Having problems with Idle .. after putting in Accufab Plenum

take the spacer off. Ive heard that if you use a spacer and the plenum, it will cause an idle issue. Also check for vacuum leaks, someone else on here experienced this problem and it was a leak

Vacuum leak

Installed the 75mm Accufab T/B and Plenum on my 03 GT about 2 weeks ago. Initially when sitting in neutral it would hang at about 1250 rpm after having blipped the throttle then settle down after about 5 secs. or so to a normal idle. Would also do this when coming to a stop. Read some good info here about vacuum lines so sealed all the line with high temp RTV and made sure I went through the proper procedure for resetting the computer. Everything is good now. Look in the section in your owners manual about changing the battery. It has the computer reset procedure and be sure you leave the negative lead from your battery off for at leat 45min. before doing it. When it resets you will see your guage needles go fully to the right, then back to normal. If you don't see this you didn't leave the battery unhooked for long enough. After you see this, follow the procedure in the manual.

Get rid of the plenum spacer!!!!
When you are removing it, disconnect the neg bat cable. When you're done, reconnect. Turn the key on and watch for all the gauges to peg then fall back down to zero. Then start the car. Mine still took a week of driving to completely settle down, but it wasn't idling at 1500 - 2000! (That's being caused by that spacer!) It was more like 1000 - 1200 for a couple secs, but now its more or less back to normal.

the spacer is the problem, take it off. the plenum has a built in spacer, so there is no need for another

I agree with canning the spacer. Also it may take the car a little while to start idling normal again. Give it some time.

No spacer needed man. After my install the car would hang at around 1300rpm when coming to a stop or when I would blip the throttle. It took around a week or so but it stopped doing it altogether. Just reset your eec or give it some time. It will drop on its own