Hummer H2. Manual - part 1483

The engine speed is too high, above red line.

The vehicle speed is too high, above rated tire speed.

During an extended, high speed, closed throttle coast down-This reduces emissions and increases engine
braking.

During extended deceleration, in order to prevent damage to the catalytic converters

Fuel Trim

The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best
possible combination of driveability, fuel economy, and emission control. The PCM monitors the HO2S signal
voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse width of the fuel injectors
based on this signal. The ideal fuel trim values are around 0 percent for both short term and long term fuel trim.
A positive fuel trim value indicates the PCM is adding fuel in order to compensate for a lean condition by
increasing the pulse width. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in
order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery
changes the short term and long term fuel trim values. The short term fuel trim values change rapidly in
response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes
coarse adjustments to the fueling in order to re-center and restore control to short term fuel trim. A scan tool can
be used to monitor the short term and long term fuel trim values. The long term fuel trim diagnostic is based on
an average of several of the long term speed load learn cells. The PCM selects the cells based on the engine
speed and engine load. If the PCM detects an excessive lean or rich condition, the PCM will set a fuel trim
diagnostic trouble code (DTC).

EVAPORATIVE EMISSION (EVAP) CONTROL SYSTEM DESCRIPTION

EVAP System Operation

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel
tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the
EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the
vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the
engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid
valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve
OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is
drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through
the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal
combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test

This tests for large leaks and blockages in the evaporative emission (EVAP) system. The control module
commands the EVAP vent solenoid valve ON and commands the EVAP purge solenoid valve ON, with the
engine running, allowing engine vacuum into the EVAP system. The control module monitors the fuel tank
pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within
a set amount of time. The control module then commands the EVAP purge solenoid valve OFF, sealing the
system, and monitors the vacuum level for decay. If the control module does not detect that the predetermined

2004 Hummer H2

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

vacuum level was achieved, or the vacuum decay rate is more than a calibrated level on 2 consecutive tests,
DTC P0455 will set.

Small Leak Test

The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the evaporative
emission (EVAP) system. While previous leak detection methods were performed with the engine running, the
EONV diagnostic monitors the EVAP system pressure or vacuum with the ignition OFF. Because of this, it may
be normal for the control module to remain active for up to 40 minutes after the ignition is turned OFF. This is
important to remember when performing a parasitic draw test on vehicles equipped with EONV.

The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the
naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the
tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then starts to
drop. The EONV diagnostic relies on this temperature change and the corresponding pressure change in a
sealed system, to determine if an EVAP system leak is present.

The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine
if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed,
a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, often little
or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.

Canister Vent Restriction Test

If the evaporative emission (EVAP) vent system is restricted, fuel vapors will not be properly purged from the
EVAP canister. The control module tests this by commanding the EVAP purge solenoid valve ON,
commanding the EVAP vent solenoid valve OFF, and monitoring the fuel tank pressure (FTP) sensor for an
increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will set.

Purge Solenoid Valve Leak Test

If the evaporative emission (EVAP) purge solenoid valve does not seal properly fuel vapors could enter the
engine at an undesired time, causing driveability concerns. The control module tests for this by commanding the
EVAP purge solenoid valve OFF and the vent solenoid valve ON, sealing the system, and monitors the fuel
tank pressure (FTP) for an increase in vacuum. If the control module detects that the EVAP system vacuum
increases above a calibrated value, DTC P0496 will set.

Check Gas Cap Message

The powertrain control module (PCM) sends a class 2 message to the driver information center (DIC)
illuminating the Check Gas Cap message when any of the following occur:

A malfunction in the evaporative emission (EVAP) system and a large leak test fails

A malfunction in the EVAP system and a small leak test fails

EVAP System Components

2004 Hummer H2

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

The evaporative emission (EVAP) system consists of the following components:

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister
until the control module determines that the vapor can be consumed in the normal combustion process.

EVAP Purge Solenoid Valve

The EVAP purge solenoid valve controls the flow of vapors from the EVAP system to the intake manifold. The
purge solenoid valve opens when commanded ON by the control module. This normally closed valve is pulse
width modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The
valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the
EVAP system.

EVAP Vent Solenoid Valve

The EVAP vent solenoid valve controls fresh airflow into the EVAP canister. The valve is normally open. The
control module commands the valve ON, closing the valve during some EVAP tests, allowing the system to be
tested for leaks.

Fuel Tank Pressure Sensor

The fuel tank pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank
and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The
FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. A high
FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high
fuel tank pressure.

EVAP Service Port

The EVAP service port is located in the EVAP purge pipe between the EVAP purge solenoid valve and the
EVAP canister. The service port is identified by a green colored cap.

ELECTRONIC IGNITION (EI) SYSTEM DESCRIPTION

The electronic ignition (EI) system is responsible for producing and controlling a high energy secondary spark.
This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal
performance, fuel economy, and control of exhaust emissions. This ignition system consists of a separate
ignition coil connected to each spark plug by a short secondary wire. The driver modules within each coil
assembly are commanded ON/OFF by the powertrain control module (PCM). The PCM primarily uses engine
speed and position information from the crankshaft and camshaft position (CMP) sensors to control the
sequence, dwell, and timing of the spark. The EI system consists of the following components:

Crankshaft Position (CKP) Sensor

The crankshaft position (CKP) sensor is a three wire sensor based on the magneto resistive principle. A
magneto resistive sensor uses two magnetic pickups between a permanent magnet. As an element such as a

2004 Hummer H2

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

reluctor wheel passes the magnets the resulting change in the magnetic field is used by the sensor electronics to
produce a digital output pulse. The PCM supplies a 12-volt, low reference, and signal circuit to the CKP sensor.
The sensor returns a digital ON/OFF pulse 24 times per crankshaft revolution.

Crankshaft Reluctor Wheel

The crankshaft reluctor wheel is mounted on the rear of the crankshaft. The wheel is comprised of four 90
degree segments. Each segment represents a pair of cylinders at TDC, and is further divided into six 15 degree
segments. Within each 15 degree segment is a notch of 1 of 2 different sizes. Each 90 degree segment has a
unique pattern of notches. This is known as pulse width encoding. This pulse width encoded pattern allows the
PCM to quickly recognize which pair of cylinders are at top dead center (TDC). The reluctor wheel is also a
dual track-or mirror image-design. This means there is an additional wheel pressed against the first, with a gap
of equal size to each notch of the mating wheel. When one sensing element of the CKP sensor is reading a
notch, the other is reading a set of teeth. The resulting signals are then converted into a digital square wave
output by the circuitry within the CKP sensor.

Camshaft Position (CMP) Sensor

The CMP sensor is also a magneto resistive sensor, with the same type of circuits as the CKP sensor. The CMP
sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor
information is used by the PCM to determine the position of the valve train relative to the CKP.

Camshaft Reluctor Wheel

The camshaft reluctor wheel is either pressed onto the camshaft or part of the timing gear depending on the
application. The feature-or target- is read in a radial or axial fashion respectively. The wheel is a smooth track,
half of which is of a lower profile than the other half. This feature allows the CMP sensor to supply a signal as
soon as the key is turned ON, since the CMP sensor reads the track profile, instead of a notch.

Ignition Coils

Each ignition coil has an ignition 1 feed and a ground. The PCM supplies a low reference and an ignition
control (IC) circuit. Each ignition coil contains a solid state driver module. The PCM will command the IC
circuit ON, this allows the current to flow through the primary coil windings for the appropriate time or dwell.
When the PCM commands the IC circuit OFF, this will interrupt current flow through the primary coil
windings. The magnetic field created by the primary coil windings will collapse across the secondary coil
windings, which induces a high voltage across the spark plug electrodes. The coils are current limited to prevent
overloading if the IC current is held high too long. The spark plugs are connected to their respective coils by a
short secondary wire. The spark plugs are tipped with iridium for long life and efficiency.

Powertrain Control Module (PCM)

The PCM controls all ignition system functions, and constantly corrects the basic spark timing. The PCM
monitors information from various sensor inputs that include the following:

The throttle position (TP) sensor

The engine coolant temperature (ECT) sensor

2004 Hummer H2

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

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