hi just put new intake manifold gaskets on my silverado 4.3 ,and its not firing on number 5 ,its got good spark had new plugs and leads ,had comprssion test on 3 and 5 they both had same reading ,swaped number 3 injector with number 5 pulled plug lead off number 3 and engine noise changed pulled number 5 lead no change ,not loosing any water not over heating starts up on first turn ,dont know what to look at next ,someone told me it could be distributer shaft could have a small crack in it but would have to knock pin out and take cog off to

see saft .any help im in uk and not getting anywhere thanks hope someone can come up with something