I replaced my seized 4.2 with a used motor that has 66,000 miles on it. Everything was fine till a couple weeks ago when i went to start it and i had no compression at all. Then after a few cranks certain cylinders would have compression but still not start till finally it would start but run very rough for a 2-3 minutes. Once it was warmed up it would run smooth and start with no issues at all. I took it to Chevrolet and had a diagnostic run on it and i was told low compression in #1. I just don't buy that it must be a single source problem. I know this 04 is the first year of V.V.T. and maybe that's where my problem is