The difference between throttle body and
The two basic types of electronic fuel injection
(EFI) in use today are Throttle Body Injection (TBI) and Multi-Point
Throttle Body Injection
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.
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 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
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:
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.
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.
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.
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.
Regulators - a device that controls fuel enrichment
during engine warm-up on Bosch CIS fuel injection systems.
Pressure Regulator - A spring-loaded diaphragm that
is used in EFI systems to control fuel pressure. Rebuild kits are available
for certain applications.
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.