I have a 2000 Taurus SEL with 109K miles that can't seem to figure out where it should be idling. It seems to be worse when the engine is still cold. When in drive, the idle will search from a state where the engine is barely running and the whole car is shaking to pulling against the brakes so hard you have to keep them firmly engaged to keep the car from moving forward. I get a CEL with a code P0151, which means system too lean in bank 1, but I have changed the O2 sensors and the spark plugs with no change. Once the engine is warm, it doesn't search in such a wide range, but you can still hear the idle changing every 30 seconds or so. I checked the EGR valve and the diaphragm seems to be OK. Anyone have any other testing I can do to try to diagnose the source of the problem?

Thanks in advance.

 

Does this vehicle have a Idle air control valve up near throttle body? If so Fords are pretty notorious for this. Could also be coolant temp sensor, mass air flow sensor. cam or crank sensor? Probably going to have to use trial and error. You might also have major vacum leak as this will also give false O2 sensor readings.