The
difference between throttle body and mulit-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
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
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
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