Ed Taylor wrote:
Been off the list for a while job change etc. Back to working on the beast.Any advise on troubleshooting the MAS and/or using the o2 sensor voltage to adjust the co pot? Question prompted since I have been under the intake replacing parts along with fuel pump and injectors. The voltage I have been reading from the o2 sensor will not remain steady. Any help will be appreciated.
Ed Taylor 87 s4
slowly making it right


It doesn't normally stay steady in most cases.

The oxygen sensor is a trimming device, located downstream. The LH ECU sets the fuel mixture by adjusting the duty cycle of the injectors, based upon input from its sensors (mass air sensor, temp, rpm, etc.) and the programmed map. It then trims the duty cycle based upon the input from the oxygen sensor.

Some over-shoot is normal, so the final ratio dances around stoichiometric.

The amount of dancing can be seriously affected by the oxygen sensor used. A sensor has a finite reaction time, largely determined by the exact material used, manufacturing procedures used, etc. The ECU has a finite reaction time, largely determined by design, but affected by aging. The more closely the reaction times match, the better the trimming matches what is needed, and the smaller the dance.

An aftermarket oxygen sensor will often cause the swings to be wider.

Unmeasured air (vacuum leaks) can also increase the swings.

Wally Plumley
928 Specialists


My goal here is to determine the health of the MAS. While I had the intake off for hose and injector rehab, I carefully removed the plastic cover on the circuit boards and co pot housing. Noticed corrosion etc inside. Sprayed with contact cleaner and resealed. In absence of a co meter I was trying to adjust co/idle mixture by reading o2 sensor voltage. Tried it with the sensor both connected and unplugged. Could not get the voltage to remain in the .47 to .50 volt range. Looking for values to check on the MAS in order to see if the corrosion I found has damaged the boards or co pot which would mean a replacement. I would like to know before I throw
money at it.


The O2 sensors' impact is largest at low rpm and low throttle position settings, like idle. At big throttle and big rpm the system runs "open loop." as Wally says. That means no input from the 02 sensor is used.
So? Please don't have any illusions about the 02 sensor correcting/adjusting your mixture for you in aggressive driving...it can't. This is why the engine mapping is so important. The 85-86 USA cars have a vacuum line that functions as a load sensor to move it "up and down" on the maps. These cars are very sensitive to vacuum changes (of course.)
The '87 and newer cars rely on a knock sensor to save the engine, should the calibration get too far out. Knock sensors don't work that well at high rpm, there are so many "noises"... The LH and EZ are fascinating systems, ahead of their time...as usual for the 928. Kim


I think you have no problem. The fluctuating O2 sensor voltage shows it and the LH are working correctly. The CO pot on the MAS won't make any difference in the O2 sensor voltage because the CO pot isn't used on your '87. Check the wiring diagram, page 10, right side at P85. The CO pot, pin 6, isn't connected. The CO pot is used on the '86/'86 cars.

You can check the operation of the MAS output at pin 5 if you want. With a cold engine at idle, the voltage will be about 3.1 to 3.2 volts. Voltage at warm engine idle will be about 2.85 volts, and will increase with engine load to a max of around 5.8 to 6 volts on a stock S4.

Louis Ott