Cas issue
 

Ok i got an s5 tii swap in a s4 fc. Been running great til past couple weeks. It has developed a stumbled under acceleration and sometimes at idle. I've changed plugs and wires and ran with leading coil only and trailing coil only and could still feel the stumble. So I got a used cas and it made a huge difference but it won't cold start on it. Ive killed my battery cranking on it. Then switched back to original cast and fired right up. Runs great hot on the used cas. But I can't wrap my head around why it won't cold start with it but will run great once it's warmed up. Anyone ever had this issue before?

close the air gap, clean the connectors or replace them. there is another connector under your brake booster too that everyone neglects to inspect.

what air gap? And what does the connector under brake boOster do? Thanks for the input