Chevrolet Silverado / GMC Sierra. Manual - part 234
COOLANT IN ENGINE OIL
ENGINE COMPRESSION TEST
1. Charge the battery if the battery is not fully charged.
2. Disable the ignition system.
Cracked cylinder liner or engine block
Replace the components, as required.
Cylinder head or engine block porosity
Replace the components, as required.
Cause
Correction
DEFINITION: Foamy or discolored oil or an engine oil overfill condition may indicate
coolant entering the engine crankcase. Low coolant levels, an inoperative cooling fan, or a
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.
2008 Hummer H2
2008 ENGINE Engine Mechanical - 6.2L - H2
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
between the cylinders.
CYLINDER LEAKAGE TEST
Tools Required
J 35667-A Cylinder Head Leakdown Tester, or equivalent. See Special Tools.
Test
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:
2008 Hummer H2
2008 ENGINE Engine Mechanical - 6.2L - H2
1. Disconnect the battery ground negative cable.
2. Remove the spark plugs. Refer to Spark Plug Replacement .
3. Rotate the crankshaft to place the piston in the cylinder being tested at top dead center
(TDC) of the compression stroke.
4. Install the J 35667-A , or equivalent. See Special Tools.
5. Apply shop air pressure to the J 35667-A and adjust according to the manufacturers
instructions. See Special Tools.
6. Record the cylinder leakage value. Cylinder leakage that exceeds 25 percent is considered
excessive and may require component service. In excessive leakage situations, inspect for
the following conditions:
Air leakage noise at the throttle body or air inlet hose that may indicate a worn or
burnt intake valve or a broken valve spring.
Air leakage noise at the exhaust system tailpipe that may indicate a worn or burnt
exhaust valve or a broken valve spring.
Air leakage noise from the crankcase, oil level indicator tube, or oil fill tube that may
indicate worn piston rings, a damaged piston, a worn or scored cylinder bore, a
damaged engine block or a damaged cylinder head.
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 .
IMPORTANT: It may be necessary to hold the crankshaft balancer bolt to
prevent the crankshaft from rotating.
2008 Hummer H2
2008 ENGINE Engine Mechanical - 6.2L - H2
Air bubbles in the cooling system may indicate a damaged cylinder head or a damaged
cylinder head gasket.
7. Perform the leakage test on the remaining cylinders and record the values.
OIL CONSUMPTION DIAGNOSIS
Checks
Causes
Excessive oil consumption, not due to leaks, is the use of 1 L (1 qt) or more of engine oil
within 3 200 kilometers (2,000 miles).
Preliminary
The causes of excessive oil consumption may include the
following conditions:
External oil leaks
Refer to Oil Leak Diagnosis.
Incorrect oil level or improper reading of the oil level
indicator
With the vehicle on a level surface, run the engine for a
few minutes, allow adequate drain down time, 2-3
minutes, and measure for the correct engine oil level.
Improper oil viscosity
Refer to the vehicle owners manual and use the
recommended SAE grade and viscosity for the prevailing
temperatures.
Continuous high speed driving and/or severe usage
Crankcase ventilation system restrictions or
malfunctioning components
Worn valve guides and/or valve stems
Worn or improperly installed valve stem oil seals
Piston rings broken, worn, or not seated properly
Allow adequate time for the rings to seat.
Replace worn piston rings, as necessary.
2008 Hummer H2
2008 ENGINE Engine Mechanical - 6.2L - H2