I have a 1998 DOHC Taurus that I bought new. It has 99,000 miles. My rough idle started Feb, 2008. I replaced the Air Control Valve first. The idle was OK, then it was rough again.
I took off the intake manifold and replaced the plugs, wires and coil pack. Installed a new intake gasket and all the vacuum hoses on the motor.
The rough idle is still happening, when I place the transmission in gear and/or when the heating/ac is on.
No Codes are being set.
I've been looking at the wiring schematic for the PCM. I am following the Gray/Red wire and what it touches=Knock Sensor, engine coolant sensor, fuel pressure regulator,
delta pressure EGR sensor, T.P.S., 4-O2 sensors, Turbine shaft speed sensor,
power steering pressure switch, transmission fluid temp sensor, transmission range sensor.

So I wonder if my problem is inside the transmission-a sensor? Or is it the fuel pressure regulator?
My engine idles OK in Park or Neutral.
If anyone has repaired such a problem I would like to hear from you.
I've checked other web sites, but have not found a solution.

 

Replied under your other post. I did not see this one. Please just post one time for the problem. In short, check MAF sensor.

 

I had the codes read today and had a misfire on cylinder #5 so I switched the coil pack to cylinder #4 and reread the code and #4 was misfiring so I ordered another coilpack. Will know tomorrow if it solves the rough idle. If not then I will get a compression test. The PCM update what is that all about...?