I have been trying to adjust the mixture on my 1983 928S (US version) and something just doesn't add up.
The voltage reading on the O2 sensor will not go above .02 volts even when the mixture screw is all the way in. Even at that setting the car doesn't run very well, it's a bit rough and seems to be lacking power. Backing the screw out (leaner) it really runs poorly.
Several questions;
1. Do all O2 sensors put out the same voltage for a given O2 level? I'm wondering if the right part is in the car.
2. Is there a way to test the airflow sensor? Has anyone ever taken one apart? Will WD-40 loosen up whatever is being adjusted by the mixture screw? Or should I just get another one?

Thanks,
Phil Silano

 

The O2 sensor may be dead!
With the engine at full operating temp, the sensor must average ( in a slow oscillation) ~0.4v when disconnected. You can drive it to the limits of its range by introducing false air( leakage not measured by the AFM), or excessive fuel ( vac line pulled from the FPR).
If no voltage response, try almost any single wire O2 sensor. If that shows life, solder it into the old harness .....

 

It takes a good 5mi at 2000 RPM for the single wire o2 to heat up and respond. Reviewing the wave form, my o2 switches at .4-.5mv. Lowest MV is .2, highest .8 mv
Using a vaccum gauge, measure the vaccum. Should be 17" hg. L-Jets have a rough idle at 14" hg.
AFM can be bench tested using a ohm meter and ohming the door swing.
If you never replaced the vaccum hoses start their first. Don't forget fuel quality and last time fuel filter was replaced.

 

Another interesting thing. On checking the AFM using an Ohm meter the readings on pins 7-6 and 7-8 are all over the board. I was expecting a linear change from low to high as the door swung open. instead the readings fluctuate up and down, and at times are out of range. Is this thing scrap??

Phil

 

The response should be linear with no 'drop-outs' : if otherwise on the correct pin numbers, it would appear that the 'wiper' contact ( moved by the door hinge) is making intermittant contact with the coiled resistor wire along its arc of travel - how nostagically analogue .
Someone here has a web page demonstrating the cover removal and internal tune-up of an AFM - will post if I remember where to look ...

 

I have already taken the cover off the AFM. This one does not have a coiled wire but a circuit board with a printed arc that the wiper moves along. I have tried polishing that section of the board with a mild abrasive with no effect. The groves that have been worn by the wiper contacts seem to be pretty deep.

Phil