Engine Compression Testing

1. Engine Compression Testing “Getting caught between a rock and a hard place”

2. Cylinder Pressure IS Torque • Pressure generated in the cylinder creates torque at the crankshaft • Torque is directly proportional to cylinder pressure • Any compromise in cylinder pressure causes a corresponding loss in torque • All driveability problems stem from inadequate cylinder pressure.

3. Cylinder Pressure Failure Modes • Cylinders can fail to produce adequate pressure for two reasons. • Blowby • Misfire • Blowby is a mechanical problem • Misfire may be either mechanical, electrical, or chemical in origin.

4. The Pathology of Blow By • An adequate explosion develops in the cylinder • The explosion slips past the compression ring seal due to worn rings or cylinders • Classic symptoms • Low Engine Power • Oil Consumption • Off-gassing from P.C.V. Port • Oil soaked P.C.V. breather

5. Mechanisms of Low Cylinder Pressure • An inadequate explosion develops • May be caused by a number of mechanisms • Mechanical Failures • Chemical Failures • Electrical Failures

6. Mechanical Failures • Mechanical failures in the cylinder lead to inadequate compression • Inadequate compression leads to fire, but not explosion • Common mechanical failures • Head gasket leak • Bent, stuck, or dirty valves • Worn piston rings or cylinders • Hole burned in piston

7. Chemical Failures • An explosion requires an exact mixture of air and fuel • Fuel must be atomized • Exhaust gases must be purged • Systems contributing • Intake • Fuel • Exhaust • Emissions

8. Electrical Failures • An effective explosion requires spark • In the right place • At the right time • With adequate intensity • Spark problems originate within the ignition system • Engine management may contribute also

9. Diagnostics • We will focus on diagnosing mechanical problems • Mechanical problems are caused by poor cylinder sealing. • Leaks • Rings • Valves • Head Gasket

10. Initial Assessment • Verify the fault • Is there really something wrong? • Signs and Symptoms • Does it match the classic presentation? • Significant History • Was the onset fast or slow? • Any events leading up to? • Cranking Test • How does it crank? • Can you hear cylinders coming up on compression?

11. Compression Testing • Disable Ignition! • Make sure key is OFF • If engine starts • Compression gage is junk • If the vehicle is in gear, you get run over • Setup remote starter switch • Clip on crank terminal of solenoid • Clip on positive battery terminal

12. Setup Continued • Test the remote starter switch • Double check ignition and gear • Loosen spark plugs • Crank engine to blow contaminants out • Remove Spark Plugs • Insert tester hose

13. Testing Procedure • Remove air filter • Make sure choke is wide open (if equipped) • Open throttle wide open. • Crank engine through at least 4 needle jumps • Note and record reading • When your done, release air pressure

14. Analyzing Results • Compare Reading to Specifications in All Data • 1972 Chevrolet small block minimum 100 p.s.i. • Test remaining cylinders • All readings should be within 20% of the highest. • Example: Highest reading = 160 p.s.i. • Lowest acceptable reading = .8 * 160 p.s.i. • Lowest acceptable reading = 128 p.s.i.

15. Differential Diagnosis • If you get a cylinder that reads low, there is a leak somewhere • You can confirm or eliminate the rings as a source of leakage by conducting a wet compression test. • Wet Compression Test • Squirt oil into sparkplug hole • Crank engine over several times • Retest compression • If compression increases, rings are worn • If No change, the leak is somewhere else