Ford Repair: Intake manifold runner control, ford taurus, imrc

Question
Hi Dennis,

My 1999 Ford Taurus (67,000 miles) had been idling rough a couple of weeks ago and so I took it to Ford to have it looked at (could feel hesitation at a stoplight in the brake pedal).  They hooked it up to the computer and there were no fault codes so they did a fuel service on it (injectors, throttle body, etc.) to see if that is what it was.  It seemed fine for about a month, but then yesterday the "service engine soon" light came on and I could feel the rough idle again.  So I took it to a local mechanic who hooked it up and the IMRC was fully stuck open of which I had to replace it at a hefty price (plus perform another fuel service since carbon had built up again).  My question is would a faulty IMRC cause rough idling?  Also, I think Ford recalibrated/retested the PCM so not sure if that did anything.  Thanks for your help.  

Answer
Yes, a faulty IMRC can cause rough idle if stuck open. With the IMRC open more air is introduced at the wrong time which has the potential of leaning out the mixture.

Of course there are other things that will cause rough idle. Doing two fuel services in a month seems a bit exessive, tho, unless it was done poorly.