The Difference Between Throttle Body and Multi-Point Injection

The difference between throttle body and multi-point injection

The two basic types of electronic fuel injection (EFI) in use today are Throttle Body Injection (TBI) and Multi-Point Injection (MPI).

Throttle Body Injection

Throttle Body

A TBI system is similar to a carburetor in that one or two injectors are located in a central throttle body that supplies fuel to the engine through the intake manifold. Instead of using engine vacuum to siphon fuel through metering circuits as a carburetor does, fuel is sprayed into the manifold through the injectors.

Multi-point Injection

Multi Point Injection

In a MPI system, each cylinder has its own individual injector. The injectors are mounted at each of the intake ports so fuel can be sprayed directly into the ports. A single throttle body meters the amount of air entering the intake manifold so the amount of fuel delivered can be matched to the engine’s needs.

Fuel Regulation


Fuel metering in both types of EFI systems is controlled by a combination of fuel pressure and injector timing. The longer injectors are on, the greater volume of fuel delivered to the engine.

Fuel delivery is also increased when there is a greater pressure differential between intake vacuum and fuel line pressure (which is controlled by a fuel pressure regulator).

Some of the other components in both types of EFI systems with which you should be familiar include:

1midle Air Control (IAC) Valve- used on EFI applications for idle speed control. The electric motor opens and closes a valve so air can bypass the throttle plates. Failure may cause stalling.

Throttle Position Sensor (TPS) – A rheostat-like device that mounts on the throttle shaft to inform the computer about throttle opening. Failure may cause hesitation. Sensor must be carefully adjusted when installed to give an accurate voltage reading.

Airflow Sensor- Used to measure how much air is entering the engine so the appropriate amount of fuel can be delivered through the injectors. Basic types include the "flap" style used on many import and domestic Bosch systems, and the "heated filament" and "hot wire" mass airflow sensors. Expensive to replace.

Cold Start Valve – An auxiliary fuel injector that provides extra fuel enrichment when a cold engine is first started. If defective, can cause hard starting. If leaks, can cause rich fuel mixture.

Warm-up Regulators – a device that controls fuel enrichment during engine warm-up on Bosch CIS fuel injection systems.

Fuel Pressure Regulator – A spring-loaded diaphragm that is used in EFI systems to control fuel pressure. Rebuild kits are available for certain applications.

Fuel Injector– Two types: mechanical and electronic. Mechanical
injectors are used in Bosch CIS/K-Jetronic import applications, while electronic injectors are used on all domestic EFI applications. The electronic variety contain a solenoid that lifts a pintle valve open so fuel can spray out of the injector. The mechanical variety is spring loaded and calibrated to open when a certain minimum pressure is achieved. Both are susceptible to clogging from dirt
and fuel residue. One new type of replacement injector has a disc valve design that resists clogging.

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