epc 4 basic engine testing

7/28/2019 Epc 4 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 twopurposes:

Sealing - during

compression andpower strokes

Opening up of ports -

during intake andexhaust strokes


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Cylinder Sealing

Performed by tightseal of valve face

against seat to

seal in

compression

pressures

Most common loss

of compression isvalve 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

Face

Seat

Guide


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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 mostcommon, cost effective

design

Hemi style very efficient

air flow, expensive

Porting & polishing

increases air flow, less

restriction & reducedturbulence 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 doesntopen 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 @idle

Cranking vacuum will be 3-5


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Vacuum Testing

Measures vacuum level inintake 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, lateignition 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 lowAdd 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 causeincreased 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

epc 4 basic engine testing

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