OK, 2000 F-150. Rough idle with A/C on only. Replaced, plugs, cleaned injectors and both O2 sensors. Still very rough only with a/c on. When I get going it smooths out. Only a problem at idle. Mechanic thinks it's an electric ground problem. Oh, tried replacing the A/c relay switch too. Has anyone had the same problem and found a fix? I did a search and found LOTS of idle problems but nothing like this with the same process of elimination. The mechanic is down to wiggling wires while another watches the scope(he's using a Genesis 3). I've got a K&N air intake, dual exhaust and a Jet chip. Has run like a top for almost 4 years until a month or so ago. The mechanic called some hotline with Ford engineers and got nowhere. Appreciate any help here. Oh, scope shows flatline on right O2 sensor when he turns on thr air. Like I said before, he just installed both new sensors.

 

I want to add that we are sure it's a grounded wire somewhere. With A/C compressor disconected the right o2 sensor flatlines when the air is turned on. When the relay for the a/c is pulled it reads normal. When it's put back the sensor flatlines. It's a new sensor and relay. So it's a bloody wire grounding out and tomorrow the mechanic is going to spend all day trying to trace it down. If anyone had this problem and knows which splice to check please let me know. This is REALLY frustrating!!!

 

OK, Ford in it's infinate wisdom placed the battery over the wiring harness for the computer and right side O2 sensor. Seems like that would have been a good place for the air cleaner and the battery could have been on the left. Anyway, the mechanic unwraped the harness leading to the O2 sensor, seperated the wires and found acid damage on one length of wire. He jumped the damages section and the thing scoped like it should. Hopefully once he does the wire repair and puts it back together it will be finally fixed. But it since it now reads the correct voltage I should be golden. Now I just need to find a way to protect the harness in the future. There is no good way to get it far enough from the battery to completely fix the problem. For all of you with unexplaned rough idles, have the harnesses check for acid damage. My grief (and money wasted on parts not needed) may work to your advantage.