The problem Id like some help with is repeated hesitation when I let off the accelerator-- this occurs at all rpms and definitely not specifically 3800. The check engine light comes on at each hesitation sequence, with the frequency it looks like the check engine light flickers. This does not happen any other time when I'm on the accelerator, if at a complete stop, or if I roll in neutral-- only when in gear and off the gas. This also did not happen at all before the install and I've had it for about 1000 miles (3 months-ish).

Am I not getting enough air flow because the air passage is not from the front anymore? TPS? Throttle is a little slow. Some other issue? The hesitation threads I've found have to do with the 3800 kind and miscellaneous ones that don't describe my specific situation.

The car is entirely stock except for the koyorad. Passenger muffler has cracked welding and buzzes off accelerator. It is NA and passed Cali smog about a month ago.

After putting the koyo in, a couple of fitment issues cropped up:
-- Stock air tube doesn't fit over the rad anymore
-- Fan shroud is not fastened onto the rad on the driver's side

 

Check codes to see why the light is coming on: http://www.johnr.com/cpucodes.html. Just switching the radiator would have no effect on drive-ability, so either something else was changed unintentionally (loose plug, etc...), or it's coincidence.

Airflow isn't an issue, but without the stock duct you'll be pulling warm air from the bay. What's preventing it from clearing? When I had the stock fan shroud and intake on my car I was able to trim some plastic to make everything fit with my S4 Koyo. The tab on the top of the shroud got cut off, and part of the duct was cut/filed to fit.

 

Thanks, the fitment issue has been taken care of and the duct is back on. I'll be checking the codes later today.