An engine vacuum leak may develop from the throttle body gasket causing a higher than normal idle speed. Failed gaskets should be replaced.
Car Problem Reports
Chevrolet G Series Van (G20) Idling Problems Due to Vacuum Leak in Throttle Body Gasket
Chevrolet G Series Van (G20) Problem
Engines Affected: 5.7L V8, Gasoline
Visitor, 1990 Chevrolet G Series Van (G20), 5.7L V8, 175,000 mi
same prob very high idle. spray injector cleaner around the base of the TB the Idle will slow down ,The base gasket is bad change gasket its under $4