Content .. 1877 1878 1879 1880 ..
Chevrolet Silverado / GMC Sierra. Manual - part 1879
CYLINDER DEACTIVATION (ACTIVE FUEL MANAGEMENT) SYSTEM DIAGNOSIS
1. Measure the engine oil level. Fill, as required.
2. Using the scan tool, inspect for diagnostic codes within the engine control module (ECM).
Refer to Diagnostic System Check - Vehicle . Repair, as required.
3. Verify proper engine oil pressure and operation of the active fuel management oil pressure
relief valve. Refer to Oil Pressure Diagnosis and Testing.
4. Verify proper operation of the active fuel management system components. Refer to
Cylinder Deactivation (Active Fuel Management) System Compression Test.
faulty thermostat may lead to an overtemperature condition which may cause engine
component damage. Contaminated engine oil and oil filter should be changed.
1. Inspect the oil for excessive foaming or an overfill condition. Oil diluted by coolant
may not properly lubricate the crankshaft bearings and may lead to component
damage. Refer to Lower Engine Noise, Regardless of Engine Speed.
2. Inspect by performing a cylinder leak-down test. During this test, excessive air
bubbles within the cooling system may indicate a faulty gasket or damaged
component.
3. Inspect by performing a cylinder compression test. 2 cylinders side-by-side on the
engine block with low compression may indicate a failed cylinder head gasket. Refer
to Engine Compression Test.
Faulty external engine oil cooler
Replace the components, as required.
Faulty cylinder head gasket
Replace the head gasket and components, as
required. Refer to Cylinder Head Cleaning
and Inspection and Cylinder Head
Replacement - Left Side or Cylinder Head
Replacement - Right Side .
Warped cylinder head
Machine the cylinder head to proper flatness,
if applicable, and replace the cylinder head
gasket. Refer to Cylinder Head Cleaning
and Inspection .
Cracked cylinder head
Replace the cylinder head and gasket.
Cracked cylinder liner or engine block
Replace the components, as required.
Cylinder head, block, or manifold porosity
Replace the components, as required.
IMPORTANT: Cylinder Deactivation (Active Fuel Management) System
Diagnosis is applicable for RPO LH6/LMG/LY5/LC9/L76.
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Mechanical - 4.8L, 5.3L, 6.0L, 6.2L, or 7.0L - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
5. Remove the valve lifter oil manifold. Refer to Valve Lifter Oil Manifold Replacement .
6. Verify proper operation of the valve lifter oil manifold. Refer to Cylinder Deactivation
(Active Fuel Management) Valve Lifter Oil Manifold Diagnosis and Testing.
7. Replace the active fuel management valve lifters, as required. Both intake and exhaust lifters
should be replaced in pairs. Refer to Valve Lifter Replacement (L92 - First Design) or
Valve Lifter Replacement (L92 - Second Design) or Valve Lifter Replacement (LY2
and LY6) or Valve Lifter Replacement (LH6, LMG, LY5 and L76) .
ENGINE COMPRESSION TEST
1. Charge the battery if the battery is not fully charged.
2. Disable the ignition system.
3. Disable the fuel injection system.
4. Remove all spark plugs.
5. Turn the ignition to the ON position.
6. Depress the accelerator pedal to position the throttle plate wide open.
7. Start with the compression gage at zero and crank the engine through 4 compression strokes,
4 puffs.
8. Measure the compression for each cylinder. Record the readings.
9. If a cylinder has low compression, inject approximately 15 ml (1 tablespoon) of engine oil
into the combustion chamber through the spark plug hole. Measure the compression again
and record the reading.
10. The minimum compression in any 1 cylinder should not be less than 70 percent of the
highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the
highest pressure in any 1 cylinder is 1 035 kPa (150 psi), the lowest allowable pressure for
any other cylinder would be 725 kPa (105 psi). (1 035 x 70% = 725) (150 x 70% = 105).
Normal - Compression builds up quickly and evenly to the specified compression for
each cylinder.
Piston Rings Leaking - Compression is low on the first stroke. Compression builds up
with the following strokes, but does not reach normal. Compression improves
considerably when you add oil.
Valves Leaking - Compression is low on the first stroke. Compression usually does not
build up on the following strokes. Compression does not improve much when you add
oil.
If 2 adjacent cylinders have lower than normal compression, and injecting oil into the
cylinders does not increase the compression, the cause may be a head gasket leaking
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Mechanical - 4.8L, 5.3L, 6.0L, 6.2L, or 7.0L - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
between the cylinders.
CYLINDER DEACTIVATION (ACTIVE FUEL MANAGEMENT) SYSTEM COMPRESSION TEST
TEST
The Cylinder Deactivation (Active Fuel Management) System Compression Test is a functional
test of the valve lifter oil manifold and valve lifters. Using a compression gage, the technician can
monitor the specific cylinders ability to go into and out of active fuel management as commanded
by the scan tool. The actual pressure values on the compression gage are not as critical as is
observing the on-off switching action of the solenoid and valve lifters.
1. Measure the engine oil level. Fill, as required.
2. Perform the Diagnostic System Check - Vehicle. Refer to Diagnostic System Check -
Vehicle .
3. Disable the ignition system for the cylinder to be tested, by disconnecting the electrical wire
harness to the ignition coil.
4. Disable the fuel injection system for the cylinder to be tested, by disconnecting the electrical
wire harness to the fuel injector.
5. Remove the spark plug and wire for the cylinder to be tested.
6. Install a 0-1 378 kPa (0-200 psi) compression gage.
7. Start the engine.
8. Using the scan tool output controls, command the solenoid ON, for the cylinder to be
deactivated.
9. Depress the Schrader valve on the compression gage, in order to release the pressure and
zero the gage. With the engine running and the cylinder in active fuel management mode, the
compression reading should be less than 172 kPa (25 psi).
10. Using the scan tool, de-energize the solenoid while observing the reading on the
compression gage. With the engine running and the cylinder NOT in active fuel management
mode, the compression gage reading should increase quickly to greater than 345 kPa (50
psi).
IMPORTANT: Cylinder Deactivation (Active Fuel Management) System
Compression Test is applicable for RPO LH6/LMG/LY5/LC9/L76.
IMPORTANT: Only 1 cylinder can be tested at a time. When testing has
been completed on a specific cylinder, the wire harness
electrical connectors, spark plug, and spark plug wire must
be installed prior to testing of each additional cylinder.
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Mechanical - 4.8L, 5.3L, 6.0L, 6.2L, or 7.0L - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
11. Test each of the 4 active fuel management cylinders, 1 at a time. If the readings are not
within specifications, remove, and test the valve lifter oil manifold, as required. Refer to
Valve Lifter Oil Manifold Replacement and Cylinder Deactivation (Active Fuel
Management) Valve Lifter Oil Manifold Diagnosis and Testing.
12. If the compression test has been performed and all active fuel management cylinders are
operating within specifications, the active fuel management system is performing as
designed and no repairs are required.
13. Clear the diagnostic trouble codes (DTCs) with a scan tool.
CYLINDER LEAKAGE TEST
Tools Required
J 35667-A Cylinder Head Leakdown Tester, or equivalent
Test
1. Disconnect the battery ground negative cable.
2. Remove the spark plugs. Refer to Spark Plug Replacement .
IMPORTANT: A leakage test may be performed in order to measure
cylinder/combustion chamber leakage. High cylinder leakage may
indicate one or more of the following conditions:
Worn or burnt valves
Broken valve springs
Stuck valve lifters
Incorrect valve lash
Damaged piston
Worn piston rings
Worn or scored cylinder bore
Damaged cylinder head gasket
Cracked or damaged cylinder head
Cracked or damaged engine block
CAUTION: Refer to Battery Disconnect Caution .
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Mechanical - 4.8L, 5.3L, 6.0L, 6.2L, or 7.0L - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
Content .. 1877 1878 1879 1880 ..