Chevrolet Silverado / GMC Sierra. Service 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