1991 LEXUS SC400 Service Repair Manual

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Page 2385 of 4087

LEXUS SC400 1991 Service Repair Manual
EFI SYSTEM
NOTE: The specification descriptions ºUSA Spec.º and
º Exc. USA Spec. º used in this section indicate the following
specifications.
USA Spec .: USA (50 States) Specifications
Exc. USA

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LEXUS SC400 1991 Service Repair Manual
DESCRIPTION
FI±2EFI SYSTEM ± Description
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LEXUS SC400 1991 Service Repair Manual
FI±3EFI SYSTEM ± Description
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$LEXUS SC400 1991 Service Repair Manual The EFI system is composed of three basic sub±systems: Fuel, Air Induction and Elect\ ronic Control Systems. FUEL SYSTEM An electric pump fuel pressure supplies sufficient fuel, under a constant pre$

The EFI system is composed of three basic sub±systems: Fuel, Air Induction and Elect\
ronic 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:
1. Electronic Fuel Injection (EFI)
The ECU receives signals from various sensors indicating changing engine op\
eration 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 nec\
essary for an optimum air±fuel
ratio.
2. Electronic Spark Advance (ESA) The ECU is programmed with data for optimum ignition timing under any and all opera\
ting conditions. Using
data provided by sensors which monitor various engine functions (rpm, coola\
nt temperature, etc.), the
Electronic Control Unit (ECU) triggers the spark at precisely right instant. (S\
ee IG section)
3. 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 adj\
usted 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 ºCHECKº 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 ºCHECKº engine
warning light when terminals TE1 and E1 are connected. The diagnostic co\
des 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 ci\
rcuit is substituted to make driving
possible.
If danger is predicted, the engine is stopped and the ºCHECKº engine\
warning light will light up.
6. Fail±Safe Function Even if an abnormality occurs in the ECU, the back±up circuit uses a specifie\
d fuel injection and ignition
timing to provide vehicle driveability, and also lightsup the ºCHECKº engine warning light.
FI±4
EFI SYSTEM
± Description
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LEXUS SC400 1991 Service Repair Manual
OPERATION
FI±5 ± OperationEFI SYSTEM
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LEXUS SC400 1991 Service Repair Manual
FI±6EFI SYSTEM ± Operation
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$LEXUS SC400 1991 Service Repair Manual FUEL SYSTEM Fuel is pumped up by the fuel pump, which flows through the fuel filter under pr\ essure through the fuel pipe to the delivery pipe where it is distributed to each injector and the cold\$

Page 2392 of 4087

$LEXUS SC400 1991 Service Repair Manual AIR INDUCTION SYSTEM Air filtered through the air cleaner passes through the air flow meter and \ the amount flowing to the air intake chamber is determined according to the throttle valve opening in$

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