Check the electrical connectors at the t'stat and the air intake hose near the air filter. I forgot that one once and caused myself some grief.

How many miles on your truck?

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2004 super cab, 4 door xlt with appearance package and 4.0 cammer V-6 with auto trans and 2wd fitted with a Leer topper. Sounds via Alpine head unit and power pack through infinity Kappa components front and Kappas rear.