chrism928

I need some help here (of course). I have an 85s, ROW spec. It wasn't born with an o2 sensor. However, some time in its past lives it has had a potentiometer added which varies the fuel mixture. Ransacking the archives I have found a few posts that mention voltages across o2 sensors etc, which makes me believe that the pot has been added in lieu of the o2 sensor. Someone told me it was not an uncommon mod to XJSs of the same age.
I have just had a new air mass meter installed and would like to be rid of the pot of possible to get everything back to spec. However, just disconnecting the wire that goes to the LH brain results in a very unhappy engine indeed.
Anybody heard of this modification, how it was installed etc? Should I just leave it alone? Runs great with in, but exhaust gas is rich. Thanks,
CM

ColinB

Chris
I'm no expert but I believe the ROW (non catalytic converter) cars were fitted with the pot you describe as standard. In my RHD 89GT it's mounted on the right bracket of the ABS control box. I'm told it's for (idle?) CO adjustment.

Colin

Max

Chris, I think Collin is correct. I have seen referance to this in the wire diagrams, but have never seen or heard of one hooked up this way.

Your mechanic would be right about the use of the pot (at idle) and I think to some point before WOT.

I as well as others here would be most interested in the deatails of this Pot(values)and how it is hooked up. Does it have a Porsche Part number on it?

The O2 senor that the 928 uses is a narrow band sensor and by todays standard tells the LH\EZK very little. If your car runs fine with the pot I would leave well enough alone.


Cheers

Max

Mike Schmidt

Some cars came with the adjusting pot as described above. The pot is not there in place of the O2 sensor, but rather adjusts the signal from the MAF sensor. This adjusts the mixture. Some MAF sensors have the adjustment as an allen screw right in the sensor. The screw usually has a plug over it to prevent tampering. In other cases the pot is used instead of the MAF having the adjutment on it. When you unplugged the pot, you gave the fuel injection computer the indication that the MAF wasn't working. In that case it goes to a default mapping that's a rich mixture. If you don't have an O2 sensor in the car, the pot should be adjusted with an exhaust gas analyzer hooked up to the car.

chrism928

Thanks all. My pot is definitely of the apre-factory variety, and is mounted under the parcel shelf in the passenger footwell.
I will fiddle with the settings and let you know what voltages apply and resulting mixture ratios.
Chris.