LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> GMC >> 1990 >> Forward Control 3.9 L >> Repair and Diagnosis >> Engine Performance >> Engine Controls - Adjustments >> Idle Speed & Mixture (Gasoline)
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Idle Speed & Mixture (Gasoline)

NOTE: Idle mixture and idle speed is controlled by the Electronic Control Module (ECM). DO NOT  attempt to adjust them. Incorrect idle speeds are normally caused by dirty throttle plate or vacuum leaks. Ensure all vacuum components are functioning properly.