2001 INFINITI QX4 engine

Mixture Ratio Feedback Control (Closed loop control)NBEC0014S04
SEF336WA
The mixture ratio feedback system provides the best air-fuel mixture ratio for driveability and emission con-
trol. The warm-up three way catalyst can then better reduce CO, HC and NOx emissions. This system uses
a heated oxygen sensor 1 (front) in the exhaust manifold to monitor if the engine operation is rich or lean. The
ECM adjusts the injection pulse width according to the sensor voltage signal. For more information about the
heated oxygen sensor 1 (front), refer to EC-192. This maintains the mixture ratio within the range of stoichio-
metric (ideal air-fuel mixture).
This stage is referred to as the closed loop control condition.
Heated oxygen sensor 2 (rear) is located downstream of the warm-up three way catalyst. Even if the switch-
ing characteristics of the heated oxygen sensor 1 (front) shift, the air-fuel ratio is controlled to stoichiometric
by the signal from the heated oxygen sensor 2 (rear).
Open Loop ControlNBEC0014S05The open loop system condition refers to when the ECM detects any of the following conditions. Feedback
control stops in order to maintain stabilized fuel combustion.
IDeceleration and acceleration
IHigh-load, high-speed operation
IMalfunction of heated oxygen sensor 1 (front) or its circuit
IInsufficient activation of heated oxygen sensor 1 (front) at low engine coolant temperature
IHigh engine coolant temperature
IDuring warm-up
IAfter shifting from ªNº to ªDº
IWhen starting the engine
Mixture Ratio Self-learning ControlNBEC0014S06The mixture ratio feedback control system monitors the mixture ratio signal transmitted from the heated oxy-
gen sensor 1 (front). This feedback signal is then sent to the ECM. The ECM controls the basic mixture ratio
as close to the theoretical mixture ratio as possible. However, the basic mixture ratio is not necessarily con-
trolled as originally designed. Both manufacturing differences (i.e., mass air flow sensor hot wire) and char-
acteristic changes during operation (i.e., injector clogging) directly affect mixture ratio.
Accordingly, the difference between the basic and theoretical mixture ratios is monitored in this system. This
is then computed in terms of ªinjection pulse durationº to automatically compensate for the difference between
the two ratios.
ªFuel trimº refers to the feedback compensation value compared against the basic injection duration. Fuel trim
includes short term fuel trim and long term fuel trim.
ªShort term fuel trimº is the short-term fuel compensation used to maintain the mixture ratio at its theoretical
value. The signal from the heated oxygen sensor 1 (front) indicates whether the mixture ratio is RICH or LEAN
compared to the theoretical value. The signal then triggers a reduction in fuel volume if the mixture ratio is
rich, and an increase in fuel volume if it is lean.
ªLong term fuel trimº is overall fuel compensation carried out long-term to compensate for continual deviation
of the short term fuel trim from the central value. Such deviation will occur due to individual engine differences,
wear over time and changes in the usage environment.
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ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Multiport Fuel Injection (MFI) System (Cont'd)
EC-29

Fuel Injection TimingNBEC0014S07
SEF179U
Two types of systems are used.
Sequential Multiport Fuel Injection System
NBEC0014S0701Fuel is injected into each cylinder during each engine cycle according to the firing order. This system is used
when the engine is running.
Simultaneous Multiport Fuel Injection System
NBEC0014S0702Fuel is injected simultaneously into all six cylinders twice each engine cycle. In other words, pulse signals of
the same width are simultaneously transmitted from the ECM.
The six injectors will then receive the signals two times for each engine cycle.
This system is used when the engine is being started and/or if the fail-safe system (CPU) is operating.
Fuel Shut-offNBEC0014S08Fuel to each cylinder is cut off during deceleration or operation of the engine at excessively high speeds.
Electronic Ignition (EI) System
DESCRIPTIONNBEC0015Input/Output Signal ChartNBEC0015S01
Sensor Input Signal to ECMECM func-
tionActuator
Crankshaft position sensor (POS) Engine speed (POS signal)
Ignition
timing con-
trolPower transistor Crankshaft position sensor (REF) Engine speed (REF signal)
Camshaft position sensor (PHASE) Piston position
Mass air flow sensor Amount of intake air
Engine coolant temperature sensor Engine coolant temperature
Throttle position sensorThrottle position
Throttle valve idle position
Vehicle speed sensor Vehicle speed
Ignition switch Start signal
Knock sensor Engine knocking
Park/neutral position (PNP) switch Gear position
Battery Battery voltage
ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Multiport Fuel Injection (MFI) System (Cont'd)
EC-30

System DescriptionNBEC0015S02
SEF742M
The ignition timing is controlled by the ECM to maintain the best air-fuel ratio for every running condition of
the engine. The ignition timing data is stored in the ECM. This data forms the map shown.
The ECM receives information such as the injection pulse width and camshaft position sensor signal. Com-
puting this information, ignition signals are transmitted to the power transistor.
e.g., N: 1,800 rpm, Tp: 1.50 msec
A ÉBTDC
During the following conditions, the ignition timing is revised by the ECM according to the other data stored
in the ECM.
IAt starting
IDuring warm-up
IAt idle
IAt low battery voltage
IDuring acceleration
The knock sensor retard system is designed only for emergencies. The basic ignition timing is programmed
within the anti-knocking zone, if recommended fuel is used under dry conditions. The retard system does not
operate under normal driving conditions. If engine knocking occurs, the knock sensor monitors the condition.
The signal is transmitted to the ECM. The ECM retards the ignition timing to eliminate the knocking condition.
Air Conditioning Cut Control
DESCRIPTIONNBEC0016Input/Output Signal ChartNBEC0016S01
Sensor Input Signal to ECM ECM function Actuator
Air conditioner switch Air conditioner ªONº signal
Air conditioner
cut controlAir conditioner relay Throttle position sensor Throttle valve opening angle
Crankshaft position sensor (POS) Engine speed (POS signal)
Crankshaft position sensor (REF) Engine speed (REF signal)
Engine coolant temperature sensor Engine coolant temperature
Ignition switch Start signal
Vehicle speed sensor Vehicle speed
Refrigerant pressure sensor Refrigerant pressure
Power steering oil pressure switch Power steering operation
System DescriptionNBEC0016S02This system improves engine operation when the air conditioner is used.
Under the following conditions, the air conditioner is turned off.
IWhen the accelerator pedal is fully depressed.
IWhen cranking the engine.
IAt high engine speeds.
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ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Electronic Ignition (EI) System (Cont'd)
EC-31

IWhen the engine coolant temperature becomes excessively high.
IWhen operating power steering during low engine speed or low vehicle speed.
IWhen engine speed is excessively low.
IWhen refrigerant pressure is excessively low or high.
Fuel Cut Control (at no load & high engine
speed)
DESCRIPTIONNBEC0017Input/Output Signal ChartNBEC0017S01
Sensor Input Signal to ECMECM func-
tionActuator
Vehicle speed sensor Vehicle speed
Fuel cut
controlInjectors Park/neutral position (PNP) switch Neutral position
Throttle position sensor Throttle position
Engine coolant temperature sensor Engine coolant temperature
Crankshaft position sensor (POS) Engine speed (POS signal)
Crankshaft position sensor (REF) Engine speed (REF signal)
If the engine speed is above 1,800 rpm with no load (for example, in neutral and engine speed over 1,800
rpm) fuel will be cut off after some time. The exact time when the fuel is cut off varies based on engine speed.
Fuel cut will operate until the engine speed reaches 1,500 rpm, then fuel cut is cancelled.
NOTE:
This function is different from deceleration control listed under ªMultiport Fuel Injection (MFI) Systemº, EC-28.
Evaporative Emission System
DESCRIPTIONNBEC0018
SEF927U
The evaporative emission system is used to reduce hydrocarbons emitted into the atmosphere from the fuel
system. This reduction of hydrocarbons is accomplished by activated charcoals in the EVAP canister.
The fuel vapor in the sealed fuel tank is led into the EVAP canister which contains activated carbon and the
vapor is stored there when the engine is not operating or when refueling to the fuel tank.
The vapor in the EVAP canister is purged by the air through the purge line to the intake manifold when the
engine is operating. EVAP canister purge volume control solenoid valve is controlled by ECM. When the engine
ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Air Conditioning Cut Control (Cont'd)
EC-32

operates, the flow rate of vapor controlled by EVAP canister purge volume control solenoid valve is propor-
tionally regulated as the air flow increases.
EVAP canister purge volume control solenoid valve also shuts off the vapor purge line during decelerating and
idling.
SEF428T
INSPECTIONNBEC0019EVAP CanisterNBEC0019S01Check EVAP canister as follows:
1. Pinch the fresh air hose.
2. Blow air into portAand check that it flows freely out of portB.
SEF231SB
Tightening TorqueNBEC0019S02Tighten EVAP canister as shown in the figure.
Make sure new O-ring is installed properly between EVAP can-
ister and EVAP canister vent control valve.
SEF427N
SEF943S
Fuel Tank Vacuum Relief Valve (Built into fuel filler cap)NBEC0019S031. Wipe clean valve housing.
2. Check valve opening pressure and vacuum.
Pressure:
15.3 - 20.0 kPa (0.156 - 0.204 kg/cm
2, 2.22 - 2.90 psi)
Vacuum:
þ6.0 to þ3.3 kPa (þ0.061 to þ0.034 kg/cm
2, þ0.87 to
þ0.48 psi)
3. If out of specification, replace fuel filler cap as an assembly.
CAUTION:
Use only a genuine fuel filler cap as a replacement. If an incor-
rect fuel filler cap is used, the MIL may come on.
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ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Evaporative Emission System (Cont'd)
EC-33

Vacuum Cut Valve and Vacuum Cut Valve Bypass ValveNBEC0019S04Refer to EC-591.
Evaporative Emission (EVAP) Canister Purge Volume
Control Solenoid Valve
NBEC0019S05Refer to EC-367.
Fuel Tank Temperature SensorNBEC0019S06Refer to EC-309.
SEF462UA
Evap Service PortNBEC0019S07Positive pressure is delivered to the EVAP system through the
EVAP service port. If fuel vapor leakage in the EVAP system
occurs, use a leak detector to locate the leak.
SEF200U
PEF838U
PEF917U
How to Detect Fuel Vapor LeakageNBEC0019S08CAUTION:
INever use compressed air or a high pressure pump.
IDo not exceed 4.12 kPa (0.042 kg/cm
2, 0.6 psi) of pressure
in EVAP system.
NOTE:
IDo not start engine.
IImproper installation of EVAP service port adapter to the EVAP
service port may cause a leak.
With CONSULT-IINBEC0019S08011) Attach the EVAP service port adapter securely to the EVAP
service port.
2) Also attach the pressure pump and hose to the EVAP service
port adapter.
3) Turn ignition switch ªONº.
4) Select the ªEVAP SYSTEM CLOSEº of ªWORK SUPPORT
MODEº with CONSULT-II.
5) Touch ªSTARTº. A bar graph (Pressure indicating display) will
appear on the screen.
6) Apply positive pressure to the EVAP system until the pressure
indicator reaches the middle of the bar graph.
7) Remove EVAP service port adapter and hose with pressure
pump.
8) Locate the leak using a leak detector. Refer to ªEVAPORATIVE
EMISSION LINE DRAWINGº, EC-36.
ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Evaporative Emission System (Cont'd)
EC-34

SEF462UA
SEF598U
Without CONSULT-IINBEC0019S08021) Attach the EVAP service port adapter securely to the EVAP
service port.
2) Also attach the pressure pump with pressure gauge to the
EVAP service port adapter.
3) Apply battery voltage to between the terminals of both EVAP
canister vent control valve and vacuum cut valve bypass valve
to make a closed EVAP system.
4) To locate the leak, deliver positive pressure to the EVAP sys-
tem until pressure gauge points reach 1.38 to 2.76 kPa (0.014
to 0.028 kg/cm
2, 0.2 to 0.4 psi).
5) Remove EVAP service port adapter and hose with pressure
pump.
6) Locate the leak using a leak detector. Refer to ªEVAPORATIVE
EMISSION LINE DRAWINGº, EC-36.
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ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Evaporative Emission System (Cont'd)
EC-35

EVAPORATIVE EMISSION LINE DRAWINGNBEC0020
SEF932Y
ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Evaporative Emission System (Cont'd)
EC-36