BASIC ENGINE TESTING

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BASIC ENGINE TESTING
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Four Stroke Cycle

• Intake stroke
• Piston going down - intake valve open.
• Low pressure created in cylinder.
• Volumetric Efficiency maximum at w.o.t.
• Compression stroke
• Piston going up - both valves closed.
• Mixture squeezed to increase combustibility,
  molecules, raises temp.
• Needs good seal to compress and combust.
• Power stroke
• Piston going down - both valves closed.
• Work is performed by expanding gasses.
• Exhaust stroke
• Piston going up - exhaust valve open.
• Exhaust gasses exit.
• Any exhaust left over dilutes incoming a/f
  mixture with inert gas (exhaust)
• Causes poor quality and quantity of charge.

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Valves

• Valves serve two purposes
• Sealing - during compression and power strokes
• Opening up of ports - during intake and exhaust
  strokes

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Cylinder Sealing

• Performed by tight seal of valve face against
  seat to seal in compression pressures
• Most common loss of compression is valve not
  sealing properly

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Broken Valve Spring

• Valve not fully closed

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Bent Valve

• Bent due to piston contact

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Burned Valve

• Face burnt due to poor contact with seat (leaking)

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Proper Valve Seat Position
Guide

• Seat

Face
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Opening of Valves

• Controlled directly or indirectly by camshaft

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Volumetric Efficiency

• Three factors affect VE
• How far the valve opens (lift)
• The amount of time that it is open (duration
• Size of opening
• A problem with valve opening will affect
  volumetric efficiency or the quantity of
  incoming fuel mixture

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Multiple valves

• Multiple valve heads will act same as big valve
  heads
• Secondary throttle plates can be used to
  eliminate loss of ram effect

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Lobe Lift Duration

• Lift - How far valve will open
• Duration How long it stays open

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Head Design

• Wedge style most common, cost effective design
• Hemi style very efficient air flow, expensive
• Porting polishing increases air flow, less
  restriction reduced turbulence in ports

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Carbon Buildup

• Carbon build up on intake valve
• Worn valve guide/seal (oil)
• Fuel deposits
• Affects V.E.
• Flat cams
• Valve doesnt open fully

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Testing
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Vacuum gauge testing

• Provides quick test for compression problems
• Should read 15-20 of manifold vacuum _at_ idle
• Cranking vacuum will be 3-5

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Vacuum Testing

• Measures vacuum level in intake manifold
• Indicator of basic engine condition

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Normal

• Ave., steady readings between 15 22 in Hg

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Leaking Rings

• Low but steady, 12 15 in Hg, late ignition
  timing, late valve timing

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Single Cylinder Misfire

• Fluctuations between 1 2 in Hg
• Possible burned valve

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Sticking Valve

• Irregular needle drop between 1 3 in Hg.
• Multiple cylinder misfire rich or lean mixture

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Weak Valve Springs

• Normal at idle
• Excessive vibrations at higher rpm
• Sticking valve guides

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Worn Valve Guides

• Excessive vibrations at idle
• Steadies at higher rpm

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Leaking Head Gasket

• Excessive vibrations at any rpm
• Loss of compression

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Leaking Intake

• Needle oscillates slowly, drifts between 3 9 in
  Hg.

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Restricted Exhaust

• Normal at idle
• Drops near 0, may rise to lower than normal

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Cranking Compression Test

• Used for checking engine seal
• Must set up conditions
• W.O.T., bat. charger, disable ignition, all plugs
  out
• Compare readings - 10 max difference
• Typical range 110 - 150 psi
• Wet test is performed when cranking compression
  is low
• Add SMALL amount of oil to cylinder, helps seal
  worn rings

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Leak Down Test

• Used to identify location of sealing problem
• Cylinder must be at TDC, comp. stroke
• Problem is detected if loss is greater than 20

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Identifying Location of Leak

• Air escaping through exhaust, open valve
• Air escaping through intake, open valve
• Air escaping through valve cover, worn piston
  rings
• Air escaping through radiator, blown head gasket

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Running Compression Test

• Test cylinder seal at idle
• Readings will be low, less time to fill cylinder
• Compare readings - no specs
• Identifies compression loss apparent while
  running
• Broken valve springs
• Worn valve guided
• Burned valve

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Snap Compression Test

• Tests for problems with v.e.
• Allows rush of air at relatively low rpm, nearly
  filling cylinder which will cause increased
  compression pressures
• If a cylinder has a restricted intake snap
  pressures will be lower
• Compare readings - no specs

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Improper Valve Lash

• Too little will cause sealing problems
• Too much will cause excessive noise, wear and
  loss of lift

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Valve Lash Adjustment