I would start with retracing what you had to disassemble. On my ‘04, which I believe is the same as yours, the intake manifold has to come off to get to the valve covers properly. The disassembly process involves multiple associated electrical and vacuum connections that could have been missed, along with the 6 o-rings that must be seated properly, and some aren’t highly visible. I failed to connect my EGR vacuum lines when I had mine off last time. If you missed a vacuum connection you will likely hear some kind of hiss, more or less depending on if it is a 1/8” or 3/8” connection. Most of these connections won’t cause it not to start at all though, they will more likely cause CEL to come on or run less than desirably.
