2005 Chevy Trailblazer Valves

Try some carbon removal procedures with chemicals first. Combustion naturally leaves carbon as a by-product and should get cleaned every 20,000 in my opinion. As it builds up it gets harder to chemically clean.

Good news is there are some good cleaners available. Both Castle and BG make such products. While adding an additive helps it is only part of the procedure.
These products are fed into the throttle body in an atomized form that reach deeper into the motor to start to brake down the carbon build up.

Motorvac also has a system that is good for cleaning.