CompressionTesting
Description
This Activity Plan is designed as one among many through which studentswillrotate insmallgroups.
An automotive compressiontest is veryimportant for determining the internalconditionofanengine. In addition,the compressiontestallows automotive techniciansto evaluate engineproblems thatmayexistina wide rangeofengineareas.The compression testgives theautomotive technicianvitalinformationabout engine health,performance and/or problems.
LessonOutcomes
The student willbeableto:
•Properly usetoolsthat are requiredto doacompression test
•Removeand replace sparkplugs in an effective manner
•Retain an understanding of the propersteps or proceduresrequired todo a compressiontest
•Usecompressiontesting equipment ina safe,effective, andaccurate process.
Assumptions
The student will havea generalunderstanding of basic four-stroke engine theory.
Terminology
Compressiontester: aspecific compression gauge usedto determinehowmuch pressureanenginecan produce.
Figure1—Compressiontester
Thisworkis licensedunder aCreativeCommonsAttribution-NonCommercial-ShareAlike4.0InternationalLicenseunlessotherwiseindicated.
Fuel pumprelay:theelectrical componentthatenergizes thefuelpump.
Ignition system:theelectrical system withinavehicle that delivers spark or ignitiontotheengine.
Sparkplug socket: aspecificsocketdesigned to removeand install a spark plug. Although itmaylook likea regular deep socket,it isdifferent. Inside the cavity ofthe socketis a rubber sockthat protects the spark plug from scratching,crackingor breaking.Inaddition,most sparkplugsocketshaveahex onthe top ofthe socket soa wrenchcanbe used onitforhard toreach orawkwardly-placedsparkplugs.
Figure2—Sparkplugsocket
Sparkplugwirepliers: pliersthat aredesignedto remove thesparkplug wirefromthe sparkplugwithoutcausing anydamage.
Figure3—Sparkplugboot pullerFigure4—Sparkplug boot pliers
Wideopenthrottle:aconditionwherethemaximum amount of air isallowed into theengine.
EstimatedTime
1 hour
RecommendedNumberofStudents
20, basedontheBCTechnology Educators’Best PracticeGuide
Facilities
Automotive shopor atechnologyeducationshop that will accommodatea vehicle
Tools
•Spark plug socket
•Spark plug wire pliers
•12" 3⁄8"extension
•3⁄8"ratchet
•Compression tester
•Oilsquirtcan
Materials
Youwill need avehiclein ordertodo the compressiontest.Ideallythe vehicle will be smallerand preferably afourcylinder. Itis much easiertodo acompressiontest with thistypeofvehicle.
Resources
CompressionTestingandWhatYouCanLearnfromIt
HowtoDoaCompressionTest
Activity1:Dry CompressionTest
1.Theteacher willdemonstratehowto correctly doacompressiontest. After the studentshaveviewed thedemonstration they will begiven the opportunity to dothe compressiontest. This isusually done inalab-basedformat with instructor signaturesrequiredatvariousstages.
2.Start thevehicleandwarmit up until the engineisat normaloperating temperature(approximately5–10minutes).Note:Onlytheinstructorshouldoperatethevehicle.
Figure5—Sparkplug wires attached toengine
3.Removeall ofthesparkplug wires from the sparkplugs. Itis agood idea to labelthe sparkplugwires so you knowwhich cylinder they camefrom.Note: Graspand twist the sparkplugboot(not the wire) in order to remove. Sometimesdueto the heat ofthe engine,sparkplugbootsgetstuck on the sparkplugs. In such cases itmaybe necessary to use sparkplugboot puller/pliers.
4.Disable the ignition systembyremovingand thengrounding the high-tension lead.
5.Removethe fuel pumprelay.Thisrelayis usuallyfound in thefuse boxunder the hood andis labelled assuch.
Figure6—Typical fuelpumprelaylocation
Figure7—Label thespark plug wires sothey do notget mixed up.Thenremovethecentre wireandground this high-tensionlead.
6.Removeall ofthe spark plugsusing a spark plug socket.
7.Insert thecompressiontesterinto oneofthesparkplugholes.
Figure8—Removecentrewire
Figure9—Compression gauge inserted into sparkplug hole
8.Withapartner cranktheengine over for 5seconds. Oneperson willbe holdingthecompression testerand the other will be insidethe vehicle cranking theengine over.Note: Makesuretohavethegaspedalallthewaytothe floor when performing thisoperation, asthis will allowformaximumair into the engine.
9.Write downthereading andrepeat fortherest ofthecylinders.
10.Compareall of the compressiontest results.They should be within20% of each other.
Activity2:Wet CompressionTest
Awet compression testis similartoadrycompressiontestexceptit hastheaddition of oilplacedin eachcylinder. Awet compression testis done for tworeasons:
•Oneor more cylinders has areading of less than 100 psion the drycompression test.
•Oneor morecylindersismorethan 20%different from theothercylinders onadrycompressiontest.
Theextra oilin thecylindershouldtemporarilysealthe pistonringsinthe cylinder. The resultscanhelp diagnose possibleengine problems.
Procedure:
1.Performadry compression test and recordtheresults.
2.With anoil squirt can,placeapproximatelyonetablespoonofoil(15mL)intothefirstcylinder—this is roughlytwosquirts of oil.
Note: Do not putexcess oil inthe cylinder!
3.Crank the engine approximatelyfour timesto move the oilaround the cylinder.
4.Insert the compressiongauge andcheck the compressionasbefore.
5.Record thenewreading.
6.Repeat this processon eachcylinder.Note:Squirt oil into one cylinder at atime. Squirtingoil into allthe cylindersat once andthen doing thecompressiontestwillskew theresults,sincebythetime you gettothe last cylindertheoil willhave lost itseffectiveness.
7.Comparetheresultsof the wettest withthose of thedrytest. In particular,pay attentiontothecylindersthat were questionable from thedry compressiontest. One of two thingsshould happen:
a.Theresults remainabout thesame.Example:
Drytest:115
Wet test: 120
Diagnosis: Thelow compressionislikely dueto wear inthe valvesor valve guides.
b.The results improvesignificantly.Example:
Drytest:115
Wet test: 135
Diagnosis: Thelowcompression is likely dueto wearinthepistonrings.
Note: When starting thevehicleforthefirsttime after a wetcompression test,excesssmokemaycome out of the exhaust. Thisis perfectly normaland shouldonlylast for2–5 minutes.
EvaluationGuidelines
•The students use therequiredtoolsina safeandeffective manner.
•Theignition systemis disabled properly.
•Thefuel systemis disabled properly.
•The studentscanremove and replacethe spark plugs correctly withoutdamagingthem.
•Thecompression testitself is carried out in asafeand effectivemanner.
•The student’sevaluation of the compression test datais acorrect interpretationofthehealth of the engine.