I have recently aquired a 2002 Trailblazer. It has 6 new coil packs and 6 new spark plugs. The parts are from a reputable company. The vehicle will start but runs very poorly.
I removed the coil packs and spark plugs and found that only cylinders 3 and 4 are firing. Any suggestions on how to diagnose the cause of this condition would be greatly appreciated.

 

Welcome from Florida. Try putting different coil packs on cylinders 3 and 4 and see if they still fire. If different cylinders fire, then you have some bad coil packs. Just because they are new doesn't mean they are good. Also check the wiring going to each coil pack for good connection and damage to the wires.

 

A couple of things.

One, unless you used ACDelco 41-103 (that were purchased at a local auto parts store or rockauto.com) that could be the problem. These engines are darn picky about which spark plugs go into them.

Two, double check and make sure the ignition coils are properly seated on to the spark plugs and that the ignition coil electrical connectors are properly seated.

Now, you said you purchased the parts from a reputable company. If so, then what are the brand names and part numbers? If you purchased them from Amazon or ebay, chances are really high that you purchased counterfeit parts.

Good Luck!