l have a 2002 trailblazer that I have owned since new. I changed the water pump 2 weeks ago. When I started the vehicle it developed a very slight miss. It doesn't happen all of the time and it only happens at idle. I retorqued the pulley bolts. There is no noticeable wobble or play in the pulley. It does feel like a miss not an out of balance condition. It has 160,000 miles. I changed plugs (110,000 miles on the plugs). If it is coils or injectors whats the best way to test without throwing parts at it.