Hal48901 Ch31 - Pearson

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31Engine PerformanceDiagnosis and TestingOBJECTIVES: After studying Chapter 31, you shouldbe able to:1. Prepare for the interprovincial Red Seal certificationexamination in Appendix VIII (Engine Performance)on the topics covered in this chapter.2. List the steps of the diagnostic process.3. Describe the simple preliminary tests that shouldbe performed at the start of the diagnostic process.4. List six items to check as part of a thorough visualinspection.5. List the six fundamental troubleshooting principles.6. List the precautions that should be taken whenworking on computerized engine control systems.7. Explain the troubleshooting procedures to follow ifa diagnostic trouble code has been set.8. Explain the troubleshooting procedures to follow ifno diagnostic trouble codes have been set.9. Discuss the diagnosis of a vehicle equipped with thesecond generation of on-board diagnostics (OBD II).10. List acceptable levels of HC, CO, CO2, and O2 withand without a catalytic converter.11. List four possible causes of high readings for HC,CO, and NOx.method that eliminates known good components orsystems in order to find the root cause of automotiveengine performance problems. All vehicle manufacturers recommend a diagnostic procedure, and the plansuggested in this chapter combines most of the features of these plans plus additional steps developedover years of real-world problem solving.THE EIGHT-STEPDIAGNOSTIC PROCEDUREMany different things can cause an engine performance problem or concern. The service technicianhas to narrow the possibilities until the cause of theproblem is found and corrected. A funnel is a way ofvisualizing a diagnostic procedure. See Figure 31–1.At the wide top are the symptoms of the problem; thefunnel narrows as possible causes are eliminateduntil the root cause is found and corrected at the bottom of the funnel.All problem diagnosis deals with symptoms thatcould be the result of many different causes. Thewide range of possible solutions must be narrowed tothe most likely and these must eventually be furthernarrowed down to the actual cause. The followingsection describes eight steps the service techniciancan take to narrow the possibilities to one cause.It is important that all automotive service techniciansStep #1. Verify the Problemknow how to diagnose and troubleshoot engine computer systems. The diagnostic process is a specificBefore one minute is spent on diagnosis, be certainthat a problem exists. If the problem cannot be739

740CHAPTER 31T E C HT I P “Original Equipment” Is Nota Four-Letter WordMany problems can be traced to the use of an aftermarket part that has failed early in its service life.Technicians who work at dealerships usually go immediately to an aftermarket part that is observed duringa visual inspection. It has been their experiencethat simply replacing the aftermarket part with the factory original equipment (OE) part often solves theproblem.Original equipment parts are required to pass qualityand durability standards and tests that are not requiredof aftermarket parts. The technician should be aware thatthe presence of a new part does not necessarily meanthat the part is good.Figure 31–1 A funnel is a visual method of thinking of thediagnostic process. The goal, of course, is to find the rootcause and to repair it in order to achieve customersatisfaction.verified, the problem cannot be solved or tested toverify that the repair was complete. See Figure 31–2.The driver knows the vehicle and how it isdriven. Before diagnosis is started, always ask thefollowing questions: Is the malfunction indicator light (check engine)on?What was the temperature outside?Was the engine warm or cold?Was the problem during starting, acceleration,cruise, etc.?How far had the vehicle been driven?Were any dash warning lights on? If so, which one?Has there been any service or repair workperformed on the vehicle lately?After the nature and scope of the problem are determined, the complaint should be verified beforefurther diagnostic tests are performed. A sample of aform that customers could fill out with details of theproblem can be seen in Figure 31–3. Perform a thorough test drive under similar conditions to verify thecomplaint (problem or concern).Step #2. Perform a ThoroughVisual Inspection and Basic TestsThe visual inspection is the most important aspectof diagnosis! Most experts agree that between 10and 30% of all engine performance problems canbe found simply by performing a thorough visualinspection.NOTE: The purpose of any fault diagnosis is the elimination of known good components.The inspection should include the following: NOTE: This last question is very important! Many engine performance faults are often the result of something being knocked loose or a hose falling off duringrepair work, etc. Knowing that the vehicle was justserviced before the problem began may be an indicator.Check for obvious problems.Fuel leaksVacuum hoses disconnected or split (see Figure31–4)Corroded connectorsUnusual noises, smoke, or smellCheck the air cleaner and air duct forrestrictions. See Figure 31–5.

Engine Performance Diagnosis and Testing741Figure 31–2 Diagnostic funnel. Step#1 is to verify the problem. If theproblem cannot be duplicated,obviously the repair cannot beperformed. Check everything that does and does notwork. This step involves turning things on andobserving that everything is working properly.Look for evidence of previous repairs. Anytime work is performed on a vehicle, there isalways a risk that something will be disturbed,knocked off, or left disconnected.Check oil level and condition. Another areafor visual inspection is oil level and condition.Oil level—Oil should be to the proper level.Oil condition—Using a match or lighter, try tolight the oil on the dipstick; if the oil flamesup, gasoline is present in the engine oil. Dripsome engine oil from the dipstick onto thehot exhaust manifold. If the oil bubbles orboils, coolant (water) is present in the oil.Check for grittiness by rubbing the oilbetween your fingers.NOTE: Gasoline in the oil will cause the engine to runrich by drawing fuel through the positive crankcaseventilation (PCV) system. Check coolant level and condition. Mostmechanical engine problems are caused byoverheating. The proper operation of the coolingsystem is critical to the life of any engine.

742CHAPTER 31ENGINE PERFORMANCE DIAGNOSIS WORKSHEET(To Be Filled Out By the Vehicle Owner)Name:Make:Kilometres (Mileage):Model:Date:Year:Engine:(Please Circle All That Apply in All Categories)Describe Problem:When Did the ProblemFirst Occur? Just Started Other Last Week Last MonthList Previous Repairsin the Last 6 Months:Starting Problems Will Not Crank Cranks, but Will Not Start Starts, but Takes a Long TimeEngine Quits or Stalls Right after Starting When Put into Gear During Steady Speed Driving Right after Vehicle Comes to a Stop While Idling During Acceleration When ParkingPoor Idling Conditions Is Too Slow at All Times Is Too Fast Intermittently Too Fast or Too Slow Is Rough or Uneven Fluctuates Up and DownPoor Running Conditions Runs Rough Lacks Power Bucks and Jerks Poor Fuel Economy Hesitates or Stumbles on Acceleration Backfires Misfires or Cuts Out Engine Knocks, Pings, Rattles Surges Dieseling or Run-OnAuto. Transmission Problems Improper Shifting (Early/Late) Changes Gear Incorrectly Vehicle Does not Move When in Gear Jerks or BucksUsually Occurs Morning Afternoon AnytimeEngine Temperature ColdDriving Conditions DuringOccurrence Short—Less Than 3 km (2 mi) 3–15 km (2–10 mi) Long—More Than 15 km (2–10 mi) Stop and Go While Turning While Braking At Gear Engagement With A/C Operating With Headlights On During Acceleration During Deceleration Mostly Downhill Mostly Uphill Mostly Level Mostly Curvy Rough RoadDriving Habits Mostly City Driving Highway Park Vehicle Inside Park Vehicle OutsideDrive Per Day: Less Than 15 km (10 mi) 15 to 80 km (10–50 mi) More Than 80 km (50 mi)Gasoline UsedFuel Octane: 87 89 91 More Than 91Brand:Temperature When ProblemOccurs 0–13 C (32–55 F) Below Freezing 0 C (32 F) Above 13 C (55 F) Warm HotCheck Engine Light/Dash Warning Light Light On SometimesSmells "Hot" Gasoline Oil Burning ElectricalNoises Rattle Knock Squeak Other Light On Always Light Never OnFigure 31–3 Form that can be given to a customer to be filled out before attemptingto diagnose an engine performance problem.

(a)Figure 31–4 All vacuum hoses should be checked to seeif they are cracked, swollen, or split.T E C HT I P Smoke Machine TestingVacuum (air) leaks can cause a variety of driveabilityproblems and are often difficult to locate. One methodthat works well is to use a machine that generates aburst of smoke. Connecting the outlet of the machine tothe hose removed from the vacuum brake booster allows smoke to enter the intake manifold. Any vacuumleaks will be spotted by observing smoke coming out ofthe leak. See Figure 31–6. A theatre smoke machineworks well.(a)(b)Figure 31–5 (a) This is what was found as the air filterhousing was opened during service. The nuts wereobviously deposited by squirrels (or some other animal).(b) Not only was the housing filled with nuts, but this airfilter was extremely dirty, indicating that this vehicle hadnot been serviced for a long time.(b)Figure 31–6 (a) A shot of smoke from a smoke machine. In actual use, this outlet isconnected to a disconnected vacuum hose on the engine being tested. A convenient hoseto use is the hose at the vacuum brake booster. The machine forces smoke into the intakemanifold through the hose. (b) To keep the smoke from escaping through the throttle plateopening, a plastic bag can be used to seal the opening. Here a rubber glove is used, andwhile it looks strange, it worked well.743

744CHAPTER 31Figure 31–7 The paper test involves holding a piece ofpaper near the tailpipe with a warm engine at idle. A goodengine should produce an even, outward flow of exhaust. Ifthe paper is sucked in toward the tailpipe, a burned valve isa possibility.(a)NOTE: Check the coolant level in the radiator only ifthe radiator is cool. If the radiator is hot and the radiator cap is removed, the drop in pressure above thecoolant will cause the coolant to boil immediately,which can cause severe burns because the coolant expands explosively upward and outward from the radiator opening. Perform the paper test. A soundly runningengine should produce even and steadyexhaust at the tail pipe. Hold a piece of paperor a 75 125 mm (3 5 in.) card within25 mm (1 in.) of the tail pipe with the enginerunning at idle. See Figure 31–7. The papershould blow evenly away from the end of thetail pipe without puffing or being drawninward toward the end of the tail pipe. If thepaper is at times drawn toward the tail pipe,the valves in one or more cylinders could beburned. Other reasons why the paper might bedrawn toward the tail pipe include thefollowing:1. The engine could be misfiring because of alean condition or an ignition system fault suchas a bad spark plug wire.2. A faulty fuel injector.3. Pulsing of the paper toward the tail pipe couldalso be caused by a hole in the exhaust system.If exhaust escapes through a hole in theexhaust system, air could be drawn—in theintervals between the exhaust puffs—from thetail pipe to the hole in the exhaust, causing thepaper to be drawn toward the tail pipe.Ensure adequate fuel level. Make certain thatthe fuel tank is at least one-fourth to one-halffull; if the fuel level is low it is possible that anywater or alcohol at the bottom of the fuel tank ismore concentrated and can be drawn into thefuel system.(b)Figure 31–8 (a) A typical spark tester that uses a clearplastic shield that protects the spark from igniting anyflammable substance that may be near while testing for spark.(b) A spark tester that is adjustable for different voltages.Most electronic ignition systems should be able to jump a25 mm (1 in.) gap, which is equal to about 40 000 volts. Check the battery voltage. The voltage of thebattery should be at least 12.4 volts and thecharging voltage (engine running) should be13.5 to 15.0 volts at 2000 rpm. Low batteryvoltage can cause a variety of problemsincluding reduced fuel economy and incorrect(usually too high) idle speed.Check the spark using a spark tester.Remove one spark plug wire and attach theremoved plug wire to the spark tester. Attach thegrounding clip of the spark tester to a good cleanengine ground, start or crank the engine, andobserve the spark tester. See Figure 31–8. Thespark at the spark tester should be steady andconsistent. If an intermittent spark occurs, thenthis condition should be treated as a no-sparkcondition. If this test does not show satisfactoryspark, carefully inspect and test all componentsof the primary and secondary ignition systems.See Chapter 24 for details.

Engine Performance Diagnosis and Testing745Figure 31–9 Step #3 in thediagnostic process is to retrieve anystored diagnostic trouble codes(DTCs). The vehicle computer isconstantly monitoring all sensorsand control devices and, therefore, ifa DTC was stored, it makes senseto use the ability of the vehiclecomputer to help narrow the focusof what could be wrong.NOTE: Do not use a standard spark plug to check forproper ignition system voltage. An electronic ignitionspark tester is designed to force the spark to jumpabout 20 mm (0.75 in.). This amount of gap requires between 25 and 30 kilovolts (kV) at atmospheric pressure, which is enough voltage to ensure that a sparkcan occur under compression inside an engine. Check the fuel pump pressure. Checking thefuel pump pressure is relatively easy on manyport fuel-injected engines. Often the cause ofintermittent engine performance is due to aweak electric fuel pump. Checking fuel pumppressure early in the diagnostic processeliminates low fuel pressure as a possibility.Step #3. Retrieve the DiagnosticTrouble CodesIf a diagnostic trouble code (DTC) is present in thecomputer memory, it is signalled by illuminating amalfunction indicator lamp (MIL), commonly labelled“check engine” or “service engine soon.” See Figure31–9. The code(s) displayed if the MIL is on is called ahard code. Any code(s) that is displayed when theMIL is not on is called a soft code. A soft code is sometimes called an intermittent code and indicatesthat the computer detected a fault in the circuit

746CHAPTER 31represented by the DTC. Because the MIL is not on,this indicates that the fault is no longer present. Although this soft code is helpful to let the technicianknow that a fault has, in the past, been detected, further testing will be needed to find the root cause of theproblem. Most vehicle manufacturers state that thediagnostic procedure for a DTC is for a hard code only.Step #4. Check for TechnicalService Bulletins (TSBs)Check for corrections in bulletins that match thesymptoms. See Figure 31–10. According to studiesperformed by automobile manufacturers, as manyas 30% of vehicles can be repaired following the information, suggestions, or replacement parts foundin a service bulletin. (DTCs must be known beforesearching for service bulletins because bulletins often include information on solving problems thatinvolve a stored diagnostic trouble code.)Step #5. Look at Scan Tool DataStarting in 1981, General Motors and Chrysler vehicle manufacturers have been giving the technician more and more data on a scan tool connectedto the data link connector or DLC. See Figure31–11. Beginning technicians are often observedscrolling through scan data without a real clue towhat they are looking for. When asked, they usuallyreply that they are looking for something unusual,as if the screen will flash a big message “LOOKHERE—THIS IS NOT CORRECT.” That statementdoes not appear on scan tool displays. See Figure31–12. The best way to look at scan data is in a definite sequence and with specific, selected bits ofdata (also called parameter identification orPID) that can tell the most about the operation ofthe engine such as:1. Engine coolant temperature (ECT) is the sameas intake air temperature (IAT) after thevehicle sits for several hours.2. Idle air control (IAC) valve is being commandedto an acceptable range.3. Oxygen sensor (O2S) is operating properly: Readings below 200 millivolts at times Readings above 800 millivolts at times Rapid transitions between rich and lean At least eight cross counts on a fuel-injectedengineStep #6. Narrow the Problem toa System or CylinderNarrowing the focus to a system or individual cylinder is the hardest part of the entire diagnostic process.Figure 31–10 After checking for stored diagnostictrouble codes (DTCs), the technician checks to see if thereare any technical service bulletins (TSBs) that relate to thevehicle being serviced.T E C HT I P The Five WhysWhenever a problem is detected, the smart technicianshould ask, “Why did this part or component fail?” Forexample, consider a vehicle that misfired under load. Athorough inspection revealed a cracked spark plug. Replacing the spark plugs solved the misfire problem butonly for a few weeks. Again, a spark plug was found to becracked. Now the technician has to ask, “why?” Obviously, the cause of the engine misfire has been determined, but what can cause the recurring cracked sparkplug? A missing inner-fender splash shield could be lettingwater splash onto a hot spark plug, causing it to crack. Ifthe shield is missing, the technician could ask why it wasmissing. Perhaps some other body parts have been damaged by an accident or other cause.Usually by the time the technician has asked “why”five times, the root cause of the problem has been determined. Asking yourself the five whys is being truly professional. Customers expect their vehicle to be repairedright the first time. Correcting the root cause is the keyto customer satisfaction.

Figure 31–11 Scan tool data is apowerful tool to use to find engineperformance malfunctions.Figure 31–12 Using a Snap-On scan tool to check forengine data that may give an indication as to the root causeof the problem.747

748CHAPTER 31 The radio is turned off. The clock is set to the right time and theradio stations have been restored if thebattery was disconnected during the repairprocedure.T E C HT I P One Test Is Worth 1000“Expert” OpinionsWhenever any vehicle has an engine performance or driveability concern, certain people always say:“Sounds like it’s the ignition coil.”“I’ll bet you it’s a bad computer.”“I had a problem just like yours yesterday and it was abad EGR valve.”Figure 31–13 An engine analyzer can also be used tohelp narrow the problem to a particular cylinder orsystem. Perform a cylinder power balance test. SeeFigure 31–13.If a weak cylinder is detected, perform acompression and a cylinder leakage test todetermine the probable cause (see Chapter 5 fordetails).Step #7. Repair the Problem andDetermine the Root CauseThe repair or part replacement must be performedfollowing vehicle manufacturer’s recommendations.Read the Tech Tip “The Five Whys” to be certain thatthe root cause of the problem has been found. Alsofollow manufacturers’ recommended repair procedures and methods.Step #8. Verify the Repair andClear Any Stored DTCsSee Figure 31–14.1. Test drive to verify that the original problem(concern) is fixed.2. Verify that no additional problems have occurredduring the repair process.3. Clear all diagnostic trouble codes. (This stepensures that the computer will not make anychanges based on any stored DTC.)4. Before returning the vehicle to the customerdouble check that The vehicle is clean.Regardless of the skills and talents of those people,it is still more accurate to perform tests on the vehiclethan to rely on feelings or opinions of others who havenot even seen the vehicle. Even your own opinion shouldn

743 Figure 31–4 All vacuum hoses should be checked to see if they are cracked,swollen,or split. Figure 31–5 (a) This is what was found as the air filter housing was opened during service. The nuts were obviously deposited by squirrels (or some other animal). /p div class "b_factrow b_twofr" div class "b_vlist2col" ul li div strong File Size: /strong 807KB /div /li /ul ul li div strong Page Count: /strong 33 /div /li /ul /div /div /div div data-tag ck" /div

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