Compression Test 101

Original Author: c.d.iesel

How to perform a compression test?

  • ALUMINUM HEADS? loosen the spark plugs when cold, and snug up to warm up the motor
  • remove all the plugs
  • battery must be fully charged and starter motor spinning like mad
  • disconnect your ignition primary - even more important if you have a HugeBox-o-Transistor mega-spark system
  • also for all of you with powered fuel pumps and injection systems - de-power yer pumps so the possibility of FIRE is eliminated!!
  • hold the throttle fully open during cranking
  • valve clearance should have been adjusted first- COLD - only crank each tested cylinder 4 revolutions
  • record each of the 4 pulse readings of each of the 4 revolutions
  • the 1st pulse of the needle should be more than 1/2 of the 4th pulse
  • for example, if 185 is the target: 30-60-90-120= there's a leak all the time and uniform in size like a burnt valve, or severly burned out head gasket
  • 90-100-110-120 = that cylinder is sealing up to a point but begins to leak - rings, leaking head gasket....
  • 120-145-165-185 = is normal
  • adding a little oil to a weak cylinder and re testing will only show some sealing by the rings - but indicate a direction to look
  • proceed to the Cylinder Leak Test

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