Lexus SC300 / Lexus SC400. Service manual

The EFI system is composed of three basic sub–systems: Fuel, Air Induction and Electronic Control

Systems.

FUEL SYSTEM

An electric pump fuel pressure supplies sufficient fuel, under a constant pressure, to the EFI injectors. In

accordance with signals from the ECU (Electronic Control Unit), these injectors inject the quantity of fuel most
appropriate for the engine condition into the intake manifold.

AIR INDUCTION SYSTEM

The air induction system provides sufficient air for engine operation.

ELECTRONIC CONTROL SYSTEM

The 1UZ–FE engine (ECU–formerly EFI computer) with a microcomputer centrally controls the EFI, ESA,

ISC and Diagnosis system, etc.

The ECU controls the following functions:

Electronic Fuel Injection (EFI)

The ECU receives signals from various sensors indicating changing engine operation conditions such as:

Intake air volume
Intake air temperature
Coolant temperature
Engine rpm
Acceleration/deceleration
Exhaust oxygen content etc.

The signals are utilized by the ECU to determine the injection duration necessary for an optimum air–fuel
ratio.

Electronic Spark Advance (ESA)
The ECU is programmed with data for optimum ignition timing under any and all operating conditions. Using
data provided by sensors which monitor various engine functions (rpm, coolant temperature, etc.), the
Electronic Control Unit (ECU) triggers the spark at precisely right instant. (See IG section)

Idle Speed Control (ISC)
The ECU is programmed with idle speed data for various engine conditions (coolant temperature, air condi-
tioner ON/OFF, etc.).
The air volume flowing through the throttle valve by–pass passage is adjusted according to the signal from
each sensor and the idle speed is kept at the set value.

4. Diagnosis

Function

When the ECU detects any malfunctions or abnormalities in the sensor network, it lights the ”CHECK” en-
gine warning light in the combination meter. At the same time, the trouble is identified and a diagnostic code
is recorded by the ECU. The diagnostic code can be read by the number of blinks of the ”CHECK” engine
warning light when terminals TE1 and E1 are connected. The diagnostic codes are refer to the later page.
(See TR section)

5. Self–Correction

Function

If any sensor malfunctions, an average value recorded in the back–up circuit is substituted to make driving
possible.
If danger is predicted, the engine is stopped and the ”CHECK” engine warning light will light up.

6. Fail–Safe

Function

Even if an abnormality occurs in the ECU, the back–up circuit uses a specified fuel injection and ignition
timing to provide vehicle driveability, and also lightsup the ”CHECK” engine warning light.

FI–4

EFI SYSTEM – Description

Lexus SC300 / Lexus SC400. Service manual - part 597