1991 ACURA NSX coolant temperature

14. Relieve fuel pressure (see section 11).
Do not smoke while working on fuel
system; keep open flame or spark away from work
area. Drain fuel only into an approved container.
1 5. Disconnect the fuel feed pipe and the return hose
(page 5-21).

16. Remove the ignition coil covers, the wire harness covers, the ignition coils and the connectors (page
6-23).
17. Remove the water hoses, then remove the expan- sion tank (page 5-21).
18. Remove the breather hose, and the air cleaner housing (page 5-22).
19. Remove the brake booster hose, the evaporative emission (EVAP) control canister hose and other
hoses from the intake manifold and throttle body
(page 5-22).
20. Remove the emission control box (page 5-23). Do not disconnect emission hose.Disconnect the three connectors before
removing.
21. Remove the connector, the terminal and the alternator.
22. Disconnect the following engine wire harness con- nectors, then remove the harness clamps from thecylinder head and the intake manifold.
Front cylinder head:

Crankshaft position (CKP)/cylinder position
(CYP) sensor connector
Air conditioning (A/C) compressor magnetic clutch connector
Heated oxygen sensor (H02S) connector
Ground cable terminals Three injector connectors (cylinders No. 4, 5

and 6)

Intake air temperature (IAT) sensor connector
Ignition control module (ICM) connector
Condenser connector

Engine oil pressure gauge sending unit

connector

VTEC solenoid valve connector
VTEC pressure switch connector
Engine coolant temperature (ECT) sensor

connector
ECT sending unit connector
Accelerator pedal opening sensor connector

(throttle body)

Radiator fan control sensor connector
Rear cylinder head: Engine oil pressure switch connector (oil cooler

base)

Heated oxygen sensor connector
Three injector connectors (cylinders No. 1, 2

and 3)
Idle air control (IAC) valve connector
Knock sensor connector
Throttle actuator motor connector

Throttle position sensor connector

VTEC solenoid valve connectorVTEC pressure switch connector
ICM connector

Ground terminal

ICM CONNECTORS THROTTLE ACTUATOR

MOTOR CONNECTOR

THROTTLE POSITION
SENSOR CONNECTOR
ACCELERATOR PEDAL OPENING
SENSOR CONNECTORProCarManuals.com

Component Locations

Index

ENGINE COOLANT TEMPERATURE (ECT) SENSOR

Troubleshooting, page 11 -66
INTAKE AIR TEMPERATURE (IAT) SENSOR
Troubleshooting, page 11-70
FUEL PUMP RESISTER

Troubleshooting, page 11-124

CRANKSHAFT POSITION/CYLINDER POSITION
(CKP/CYP) SENSOR

Troubleshooting, page 11-64

FUEL

INJECTOR

RESISTER
Testing,

page 11-112
FRONT HEATED OXYGEN SENSOR (HO2S)
Troubleshooting, page 11-48
REAR KNOCK SENSOR (KS)

Troubleshooting, page 11-80

FRONT KNOCK SENSOR (KS)

Troubleshooting, page 11-80

REAR HEATED OXYGEN SENSOR (HO2S)
Troubleshooting, page 11-48
IDLE AIR CONTROL (IAC) VALVE

Troubleshooting, page 11 -88

FAST IDLE THERMO VALVE

Inspection, page 11-101

NEUTRAL SWITCH (M/T)

Troubleshooting, page 11-96

SERVICE CHECK CONNECTOR (2P)
Self-diagnostic Procedures, page 11-34
ENGINE CONTROL MODULE (ECM)
Self-diagnostic Procedures, page 11-34
Troubleshooting, page 11-42
PGM-FI

MAIN RELAY

Relay Testing, page 11-120

Troubleshooting, page 11-122
THROTTLE POSITION (TP) SENSOR

Troubleshooting, page 11 -68
CONTROL BOX
page, 11-7
ACCELERATOR PEDAL
ANGLE SENSOR

Troubleshooting, section 19
EXHAUST GAS RECIRCULATION (EGR) VALVE

Troubleshooting, page 11-145
EXHAUST GAS RECIRCULATION (EGR)
VALVE LIFT SENSOR

Troubleshooting, page 11-145ProCarManuals.com

PGM-FI System

System Description

INPUTS
ENGINE CONTROL MODULE (ECM)

OUTPUTS

FRONT HO2S

REAR HO2S

MAP Sensor

CKP/CYP Sensor

ECT Sensor

TP Sensor

IAT Sensor

VSS

FRONT KS

REAR KS

Ignition Timing Adjuster

EGR Valve Lift Sensor

A/T Fl Signals

TCS Signals

Starter Signal

ALT FR Signal

Air Conditioning Signal

A/T Gear Position Switch Signal

Neutral Switch Signal (M/T)

Clutch Switch Signal (M/T)

VTEC Pressure Switch

Battery Voltage (IGN.1)

Fuel Pump Terminal Voltage
Fuel Injectors

PGM-FI Main Relay (Fuel Pump)

Malfunction Indicator Lamp

IAC Valve

A/C Compressor Clutch Relay

ICM

EVAP Purge Control Solenoid Valve

Fuel Pump Relay

EGR Control Solenoid Valve

IAB Control Solenoid Valve

Engine Compartment Fan Relay

VTEC Solenoid Valve

TCS Control Signals

FRONT HO2S HEATER

REAR HO2S HEATER

PGM-FI System

The PGM-FI system on this model is a sequential multiport fuel injection system.

Fuel Injector Timing and Duration

The ECM contains memories for the basic discharge durations at various engine speeds and manifold pressures. The

basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors

to obtain the final discharge duration.

Idle Air Control

Idle Air Control (IAC) Valve

When the engine is cold, the A/C compressor is on, the transmission is in gear (A/T only) or the alternator (ALT) is charg-

ing, the ECM controls current to the IAC valve to maintain correct idle speed.

Ignition Timing Control

The ECM contains memories for basic ignition timing at various engine speeds and manifold pressures. Ignition timing

is also adjusted for engine coolant temperature.

A knock control system is also used. When detonation is detected by the knock sensor (KS), the ignition timing is

retarded.

Other Control Functions

1. Starting Control

When the engine is started, the ECM provides a rich mixture by increasing fuel injector duration.

2. Fuel Pump Control

When the ignition switch is initially turned on, the ECM supplies ground to the PGM-FI main relay that supplies

current to the fuel pump for two seconds to pressurize the fuel system.

When the engine is running, the ECM supplies ground to the PGM-FI main relay that supplies current to the fuel pump.

When the engine is not running and the ignition is on, the ECM cuts ground to the PGM-FI main relay which cuts

current to the fuel pump.

Excellent engine performance is achieved through the use of VTEC (Variable Valve Timing and Valve Lift Electron-

ic Control System), intake air bypass control and discharge volume control of the fuel pump.ProCarManuals.com

3. Fuel Cut-off Control

During deceleration with the throttle valve closed, current to the fuel injectors is cut off to improve fuel economy

at speeds over 1,500 rpm.

Fuel cut-off action also takes place when engine speed exceeds, 8,300 rpm, regardless of the position of the

throttle valve, to protect the engine from over-revving.

4. A/C Compressor Clutch Relay

When the ECM receives a demand for cooling from the air conditioning system, it delays the compressor from being

energized, and enriches the mixture to assure smooth translation to the A/C mode.

5. Evaporative Emission (EVAP) Purge Control Solenoid Valve

When the engine coolant temperature is below 1 58 °F (70°C) the ECM supplies a ground to the EVAP purge control

solenoid valve which cuts vacuum to the EVAP purge control diaphragm valve.

6. Intake Air Bypass (IAB) Control Solenoid Valve

When the engine speed is below 4,800 rpm, the IAB control solenoid valve is activated by a signal from the ECM.

Intake air then flows through the smaller chamber, and hight torque is delivered. To increase air flow at engine speeds

higher than 4,800 rpm, the solenoid valve is deactivated by the ECM, and the intake air flows through the larger

chamber.

7. Exhaust Gas Recirculation (EGR) Control Solenoid Valve

When the EGR is required for control of oxides of nitrogen (NOx) emissions, the ECM supplies ground to the EGR

control solenoid valve which supplies regulated vacuum to the EGR valve.

ECM fail-safe/back-up Functions

1. Fail-Safe Function

When an abnormality occurs in a signal from a sensor, the ECM ignores that signal and assumes a pre-programmed

valve for that sensor that allows the engine to continue to run.

2. Back-up Function

When an abnormality occurs in the ECM itself, the fuel injectors are controlled by a back-up circuit independent of

the system in order to permit minimal driving.

3. Self-diagnosis Function [Malfunction Indicator Lamp (MIL)]

When an abnormality occurs in a signal from a sensor, the ECM lights the MIL and stores the diagnostic trouble code

in erasable memory. When the ignition is initially turned in, the ECM supplies ground for the MIL for two seconds

to check the MIL bulb condition.ProCarManuals.com

Fast Idle Thermo Valve

Description

To prevent erratic running when the engine is warming
up, it is necessary to raise the idle speed. The fast idle
thermo valve is controlled by a thermowax plunger.
When the engine is cold, the engine coolant surround-ing the thermowax contracts the plunger, allowing ad-
ditional air to be bypassed into the intake manifold so
that the engine idles faster. When the engine reaches operating temperature, the valve closes, reducing theamount of air bypassing into the manifold.

IAC VALVE

FAST

IDLE

THERMO

VALVE

AIR BYPASS VALVE

WAX
Inspection

NOTE: The fast idle thermo valve is factory adjusted; it
should not be disassembled. 1. Start the engine.
2. Remove the cover of the fast idle thermo valve.
3. Put your finger on the valve seat area and make
sure that there is air flow with the engine cold
(engine coolant temperature below 86°F (30°C)

and idling.)

FAST IDLE THERMO VALVE

If not, replace the fast idle thermo valve and
retest.

12 N.
m

(1.2 kg-m, 9 Ib-ft)

O-RINGS

Replace
4. Warm up the engine (the cooling fan comes on).
5. Check that the valve is completely closed. If not, air suction can be felt in the valve seat area.If any suction is felt, the valve is leaking.
Check engine coolant level and for air in the
engine coolant system. If OK, replace the fast idle thermo valve and recheck.ProCarManuals.com

Emission Control System

Description

The evaporative emission controls are designed to minimize the amount of fuel vapor escaping to the atmosphere. The

system consists of the following components:

A. Evaporative Emission (EVAP) Control Canister

An EVAP control canister is used for the temporary storage of fuel vapor until the fuel vapor can be purged from the

EVAP control canister into the engine and burned.

B. Vapor Purge Control System

EVAP control canister purging is accomplished by drawing fresh air through the EVAP control canister and into a port

on the throttle body. The purging vacuum is controlled by the EVAP purge control diaphragm valve and the EVAP

control solenoid valve.

C. Fuel Tank Vapor Control System

When fuel vapor pressure in the fuel tank is higher than the set value of the EVAP two way valve, the valve opens

and regulates the flow of fuel vapor to the EVAP control canister.
EVAP PURGE CONTROL SOLENOID VALVE OFF AFTER

STARTING ENGINE

ENGINE COOLANT TEMPERATURE ABOVE
158°F
(70°C)
Evaporative Emission (EVAP) ControlsProCarManuals.com

Description
Electronic Control System (cont'd)
TRANSMISSION CONTROL MODULE (TCM)

ENGINE
CONTROL
MODULE
(ECM) Shift Control

Lock-up

Control

TCS Control Unit
EPS Control Unit Engine RPM
Air Conditioning Signal Brake Switch Signal
Vehicle Speed Sensor Signal
Mainshaft Speed Sensor Signal
Countershaft Speed Sensor Signal

Self-Diagnosis
Function

Self-Diagnosis Indicator

INTERLOCK
CONTROL UNIT
Lock-up Control Solenoid
Valve B
Lock-up Control Solenoid
Valve A Shift Control Solenoid

Valve B

Shift Control Solenoid
Valve A

Linear Solenoid
Throttle Position
Sensor Signal
Engine Coolant

Temperature
Sensor Signal
Accet Pedal

Angle Signal

Service Check
Connector
ProCarManuals.com

Component Location

LOCK-UP CONTROL

SOLENOID VALVE

ASSEMBLY

MAINSHAFT SPEED SENSOR
FRONT
SHIFT CONTROL

SOLENOID VALVE

ASSEMBLY

ENGINE COOLANT TEMPERATURE (ECT) SENSOR

Engine Compartment

COUNTERSHAFT SPEED SENSOR

TRANSMISSION CONTROL MODULE (TCM)

TCS CONTROL UNIT

ENGINE CONTROL MODULE (ECM)
THROTTLE POSITION (TPI

SENSOR
ACCEL PEDAL

SENSORProCarManuals.com