My 2004 blazer with 202,000 miles has a miss only after it is driven for about 10 miles and warmed up. I pulled each of the plug wires while it was running and had a miss on #3 cylinder so I done a tune-up with ac plugs, wires, distributer cap and rotor. Still have the same problem with a miss on #3 after it is warmed up. It runs great until it warms up then a miss. Was wondering if this could be a bad head gasket or possibly bad head or valve? I haven't done a compression check yet. Thanks for any help.

 

have you ever replaced your injectors? i had mine replaced 150k, which was causing the misfire in cylinder 3 and 6. according to the "dealer" he said this is common around 150k

 

No. They have never been replaced as long as I have owned it and I bought it with 30xxx miles on it. Just curious, wouldn't it show a code for a bad injector? Was throwing me off as to why it only done it when it was driven a few miles but ran great when starting after it cooled off overnight...unless it may be a bad o-ring on the injector?? Thanks for the info though. It is definitely something I should check out.

 

The 2004 has the newer MPFI system that is more reliable.
Do a compression test, you may have a valve problem.