I just forgot the resistance reading on the sensor.

[MB535]

I get a check engine light rarely, but I've found the air door in the AFM stuck on its stop (the "engine off" stop) and had to remove a small build-up of crud there in the corner. After a bit it will clear tiself but if the car sits a lot (which it does0 sometimes it happens again. The car idles OK. when it happens but has no power until enough air comes in (with the engine sucking its lungs out) the flap slams open.