03 v8

I just swapped engines in my 03 v8 due to a spun main bearing and bad crank. I renewed the gaskets in the "new" engine and mated it in the Rovah. This was a running engine when I got it. However now the engine will not idle unless holding the fuel peddle at 1000rpms. I have swapped the mass air flow, tps and crank sensor, as well as the iacv. When motor went in it was fine, two days later and a new exhaust it does this. Any ideas would be great.