TBI

Throttle body injection is a fuel delivery system where a single injector (or pair of injectors) sprays gasoline directly into the throttle body, upstream of the intake manifold. The fuel mixes with incoming air before distribution to individual cylinders. This sits between the carburettor and modern multi-point port injection in terms of control and precision.

General Motors popularized TBI in the 1980s, fitting it to Chevrolet and Pontiac V8s and smaller-displacement engines as an economical alternative to tuning carburettors for emissions compliance. The system uses a single fuel pressure regulator (typically 9 to 13 psi), a mechanical or electronic injector, and basic engine management to meter fuel based on airflow, engine speed, and load. Because fuel enters the manifold as a coarse spray rather than at individual ports, mixture distribution across cylinders is less uniform than port injection.

Limitations and why it disappeared

TBI's main weakness is poor atomization and uneven cylinder charging. Heavier fuel droplets pool on manifold walls; some cylinders run lean, others rich. Cold-start stumble, hesitation during transient throttle movements, and difficulty meeting tighter emissions standards plagued these systems. Fuel economy and responsiveness lagged behind port injection, which arrived on mainstream vehicles by the mid-1990s.

The injector itself is straightforward: a solenoid coil opens a spring-loaded pintle valve, allowing fuel to spray through a single orifice. Injector pulse width (typically 2 to 10 milliseconds) controls how much fuel enters per cycle. Clogging from carbon buildup or water contamination was common, causing rough idle and lean misfire codes.

Today TBI exists mainly in older light-duty trucks, small-block Chevrolets from the 1980s and early 1990s, and some marine and small-engine conversions. Technicians occasionally encounter failing TBI systems where the injector leaks (flooding the manifold) or the fuel pressure regulator diaphragm ruptures, allowing manifold vacuum to draw fuel into the engine oil. Aftermarket EFI systems have largely replaced worn-out TBI in restoration and performance builds.