Hummer H2. Manual - part 1491

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An inaccurate MAF sensor  

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Fuel contamination - Refer to Alcohol/Contaminants-in-Fuel Diagnosis 
(without Special Tool and E85) or Alcohol/Contaminants-in-Fuel 
Diagnosis (with Special Tool) .  

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Vacuum hoses that are split, kinked, or improperly connected  

Sensor/System

Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark 
retard activity.

Ignition System

z

Soak the secondary ignition system with water from a spray bottle. Soaking the 
secondary ignition system may help locate damaged or deteriorated components. 
Look and listen for arcing or misfiring as you apply water.  

z

Test for proper ignition voltage output with the J 26792 Spark Tester.  

z

Remove the spark plugs and inspect for the following: 

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Correct heat range  

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Wet plugs  

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Cracks  

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Wear  

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Improper gap  

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Burned electrodes  

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Heavy deposits  

Refer to Spark Plug Inspection . 

z

An improper spark plug gap will cause a driveability problem. Refer to Spark 
Plug Inspection . Gap the spark plugs using a wire gauge gap tool. Refer to 
Spark Plug Replacement .  

z

Determine the cause of the fouling before replacing the spark plugs.  

z

Visually and physically inspect the secondary ignition for the following: 

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The ignition wires arcing to ground  

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The ignition wires for proper engagement to spark plug  

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The ignition coils for cracks or carbon tracking  

z

Monitor the Misfire Current Counters while driving the vehicle in the conditions 
that the misfire occurred. If a misfiring cylinder can be located, use the DTC 
P0300 table for diagnosis. Refer to DTC P0300 .  

Engine 
Mechanical

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Inspect engine mechanical for the following: 

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Inspect compression - Refer to Engine Compression Test in Engine 
Mechanical.  

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Sticking or leaking valves  

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Worn camshaft lobes  

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Valve timing  

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Bent push rods  

 

2004 Hummer H2 

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

  

POOR FUEL ECONOMY 

Poor Fuel Economy 

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Worn rocker arms  

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Broken valve springs  

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Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil 
Consumption Diagnosis in Engine Mechanical.  

z

For incorrect basic engine parts inspect the following: 

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The camshaft - Refer to Camshaft and Bearings Cleaning and 
Inspection in Engine Mechanical.  

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The cylinder heads - Refer to Cylinder Head Cleaning and Inspection in 
Engine Mechanical.  

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The pistons, etc.-Refer to Piston, Connecting Rod, and Bearings 
Cleaning and Inspection in Engine Mechanical.  

z

Inspect for excessive crankshaft endplay that will cause the crankshaft position 
(CKP) sensor reluctor wheel to move out of alignment with the CKP sensor. 
Refer to Crankshaft and Bearings Cleaning and Inspection in Engine 
Mechanical. This could result in any of the following conditions: 

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A no start  

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A start and stall  

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Erratic performance  

Refer to Symptoms - Engine Mechanical in Engine Mechanical for diagnostic 
procedures.

Additional 
Inspections

z

Inspect the exhaust system for possible restrictions. Inspect for the following: 

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Inspect the exhaust system for damaged or collapsed pipes.  

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Inspect the mufflers for heat distress or possible internal failure.  

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Inspect for possible plugged catalytic converters. Refer to Restricted 
Exhaust in Engine Exhaust.  

z

Electromagnetic interference (EMI) on the reference circuit can cause an engine 
misfire condition. A sudden increase in indicated RPM with little change in 
actual engine RPM change indicates EMI is present. Inspect for high voltage 
components near ignition control circuits if a condition exists.  

z

Inspect the intake manifold and the exhaust manifold passages for casting flash.  

Inspections

Action

DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. 
Also, fuel economy is noticeably lower than the economy was on this vehicle at one time, as previously 
shown by an actual road test.
Preliminary

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Refer to Important Preliminary Inspections Before Starting in Symptoms - 
Engine Controls .  

 

2004 Hummer H2 

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

  

z

Search for bulletins.  

z

Verify that the powertrain control module (PCM) grounds are clean, tight, and in 
the proper locations. Refer to Power and Grounding Component Views in 
Wiring Systems and Engine Controls Schematics .  

z

Inspect the owners driving habits. 

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Is the A/C ON or the Defroster mode ON full time?  

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Are the tires at the correct pressure?  

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Are the wheels and tires the correct size?  

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Are there excessively heavy loads being carried?  

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Is the acceleration rate too much, too often?  

z

Remove the air filter element and inspect for dirt or for restrictions. Refer to Air 
Cleaner Element Replacement . Replace as necessary.  

Fuel System

z

Inspect the type, quality, and alcohol content of the fuel. Oxygenated fuels have 
lower energy and may deliver reduced fuel economy. Refer to 
Alcohol/Contaminants-in-Fuel Diagnosis (without Special Tool and E85) or 
Alcohol/Contaminants-in-Fuel Diagnosis (with Special Tool) .  

z

Inspect the fuel injectors. Refer to Fuel Injector Coil Test , Fuel Injector 
Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2 .  

z

Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .  

z

Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-
Fuel Diagnosis (without Special Tool and E85) or Alcohol/Contaminants-in-
Fuel Diagnosis (with Special Tool) .  

z

Inspect that each fuel injector harness is connected to the correct injector and 
cylinder. Relocate the injector harnesses as necessary.  

z

Inspect for foreign material accumulation in the throttle bore, coking on the 
throttle valve, or on the throttle shaft.  

z

Inspect for the following that may cause the engine to run rich: 

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Water intrusion in the heated oxygen sensor (HO2S) connector  

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Engine oil contaminated by fuel  

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An evaporative emissions (EVAP) canister purge condition  

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Incorrect fuel pressure-Refer to Fuel System Diagnosis .  

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Leaking fuel injectors-Refer to Fuel System Diagnosis .  

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An inaccurate mass air flow (MAF) sensor  

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Blockage on the inlet screen of the MAF sensor-Refer to Mass Air Flow 
(MAF)/Intake Air Temperature (IAT) Sensor Replacement .  

NOTE:

Refer to Heated Oxygen and Oxygen Sensor Notice in Cautions and 
Notices.

 

2004 Hummer H2 

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

  

{

Vacuum hoses that are split, kinked, or improperly connected  

{

An air intake duct that is collapsed or restricted-Refer to Air Cleaner 
Resonator Outlet Duct Replacement .  

{

An air filter that is dirty or restricted-Refer to Air Cleaner Element 
Replacement .  

Sensor/System

z

Inspect the air intake system and crankcase for air leaks.  

z

Inspect the crankcase ventilation valve for proper operation. Refer to Crankcase 
Ventilation System Inspection/Diagnosis in Engine Mechanical.  

z

Inspect for an inaccurate speedometer. Refer to Symptoms - Instrument Panel, 
Gauges and Console in Instrument Panel, Gauges, and Console.  

z

Use a scan tool in order to monitor the knock sensor (KS) system for excessive 
spark retard activity. Refer to Knock Sensor (KS) System Description .  

Ignition System

z

Inspect for proper ignition voltage output with the J 26792 Spark Tester.  

z

Remove the spark plugs and inspect for the following: 

{

Wet plugs  

{

Cracks  

{

Wear  

{

Improper gap  

{

Burned electrodes  

{

Heavy deposits  

Refer to Spark Plug Inspection . 

z

An improper spark plug gap will cause a driveability problem. Refer to Spark 
Plug Inspection . Gap the spark plugs using a wire gauge gap tool. Refer to 
Spark Plug Replacement .  

z

Determine the cause of the fouling before replacing the spark plugs. Refer to 
Spark Plug Inspection .  

z

Visually and physically inspect the secondary ignition for the following: 

{

Ignition wires arcing to ground  

{

Ignition wires for proper routing  

z

Soaking the secondary ignition system with water from a spray bottle may help 
locate damaged or deteriorated components. Look and listen for arcing or 
misfiring as you apply water.  

z

Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System 
Diagnosis .  

Engine Cooling 
System

z

Inspect the engine coolant level for being low. Refer to Loss of Coolant in 
Engine Cooling.  

z

Inspect the engine thermostat for proper operation and for the correct heat range. 
Refer to Thermostat Diagnosis in Engine Cooling.  

 

2004 Hummer H2 

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