Engine runs rough after warming up
 

The previous owner of my vehicle was running on leaded fuel.
I was reading on the Haynes manual that in order for the oxygen sensor to work, the engine must always run on unleaded fuel.

Could the oxygen sensor be damaged causing rough idle after the engine has warmed up? The Haynes manual also says that the oxygen sensor is by-passed until the engine reaches it;s normal working temp. The engine does seem smoother when cold.

Can any of you comment please...

Yes, Leaded fuel will kill O2 sensors and also the CAT. So if your O2 sensor is dead you can expect running problems like you discribed, i.e. OK when cold and bad when the engine warms up. You can I think easily verify the O2 sensor by just disconnecting it and see if the engine runs rough without it. However, if your CAT is dead the O2 test will probably not work.

If your car is a california model, it has a heated lambda, the rest of the us has to wait for the sensor to warm up from exhaust before it starts to work. You could always unplug the sensor and see if your problem goes away. It's the quicker checker upper.

Thanks for that info. The car no longer has a cat. I think my car is a German model, but I'm not sure.

Where is the location of the Lambda sensor plug? Is it near the Battery?

If it does not have a CAT it may also not have a O2. But if it has one you will find it bolt on the exhaust pipe between the exhaust manifold and the first silencer. If you can find it then follow the wire until it reaches the first connector. At the connector you can disconnect it and check how the engine behaves when warmed up. If you don't have a CAT you will be better off to adjust the DME to run without the CAT maps and get rid of the O2 sensor altogether. You will only need to adjust the idle mixture in a traditional way using an exhaust analyser. And if you can get hold of leaded fuel where you are you can continue using it once you rid of the O2 sensor.

0₂ = Lambda = Stoichiometric (or sumthing) = a darn fine mixture