Chevrolet TrailBlazer, Service Manual

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13. Compare the highest readings from each cylinder to that of the others.
The indicated compression pressures are considered within specifications if
the lowest reading cylinder is within 75 percent of the pressure recorded for
the highest reading cylinder. For example, if your highest reading cylinder
pressure was 150 psi (1034 kPa), then 75 percent of that would be 113 psi
(779 kPa). So the lowest reading cylinder should be no less than 113 psi
(779 kPa)
14. If a cylinder exhibits an unusually low compression reading, pour a tablespoon of
clean engine oil into the cylinder through the spark plug hole and repeat the
compression test.
If the compression rises after adding oil, it means that the cylinder’s piston
rings and/or cylinder bore are damaged or worn
If the pressure remains low, the valves may not be seating properly (a valve
job is needed), or the head gasket may be blown near that cylinder.
Oil and coolant in the combustion chamber, combined with blue or constant white smoke
emitted from the tailpipe, are symptoms of a blown head gasket. There may be evidence
of water droplets on the engine dipstick and/or oil droplets in the cooling system if a head
gasket is blown. The oil will become the consistency of chocolate milk if water is present,
and the coolant will appear muddy if oil is present.
Oil Pressure Test
Check for proper oil pressure at the sending unit passage with an externally mounted
mechanical oil pressure gauge. A tachometer may also be needed, as some specifications
may require running the engine at a specific RPM.
1. While the engine is cold, locate and remove the oil pressure sending unit.
2.
Following the manufacturer’s instructions, connect a mechanical oil pressure gauge
and, if necessary, a tachometer to the engine.
3. Start the engine and allow it to idle.
4. Note and record the oil pressure reading when the engine is cold. The engine may
need to run at a specified RPM, if so check the vehicle's specification.
5. Run the engine until normal operating temperature is reached.
6. Check the oil pressure reading again with the engine hot and record the value.
Turn off the engine.
Compare the hot oil pressure reading to the specification. If the reading is low, check the
cold pressure reading against the specification. If the cold pressure is well above the
specification, and the hot reading was lower than the specification, the engine may have
the wrong viscosity oil. Change the oil, making sure to use the proper grade and quantity,
and then repeat the test.
Low oil pressure readings could be attributed to internal component wear, pump related
problems, a low oil level, or oil viscosity that is too low. High oil pressure readings could be
caused by an overfilled crankcase, too high of an oil viscosity or a faulty pressure relief
valve.