Ummm. Makes since. Well a plenum leak is a vacuum leak. Maybe theres more vacuum leaks some wear…
Go get you a can of carb cleaner… WARNING DO THIS ONLY WHEN THE ENGINE COOL.
Spry the carb cleaner around any vacuum leaks. Spry around the plenum as well. Maybe the plenum did not seal 100% or maybe its warped.
Get you a spry can of sea foam and spy it down the throttle body.(SPRY LIGHTLY – THE TRUCK WILL TRY TO DIE AND YOU WILL SEE WHITE SMOKE) after you have spryed for sometime take a look around the tail pipe area and see if there any black suds around the area…