The Model 400 Electronic Fuel Injection (EFI) system is a computer controlled system which uses a pair of Throttle Body Injection (TBI) units mounted on a single manifold cover. Each TBI unit feeds the cylinders on the opposite side of the engine, thus the name Crossfire Injection (CFI).

The major sub-assemblies of the fuel system are: fuel supply system, Throttle Body Injector (TBI) assembly, Idle Air Control (IAC) system, Electronic Control Module (ECM), data sensors, and emission controls.

Fuel is supplied to the engine through electronically pulsed (timed) injector valves located in the throttle bodies.

The ECM controls amount of fuel metered through injector valves based upon engine demand and efficiency information. The ECM is a digital electronic computer which receives and computes signals from various data sensors.

NOTE: Primary sub-systems which affect fuel system operation will be covered in this article: fuel supply system, TBI assembly, IAC system, ECM, and data sensors. Because of the interrelated functions of the Computer Command Control (CCC) system, refer to GENERAL MOTORS COMPUTER COMMAND CONTROL SYSTEM in COMPUTERIZED ENGINE CONTROLS section for more information.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Fuel Injection System - Dual Unit TBI: Description

Use the Manual With the Right Hardware

When to See a Mechanic