I have a 2004 C70 with a 2.3l 5 Cylinder HT. We recently did an engine swap where we just replaced the engine with the same one, except the new one doesn't have rod knock After the swap, the car has a really bad idle, around 1k and just barely dies. There is also smoke coming out of the exhaust, not a huge amount but it's there. We are bringing it to the mechanics to hopefully do a re-tune, but does this sound like something I should worry about? Would a retune do the job?

 

with all the uncoupling of hoses during the engine work, there's a good probability you have an air leak - check vacuum lines, intake hoses etc.