An engine vacuum leak may develop from the throttle body gasket causing a higher than normal idle speed. Failed gaskets should be replaced.Google+
Car Problem Reports
GMC 3500 Pickup Idling Problems Due to Vacuum Leak in Throttle Body Gasket
GMC 3500 Pickup Problem
Engines Affected: 5.7L V8, Gasoline
Visitor, 1998 GMC 3500 Pickup, 5.7L V8, 48,860 mi
When you start the truck, the idle goes high, and when I put in drive, it comes to normal!!
What that could be?