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

Face

Seat

Guide

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