with sea foam in the vacuum smoke came out the top of heat shield?P0171code on 2004 Chevrolet Colorado

Just after I had glass packs muffler put in and cut off and turned down the following days had a reduce in power and little of a rough idle where's The best place to startlooking for tha problem?

Asked by for the 2004 Chevrolet Colorado
Are the converter(s) still on and in original location? Changing backpressure will affect performance.