BASIC ENGINE TESTING Four Stroke Cycle Intake stroke

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 – 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