BASIC ENGINE TESTING

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.

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

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

Broken Valve Spring Valve not fully closed

Bent Valve Bent due to piston contact

Burned Valve Face burnt due to poor contact with seat (leaking)

Proper Valve & Seat Position Guide Seat Face

Opening of Valves Controlled directly or indirectly by camshaft

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

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

Lobe Lift & Duration Lift - How far valve will open Duration – How long it stays open

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

Carbon Buildup Carbon build up on intake valve Flat cams Worn valve guide/seal (oil) Fuel deposits Affects V.E. Flat cams Valve doesn’t open fully

Testing

Vacuum gauge testing Provides quick test for compression problems Should read 15-20” of manifold vacuum @ idle Cranking vacuum will be 3-5”

Vacuum Testing Measures vacuum level in intake manifold Indicator of basic engine condition

Normal Ave., steady readings between 15 – 22 in Hg

Leaking Rings Low but steady, 12 – 15 in Hg, late ignition timing, late valve timing

Single Cylinder Misfire Fluctuations between 1 & 2 in Hg Possible burned valve

Sticking Valve Irregular needle drop between 1 & 3 in Hg. Multiple cylinder misfire – rich or lean mixture

Weak Valve Springs Normal at idle Excessive vibrations at higher rpm Sticking valve guides

Worn Valve Guides Excessive vibrations at idle Steadies at higher rpm

Leaking Head Gasket Excessive vibrations at any rpm Loss of compression

Leaking Intake Needle oscillates slowly, drifts between 3 & 9 in Hg.

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

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

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%

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

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

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

Improper Valve Lash Too little will cause sealing problems Too much will cause excessive noise, wear and loss of lift

Valve Lash Adjustment