I just installed a 5/16 MD Copper ISO Thermal plenum spacer on my 03' yesterday. When I started the car I noticed it had very rough idle (after a few seconds it feels like it wants to turn off), so I followed the instructions that were provided to me by MD to recalibrate my throttle body. And after trying to recalibrate my throttle body it still has a very rough idle.

At the very last page of the link it describes the procedure I followed IOT recalibrate my TB. Any suggestions?

http://www.motordyneengineering.com/manual.pdf

 

Check for intake / plenum leaks. There lies your salvation.

 

Some '03s had a TSB on the TBs for them not wanting to recalibrate once losing calibration. I thought that may have been the case on mine, but i was actually not completing the TB reset procedure properly. Once you go through it and start the car, your Service Engine Soon light should flash as you rev your RPMs. If it wasn't flashing when you started the car after completing the procedure, then it didn't work. That did the trick for my '03 after losing calibration.

Only other thing could be a fouled install. Did you use a inch lb torque wrench? Did you follow the tightening procedures? How about the 3 hoses that connect to the plenum, are the hoses in place and snug? I would check over everything again.

 

-when idling, does it drop under 500 rpm?
First you should check the plenum for leaks. (do you hear hissing from the engine bay?)
Make sure that the copper TB mod is installed properly. I had a similar issue and when I re-installed, it went away immediately. You may also want to make sure your MAF is clean and undamaged.

 

if you touched the butterfly in the tb you need to do a throttle relearn, else check the bolts on your plenum