1991 LEXUS SC400 Throttle Position

Troubleshooting
If the CO/HC readings do not comply with regulations, pre-
form troubleshooting in the order given below.
(a) Check oxygen sensor operation.(See page FI±73)
(b) See the table below for possible cause, and then inspect
and correct the applicable causes if necessary.
 CO HC Symptoms Cause
 Normal High Rough idle 1. Faulty ignition:
     Incorrect timing
     Fouled, shorted or improperly gapped plugs
 Open or crossed high±tension cords     Cracked distributor cap    2. Incorrect valve clearance    3. Leaky EGR valve    4. Leaky intake and exhaust valves
    5. Leaky cylinder
 Low High Rough idle 1. Vacuum leak:
   (Fluctuating HC reading)  PCV hose
     EGR valve
     Intake manifold
     Air intake chamber
 Throttle body     ISC valve     Brake booster line    2 Lean mixture causing misfire High High Rough idle 1. Clogged air filter
   (Black smoke from exhaust) 2. Faulty EFI system
     Faulty pressure regulator
     Clogged fuel return line
     Defective engine coolant temp. sensor
     Defective intake air temp. sensor
     Faulty ECU
 Fault injectors     Faulty throttle position sensor     Faulty volume air flow meter
±
ENGINE MECHANICAL Idle and/or 2,500 rpm CO/HC CheckEM±21
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Troubleshooting
If the HC/CO concentration does not comply with regulations,
perform troubleshooting in the order given below.
(a) Check oxygen sensor operation.(See page FI±113)
(b) See the table below for possible cause, and then inspect
and correct the applicable causes if necessary.
 

HC 

CO 

Symptoms\
 

\
Cause
 High Normal Rough idle\
 \
1. Faulty ignitions:
   \
 \

Incorrect timing
   \
 \

Fouled, shorted or improperly gapped plugs
   \
 \

Open or crossed high±tension cords
Ckddiib\

Cracked distributor cap
Ill   \
 \
2. Incorrect valve clearance
3 L k EGR l   \
 \
3. Leaky EGR valve
4L k i k d h l   \
 \
4. Leaky intake and exhaust valves
L k li d   \
 \
5. Leaky cylinders
 High Low Rough idle\
 \
1. Vacuum leaks:
   (Fluctuating HC reading)\
 \

PCV hoses
   \
 \

EGR valve
   \
 \

Intake manifold
Ai i k h b   \
 \

Air intake chamber
Th l b d   \
 \

Throttle body
ISC l\

ISC valve
Bkb li   \
 \

Brake booster line
2L i i ifi   \
 \
2. Lean mixture causing misfire
 High High Rough idle\
 \
1. Clogged air filter g g g
(Black smoke from exhaust)\
 \
gg
2. Faulty EFI systems:   ()\
 \
yy

Faulty pressure regulator
   \
 \

yg

Clogged fuel return line
   \
 \

gg

Defective water temp. sensor
   \
 \

Defective air temp. sensor
   \
 \

Faulty ECU
   \
 \

y

Faulty injectors
\

yj

Faulty cold start injector   \
 \
yj

Faulty throttle position sensor   \
 \

Faulty air flow meter
±
ENGINE MECHANICAL Idle and/or 2,500 rpm HC/CO
Concentration Check MethodEM±29
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(b) Remove the two bolts, two nuts, adaptor and gasket.
(c) (USA Spec. only)Disconnect the EGR gas temperature sensor connec-
tor.
14. REMOVE ISC VALVE (a) Disconnect the ISC valve connector.
(b) Disconnect the water by±pass hose from the ISC valve.
(c) Remove the two nuts, ISC valve and gasket.
15. DISCONNECT HEATER WATER VALVE FROM BODY (a) Disconnect the VSV connector.
(b) Remove the bolt holding the engine wire clamp to thewater valve bracket.
(c) Remove the two bolts, and disconnect the water valve and bracket assembly.
16. REMOVE THROTTLE BODY (a) Disconnect the following connectors:(1) Throttle position sensor connector
(2) (w/ TRAC)
Sub±throttle position sensor connector
(3) (w/ TRAC) Sub±throttle actuator connector
EM±64
±
ENGINE MECHANICAL Cylinder Heads
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31. INSTALL ACCELERATOR BRACKETInstall the bracket with the bolt and stud bolt.
32. INSTALL BRAKE BOOSTER UNION Install the union with two new gaskets and the union bolt.
Torque: 29 N Vm (300 kgf Vcm, 22 ft Vlbf)
33. CONNECT VACUUM HOSES
Connect the following hoses:(1) Vacuum hose to brake booster union
(2) Vacuum hose (from VSV for heater water valve) to
air intake chamber
34. INSTALL THROTTLE BODY (a) Connect the following hoses:(1) Water by±pass hose to throttle body
(2) PCV hose to throttle body
(b) Install a new gasket and throttle body with the two bolts and two nuts.
Torque: 18 N Vm (185 kgf Vcm, 13 ft Vlbf)
HINT: Use bolts 40 mm (1.57 in.) in length.
(c) Install the water by±pass pipe (from rear water bypass
joint) to the clamp on the engine wire cover.
(d) Connect the following connectors: (1) Throttle position sensor connector
(2) (w/ TRAC) Sub±throttle position sensor connector
(3) (w/ TRAC) Sub±throttle actuator connector
±
ENGINE MECHANICAL Cylinder HeadsEM±109
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2. INSPECT ISC VALVE OPERATION(a) Apply battery voltage to terminals B1 and B2, and whilerepeatedly grounding S1±S2±S3±S4±S1 in sequence,
and check that the valve moves toward the closed
position.
(b) Aplly battery voltage to terminals B1 and B2, and while repeatedly grounding S4±S3±S2±S1±S4 in sequence,
and check that the valve moves toward the open
position.
If operation is not as specified, replace the ISC valve.
INSTALLATION OF ISC VALVE
(See Components on page FI±54)
1. INSTALL ISC VALVE
(a) Install a new O±ring on the throttle body.
(b) Install the ISC valve with the two screws.
2. INSTALL THROTTLE BODY (See page FI±45)
±
EFI SYSTEM Air Induction System (Idle Speed Control (ISC) Valve)FI±55
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REMOVAL OF THROTTLE BODY
(See Components on page FI±69)
1. DISCONNECT CABLE FROM NEGATIVE TERMINAL OF BATTERY
CAUTION: Work must be started after approx. 20 se-
conds or longer from the time the ignition switch is
turned to the ºLOCKº position and the negative (±) termi-
nal cable is disconnected from the battery.
2. DRAIN ENGINE COOLANT (See page CO±6)
3. REMOVE THROTTLE BODY COVER (a) Remove the mounting cap nut.
(b) Loosen the two bolts, and remove the throttle bodycover.
4. DISCONNECT CONTROL CABLES FROM THROTTLE BODY
Disconnect the following cables:(1) Accelerator cable
(2) A/T throttle control cable
(3) (w/ Cruise Control System)
Cruise control actuator cable
5. REMOVE INTAKE AIR CONNECTOR (a) Disconnect the following hoses:(1) Air hose from ISC valve
(2) Air hose (from PS air control valve) from intake air connector
(b) Remove the bolt holding the intake air connector to the cylinder head cover.
(c) Loosen the two hose clamps.
(d) Disconnect the intake air connector from the throttle body and air cleaner hose, and remove the intake air
connector.
FI±70
EFI SYSTEM
± Air Induction System (Throttle Body)
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6. REMOVE UPPER HIGH±TENSION CORD COVER(a) Remove the two mounting bolts.
(b) Disconnect the front side claw groove of the cord coverfrom the claw of the lower cover, and remove the cord
cover.
7. REMOVE THROTTLE BODY (a) Disconnect the following connectors:(1) Throttle position sensor connector
(2) (w/ TRAC) Sub±throttle position sensor connector
(3) (w/ TRAC) Sub±throttle actuator connector
(b) Disconnect the following hoses: (1) Heater water hose from heater water valve
(2) Water by±pass hose from ISC valve
(3) (USA Spec.) Vacuum hose from throttle body
(4) (Exc. USA Spec.) Three vacuum hoses from throttle body FI±71
EFI SYSTEM
± Air Induction System (Throttle Body)
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INSPECTION OF THROTTLE BODY
1. CLEAN THROTTLE BODY(a) Using a soft brush and carburetor cleaner, clean the cast
parts.
(b) Using compressed air, clean all the passages and apertures.
NOTICE: To prevent deterioration, do not clean the
throttle position sensor and DP.
2. INSPECT THROTTLE VALVE AND THROTTLE POSI TION
SENSOR
A. Inspect throttle valve Check that there is no clearance between the throttle stop
screw and throttle lever when the throttle valve is fully closed.
B. Inspect throttle position sensor (a) Insert a 40 mm (0.016 in.) or 0.65 mm (0.026 in.) feelergauge between the throttle stop screw and stop lever.
(b) Using an ohmmeter, measure the resistance between each terminal.
Clearance betweenClearance between
lever and stop screwBetween terminalsResistancelever and stop screwBetween terminalsResistance
0 mm (0 in.)VTA1±E20.2±0.8

0.40 mm (0.016 in.)IDL1±E22.3 k
or less
0.65 mm (0.026 in.)IDL1±E2Infinity
Throttle valve fully
openVTA1±E23.3±10.0 k

±VC±E24.0±9.0 k

C. If necessary, adjust throttle position sensor
(a) Loosen the two set screws of the sensor. FI±73
EFI SYSTEM
± Air Induction System (Throttle Body)
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