Chevrolet Silverado / GMC Sierra. Service manual — part 231

BASE ENGINE MISFIRE WITHOUT INTERNAL ENGINE NOISES

Cause

Correction

Fuel injector harness connectors are

connected to the incorrect fuel

injectors/cylinders

Relocate the fuel injector harness connectors,

as necessary.

Abnormalities, such as severe cracking,

bumps, or missing areas in the accessory drive

belt

Abnormalities in the accessory drive system

and/or components may cause engine RPM

variations and lead to a misfire diagnostic

trouble code (DTC). A misfire code may be

present without an actual misfire condition.

Replace the drive belt. Refer to Drive Belt

Replacement - Accessory.

Worn, damaged, or mis-aligned accessory

drive components or excessive pulley runout

May lead to a misfire DTC.

A misfire code may be present without an

actual misfire condition.

Inspect the components and repair or

replace, as required.

Loose or improperly installed engine flex

plate or crankshaft balancer

A misfire code may be present without an

actual misfire condition.

Repair or replace the flex plate and/or

balancer, as required. Refer to Automatic

Transmission Flex Plate Replacement, or

Crankshaft Balancer Replacement.

Restricted exhaust system

A severe restriction in the exhaust flow can

cause significant loss of engine performance

and may set a DTC. Possible causes of

restrictions include collapsed or dented pipes

or plugged mufflers and/or catalytic

converters.

Repair or replace, as required.

Improperly installed or damaged vacuum

hoses

Repair or replace, as required.

Improper sealing between the intake manifold

and cylinder heads or throttle body

Replace the intake manifold, gaskets,

cylinder heads, and/or throttle body, as

required.

Improperly installed or damaged manifold

absolute pressure (MAP) sensor

The sealing grommet of the MAP sensor

should not be torn or damaged.

Repair or replace the MAP sensor, as

required.

2008 Hummer H2

2008 ENGINE Engine Mechanical - 6.2L - H2

Worn or loose rocker arms

The rocker arm bearing end caps and/or

needle bearings should be intact and in the

proper position.

Replace the valve rocker arms, as required.

Worn or bent pushrods

Replace the pushrods.

Inspect the top of the pistons for valve

contact. If the top of the piston shows

valve contact, replace the piston and

pin assembly.

Stuck valves

Carbon buildup on the valve stem can cause

the valve to not close properly.

Repair or replace, as required.

Excessively worn or mis-aligned timing chain Replace the timing chain and sprockets, as

required.

Worn camshaft lobes

Replace the camshaft and valve lifters.

Excessive oil pressure

A lubrication system with excessive oil

pressure may lead to excessive valve lifter

pump-up and loss of compression.

1. Perform an oil pressure test. Refer to

Oil Pressure Diagnosis and Testing.

2. Repair or replace the oil pump, as

required.

Faulty cylinder head gaskets and/or cracking

or other damage to the cylinder heads and

engine block cooling system passages

Coolant consumption may or may not cause

the engine to overheat.

1. Inspect for spark plugs saturated by

coolant. Refer to Spark Plug

Inspection .

2. Inspect the cylinder heads, engine

block, and/or head gaskets. Refer to

Coolant in Combustion Chamber.

3. Repair or replace, as required.

Worn piston rings

Oil consumption may or may not cause the

engine to misfire.

1. Inspect the spark plugs for oil deposits.

Refer to Spark Plug Inspection .

2. Inspect the cylinders for a loss of

compression. Refer to Engine

Compression Test.

3. Perform cylinder leak down and

compression testing to identify the

cause. Refer to Cylinder Leakage

Test.

4. Repair or replace, as required.

2008 Hummer H2

2008 ENGINE Engine Mechanical - 6.2L - H2

BASE ENGINE MISFIRE WITH ABNORMAL INTERNAL LOWER ENGINE NOISES

A damaged crankshaft reluctor wheel

A damaged crankshaft reluctor wheel can

result in different symptoms depending on the

severity and location of the damage.

Systems with electronic

communications, DIS or coil per

cylinder, and severe reluctor ring

damage may exhibit periodic loss of

crankshaft position, stop delivering a

signal, and then sync the crankshaft

position.

Systems with electronic communication,

DIS or coil per cylinder, and slight

reluctor ring damage may exhibit no loss

of crankshaft position and no misfire

may occur. However, a P0300 DTC

may be set.

Systems with mechanical

communications, high voltage switch,

and severe reluctor ring damage may

cause additional pulses and effect fuel

and spark delivery to the point of

generating a P0300 DTC or P0336.

Replace the sensor and/or crankshaft, as

required.

Improper operation of the camshaft position

(CMP) actuator and/or control system

Repair, as required. Refer to Camshaft

Position Actuator and Solenoid Valve

Diagnosis and Testing.

Cause

Correction

Abnormalities, such as severe cracking,

bumps or missing areas in the accessory drive

belt

Abnormalities in the accessory drive system

and/or components may cause engine RPM

variations, noises similar to a faulty lower

engine and also lead to a misfire condition. A

misfire code may be present without an

Replace the drive belt. Refer to Drive Belt

Replacement - Accessory.

2008 Hummer H2

2008 ENGINE Engine Mechanical - 6.2L - H2

BASE ENGINE MISFIRE WITH ABNORMAL VALVE TRAIN NOISE

actual misfire condition.

Worn, damaged, or mis-aligned accessory

drive components or excessive pulley runout

A misfire code may be present without an

actual misfire condition.

Inspect the components and repair or replace,

as required.

Loose or improperly installed engine flex

plate or crankshaft balancer

A misfire code may be present without an

actual misfire condition.

Repair or replace the flex plate and/or

balancer, as required.

Refer to Automatic Transmission Flex

Plate Replacement, or Crankshaft

Balancer Replacement.

Worn piston rings

Oil consumption may or may not cause the

engine to misfire.

1. Inspect the spark plugs for oil deposits.

Refer to Spark Plug Inspection .

2. Inspect the cylinders for a loss of

compression. Refer to Engine

Compression Test.

3. Perform cylinder leak down and

compression testing to determine the

cause. Refer to Cylinder Leakage Test.

4. Repair or replace, as required.

Worn crankshaft thrust bearings

Severely worn thrust surfaces on the

crankshaft and/or thrust bearing may permit

fore and aft movement of the crankshaft and

create a DTC without an actual misfire

condition.

Replace the crankshaft and bearings, as

required.

Cause

Correction

Worn or loose rocker arms

The rocker arm bearing end caps and/or

needle bearings should be intact within the

rocker arm assembly.

Replace the valve rocker arms, as required.

Worn or bent pushrods

Replace the pushrods.

Inspect the top of the pistons for valve

contact. If the top of the piston shows

valve contact, replace the piston and pin

2008 Hummer H2

2008 ENGINE Engine Mechanical - 6.2L - H2