I live in CA. I have a 1989 Jeep YJ, 4.2L, Auto all stock (Carter BBD). Couple weeks ago I went to get my jeep smogged. Unfortunatly, it failed NO at low speed (15mph). It passed NO at 25 mph. All test done in Drive position.
Two years ago it failed the same way. That time the technician put the tranny in 1st gear to get the NO value down. He explained that the higher RPMs would increase the engine compression, therby lowering the NO levels.
The question I have is what causes the high NO levels at the lower speed?
Do I need to replace the O2 sensor, Pulse Air upstream and downstream air valves, checkvalve or whatever???
Please help....