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