I'm hoping this would be a install error or leak.
I removed a 70mm (Full Blown) TB and installed a 65mm ported stocker TB using the stock gasket. I do have a Rev3 intake manifold ported to 70mm, but TB didn't seem to leak.
It was not until the second day of using the car that the code appeared. Note that I did see rpm fluctuations especially when clutching in when coming to a stop. Ranging from 1k to 1.5k.
The egr sensor on the manifold has been blocked, so it was a straight fwd install.
I know the p0442 is typically the gas cap or the evaporative system. Could it be a coincidence, or the gas cap or evaporative system is bad? Or the TB has a leak causing issues?

Thanks!

 

A defective or aftermarket throttle body will not set evaporative system diagnostic trouble codes.

 

How about a leak due to the TB?
I assume the RPM fluctuations are caused by the TB having a leak? or could it be the evaporative system?

 

The evap system has nothing to do with your throttle body. It sounds like you may have 2 separate issues; P0442, which will need to be diagnosed, and an engine idle problem that needs to be diagnosed. I believe that the two are not linked.