2000 blazer with 4.3 won't start when cold. Will try to start when you let off the ignition key. If it is too cold, I have to jump it to get it to start. Seems like it needs higher voltage when it is cold. If it doesn't start when the key is first turn and released, it won't start. Doesn't seem to fire when cracking. Doesn't have any problems when warm.. ie above 40 deg F or after it has been running. Replaced the fuel pump last winter and it seemed to resolve the problem but it's back again this winter. New battery didn't help.

Check for spark at two or more plugs while cranking, must be blue, not orange spark. If you have good spark, check fuel pressure and leakdown.