Compression Test
ENGINE COMPONENT TEST
Compression Test
1. Make sure oil in crankcase is of the correct viscosity and at the proper level and that the battery is properly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove the spark plugs. See Service and Repair.
2. Place the transmission in P (Park) (A/T) or N (Neutral) (M/T) and apply the parking brake.
3. Block throttle plate in the wide-open position.
4. Install a compression gauge in the No.1 cylinder spark plug hole.
5. Install a remote starter switch in the starting circuit. With the ignition switch in the OFF position, and using the remote starter switch, crank the engine a minimum of five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
6. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.
Compression Test - Results
The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75 percent of the highest reading. The following Compression Pressure Limit Chart shows acceptable pressure ranges. Use the highest compression reading obtained from the compression test and verify that the lowest compression reading is within the range shown in the chart. See the Compression Pressure Limit Chart.
Compression Pressure Limit Chart
If one or more cylinders reads low, squirt approximately one tablespoon of clean 10W40 engine oil on top of the piston(s) in the low-reading cylinder(s). Repeat the compression pressure check on these cylinders.
Compression Test - Interpreting Compression Readings
1. If compression improves considerably, piston rings require service.
2. If compression does not improve, valves are sticking or seating improperly.
3. If two adjacent cylinders indicate low compression pressures and squirting oil on piston does not increase compression, the head gasket may be leaking between cylinders. Engine oil and/or coolant in the cylinders could result from this condition. Use the Compression Pressure Limit Chart when comparing cylinder compression readings to determine if the lowest reading is at least 75 percent of the highest reading.
Cylinder Leakage Detection
When a cylinder produces a low reading, use a leak detector to help pinpoint the exact cause.
The leak detector is inserted in the spark plug hole, the piston is brought up to dead center on the compression stroke and compressed air is admitted.
Once the combustion chamber is pressurized, read the percentage of leakage. Leakage exceeding 20 percent is excessive.
While the air pressure is retained in the cylinder, listen for the hiss of escaping air. A leak at the intake valve will be heard in the intake manifold and a leak at the exhaust valve and can be heard at the tail pipe. Leakage past the piston rings will be audible at the PCV connection or oil filler opening. If air is passing through a blown head gasket to an adjacent cylinder, the noise will be evident at the spark plug hole of the cylinder into which the air is leaking. Cracks in the cylinder block or gasket leakage into the cooling system may be detected by a stream of bubbles in the radiator.
Oil Leak and Valve Stem Seal Test
The cylinder leakage detector tests for engine oil leaks and checks the valve stem seals for leakage.
1. Plug all the crankcase openings except the one used for connecting the leakage detector.
2. Connect the leak detector to a crankcase opening (an oil level indicator tube is convenient). Adjust the air pressure to approximately 34 kPa (5 psi).
3. Brush a solution of liquid soap and water along the gasket sealing surfaces and bearing seals. Look for bubbles or foam.
4. Remove the spark plug and rotate the crankshaft slowly with a wrench. Check for large amounts of air escaping into the cylinders as each intake valve and exhaust valve opens.
5. The spark plugs on the leaking cylinders will probably show deposits of burned oil.