GM Astro/Safari 1985-1996 Repair Guide

Idle Speed and Mixture Adjustments



The different combinations of emission systems application on the various available engines have resulted in a great variety of tune-up specifications. All vehicles covered by this guide should have a decal conspicuously placed in the engine compartment giving tune-up specifications.

Because the 4.3L carbureted engine utilizes an electronically controlled feedback carburetor there are no periodic mixture adjustments are necessary or possible. Slow (curb) idle speed can be set using the speed screw at the throttle valve on the carburetor. When setting the curb idle speed, make sure the engine is at normal operating temperature and that all of the conditions on the vehicle emission control information label have been met.


Engines covered by this guide may be equipped with a variety of fuel injection systems including: Throttle Body Injection (TBI), Central Multi-Port Fuel Injection (CMFI) and Central Sequential Fuel Injection (CSFI). Although each of these systems contain some of their own unique engine control components, what they all share is full computer control of the idle air supply and of all fuel delivery. The fuel injection computer module regulates idle speeds and supplies the correct amount of fuel during all engine operating conditions. No periodic adjustments are necessary or possible. If the engine is suspected of maintaining an incorrect idle speed, refer to Emission Controls of this guide for information regarding the self-diagnostic features of the computer engine and emission control systems and to Fuel System for information regarding the throttle body and fuel delivery systems.