Engine Performance Questions and Answers with Complete Solutions

Engine Performance Questions and Answers with Complete Solutions Blue smoke is coming from the exhaust pipe of a vehicle. Tech A says that worn piston rings could be the cause. Tech B says that a bad head gasket could ...

Engine Performance Questions and Answers with Complete Solutions Blue smoke is coming from the exhaust pipe of a vehicle. Tech A says that worn piston rings could be the cause. Tech B says that a bad head gasket could be the cause. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔A. Blue smoke means the engine is burning oil, which could be caused by worn piston rings. Technician B is wrong because a bad head gasket would cause coolant to leak into one or more cylinders, resulting in white exhaust smoke. A vacuum gauge is connected to the intake manifold engine and the engine is run at 2,000 rpm. During the test, the pointer on the gauge fluctuates rapidly between readings of 10 and 22 inches of vacuum. These test results point to: A. a leaking intake manifold gasket B. worn piston rings C. worn valve guides D. a weak or broken valve spring ✔✔D. A leaking intake manifold gasket or worn piston rings would not cause a vacuum gauge needle to fluctuate. Worn valve guides would make the needle fluctuate at idle but the needle would steady when engine speed is increased. A weak or broken valve spring will cause a vacuum gauge to fluctuate or jump from low to high readings. Engine detonation (knock) could be caused by any of the following EXCEPT: A. a lean air/fuel mixture B. retarded ignition timing C. excess carbon in the combustion chambers D. a stuck-closed EGR valve ✔✔B. Advanced ignition timing will cause knock, but retarded timing will not. Tech A says that spark advance on an engine can be tested by listening to the way the engine sounds at higher rpm. Tech B says that spark advance on an engine can be tested with a timing light that incorporates an ignition advance meter. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔B. To accurately measure spark advance, use a timing light that incorporates an ignition advance meter. The spark advance cannot be determined by listening to the way the engine sounds. A regular customer presents his vehicle for service. He describes symptoms of poor drivability and says that the vehicle has been repaired for the same problem in the past. Which, of the following, should you do first? A. connect the vehicle to an engine analyzer B. check for diagnostic trouble codes C. check the vehicle service history and check for TSBs, service campaigns and recalls D. perform a cylinder balance test ✔✔C. Checking the vehicle service history may get you pointed in the right direction before you even raise the hood. What repairs have been performed and what parts were installed? What fixed the car before? If it is a recurring problem, maybe a different course of action is in order. If the vehicle really has failed in the same way, checking TSBs may provide information on updated parts that weren't available previously and may solve the problem now. When checking for service campaigns and recalls, you may find that the problem is a defect that the manufacturer will repair. Two techs are listening to a tapping noise coming from the top end of a V8 engine. Tech A says the noise could be a valve needing adjustment. Tech B says the noise could be an exhaust leak. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔C. A tapping noise that does not dissipate after the engine warms up could be caused by worn valvetrain components or excessive valvetrain clearance. A similar type noise can sometimes be caused by an exhaust leak at the exhaust manifold/cylinder head juncture. Tech A says a high spike line on an oscilloscope indicates disconnected or open plug wire. Tech B says a high spike line indicates a closed gap on a spark plug. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔A. A higher-by-comparison spike line on an oscilloscope could indicate a disconnected or open spark plug wire. If a spark plug gap were closed, the oscilloscope would indicate a very low spike line. A vehicle can be started with great difficulty, but lacks power, and will not idle. Tech A says that there could be a hole or tear in the duct between the mass air flow sensor and the throttle body. Tech B says that a restricted fuel filter is the problem. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔A. The fuel filter may be in need of service, but it is not the cause of the problem. Unmetered air entering the engine means that there is more air than the amount for which the engine is delivering fuel. A careful examination of the duct between the air flow sensor and the throttle body should reveal where the unmetered air is entering. A vehicle is towed in the with a 'cranks but won't start' condition. A 'noid' light test finds no injector pulses, and Tech A says the vehicle need a computer. Tech B finds no spark and says the the vehicle needs an ignition module. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔D. Although the vehicle may need one of these items before being returned to service, both technicians are jumping to conclusions. A defective crank sensor can cause both of these symptoms, and that is far more likely than either of the other possibilities. The correct action is to test until the defective component is located. Tech A says that evidence of cylinder wear can be found using a cylinder leakage tester. Tech B says that a blown head gasket can be found using a cylinder leakage tester. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔C. Air blowing into the crankcase during a cylinder leakage test indicates a piston, rings and/or cylinder that are worn. When performing a cylinder leakage test, air that causes bubbles in the radiator coolant indicates a cracked head, cracked block, and/or a blown head gasket. A poor running vehicle has low fuel volume. Tech A tests the electrical connector at the tank, and finding system voltage available, says the vehicle needs a new pump. Tech B says high circuit resistance may prevent the pump from operating properly. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔B. Before the energy reaches the fuel pump, it must pass through the pump harness, pass-through connector, and the pump wiring. A high resistance in any of these places can keep the pump from rumling properly. When testing for available pump voltage, remember that there is virtually no load placed on the pump circuit by a multimeter. A test light can load the circuit in place of the pump to determine if there is a resistance, which will cause a significant drop in the voltage available to the pump. A typical resistance in this instance could be relay contacts, which are burned or have insufficient pressure. An engine with an electronic fuel injection system has high fuel pressure at idle. This could be caused by a: A. low manifold vacuum B. leaking fuel pump check valve C. plugged fuel injection valve D. high manifold vacuum ✔✔A. If the manifold vacuum is low, the fuel pressure regulator will restrict the fuel return and increase the fuel pressure. While the engine is running, a tech pulls the PCV valve out of the valve opening and places his finger over the valve. No vacuum is felt. Tech A says that the PCV valve could be stuck closed. Tech B says that the hose between the intake manifold and the PCV valve could be plugged. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔C. The valve should be removed from the hose and shaken. If the valve rattles when shaken, it is not stuck open or dosed. The hose from the intake manifold to the PCV valve is most likely plugged. A vehicle gets poor fuel mileage and has poor heater performance. Which of the following could be the cause? A. a bad ECT sensor B. a defective fan clutch C. a clogged cooling system D. a stuck open thermostat ✔✔D. A thermostat that is stuck open will not allow the engine to come to the correct operating temperature. Heater performance will be poor because the engine coolant will not be hot enough. The ECT will report the lower temperature to the PCM, which will make the air/fuel mixture richer, causing poor fuel economy. A vehicle with electronic fuel injection gets poor gas mileage. Engine test show a rich mixture. Tech A says that a bad O2 sensor could be the cause. Tech B says that a bad IAT sensor could be the cause. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔C. Both the oxygen sensor and the intake air temperature sensor relay signals to the PCM/ECM that directly affects the control of fuel. A vehicle has sluggish performance and high fuel consumption. Testing reveals that the vehicle fails to enter closed loop operation. Tech A says the ECT sensor may be defective. Tech B says the TP sensor may be set incorrectly. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔A. Although there may be a slight discrepancy in the setting of the TP sensor, it is not relevant to the question. If the coolant temperature sensor tells the PCM/ECM that the engine has not yet warmed up, even though it has, the system will not go into closed loop operation and will run rich. A turbocharged vehicle performs poorly and lacks power. All of the following could be the cause EXCEPT: A. restricted intake duct B. a waste gate that is stuck open C. failed turbocharger shaft bearings D. a waste gate that is stuck closed ✔✔D. A wastegate that is stuck closed would cause overboost. A restricted intake duct, a stuck open wastegace, or failed shaft bearings, which would cause the rotating assembly to bind or drag, would all reduce boost pressure and vehicle performance. When troubleshooting a slow drain on the battery, which diagnostic tool should be used? A. ohmmeter B. ammeter C. voltmeter D. test light ✔✔B. Always use an ammeter to troubleshoot a slow drain in an automotive electrical circuit. A V6 engine with distributor less ignition breaks up on acceleration, but gets smoother when reaching cruise. Tech A says a bad coil pack is the problem. Tech B says the cam sensor must be replaced. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔A. A cam sensor will have no effect on acceleration. A defective coil pack will break down under the stresses of hard acceleration, and run more smoothly when those stresses are eliminated. However, a roughness in the idle will still be detectable. Blue-gray smoke comes from the tailpipe of a vehicle during deceleration. Tech A says that this is most likely due to bad valve seals. Tech B says that the cause could be clogged oil return passages. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔C. When a vehicle suffers from worn valve guides and/or bad valve stem seals, you'll see exhaust smoke during deceleration because the high intake vacuum that occurs during deceleration draws the oil through the worn guides or seals. However, before you blame either the valve guides or the valve seals, verify that all of the oil return holes are clean. If oil cannot drain freely back into the crankcase, it can accumulate in the head and be drawn by vacuum ihto the combustion chamber, causing exhaust smoke. Tech A says that an incorrect ignition pick-up coil gap can cause a misfire. Tech B says that when testing the pick-up coil with an ohmmeter, the meter should read infinity when the ohmmeter is connected to both pick-up coil leads. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔A. Pick-up coil gap that is too small or too large may cause engine misfire. Technician B is wrong because the normal resistance reading is 500 to 1500 ohms when an ohmmeter is connecced to the pick-up coil leads. The ohmmeter should read infinity when the ohmmeter leads are connected to ground and either pick-up coil connector lead. Which of the following would cause a spark miss? A. high engine temperature B. low fuel pressure C. dirty throttle body and IAC D. distributor cap carbon track ✔✔D. If the power if diverted to ground before reaching the spark plug, a spark miss will result. All of the following are symptoms of insufficient exhaust gas recirculation EXCEPT: A. spark knock B. stalling C. surging at cruise D. increased NOx emissions ✔✔B. Stalling is a symptom of too much EGR or EGR at the wrong time. The EVAP system is designed to: A. keep fuel vapors from escaping the fuel tank B. capture fuel vapors and route them to the catalytic converter, where they are burned C. purge fuel vapors from the charcoal canister to atmosphere D. capture fuel vapors and route them to the engine where they become part of the air/fuel mixture and are burned during combustion ✔✔D. The purpose of the evaporative emissions control system (EVAP) is to prevent HC emissions from escaping the fuel system to atmosphere. Prior to the introduction of EVAP systems, fuel vaporized and was emitted to the atmosphere from vented gas caps and carburetor float bowls, polluting the environment. The EVAP system is designed to capture these vapors and route them to the engine where they become part of the air/fuel mixture and are burned during combustion. Which of the following sensors can be checked by measuring the pulse width of the fuel injector? A. TP (Throttle Positon) sensor B. VSS (Vehicle Speed Sensor) C. MAF (Mass Air Flow) sensor D. CKP (Crankshaft Position) sensor ✔✔C. Pulse width is a measurement of how long the fuel injector is open, or how much fuel the fuel injector is delivering. The longer the PCM/ECM grounds the injector, the longer the injector sprays. The longer the injector sprays, the richer the mixture becomes. The Less the injector sprays, the leaner the mixture. Restrict the air intake and look for a change in fuel control. If a change in fuel delivery is noticed, the MAF sensor is most likely working properly. An IAT sensor is being tested. Tech A says that the resistance should be checked on the NTC (Negative Temperature Coefficient) sensor. Tech B says that the voltage drop across the sensor should be checked on this type of sensor. Who is right? A. Tech A B. Tech B C. Both D. Neither ✔✔C. Resistance varies inversely with temperature on an NTC sensor. The sensor's function is usually checked by measuring the resistance between the sensor terminals. Most manufacturers publish resistance/temperature tables, but in general the resistance should be approximately 3000 ohms at room temperature (70'F, or 21'C). Start the engine, and as it warms up check that the values change smoothly, or heat the sensor using a hair dryer to see if the values change smoothly. If the resistance value doesn't change, the sensor is probably defective. The sensor can also be checked by measuring voltage drop. More voltage is dropped across the sensor when it is cold, and less when hot. The PCM/ECM, by monitoring the voltage change, adjusts the amount of fuel injection according to the air temperature. With the sensor installed, back-probe the sensor terminals with a voltmeter connected across the sensor terminals. Compare the voltage readings at various temperatures with the mamlfacturer's voltage/temperature table. The voltage drop can also be seen using a scan tool. The MAP sensor voltage on a vehicle is measured with the engine running at idle and found to be almost 5 volts. Technician A says that this voltage is too high for idle speed. Technician B says that this voltage will cause the engine to run extremely rich. Who is right? A. Tech A only B. Tech B only C Both A and B D. Neither A or B ✔✔C. A high voltage reading at the MAP sensor should indicate atmospheric pressure inside the intake manifold, which should only happen when the engine is off or at WOT (Wide Open Throttle). Since the engine is running, the PCM/ECM will think the engine is at WOT and richen the fuel mixture accordingly. The engine in a vehicle running at normal operating temperature has a rough idle and black smoke coming from the exhaust. All of the following could be the cause EXCEPT: A. EGR valve stuck closed B. EVAP canister purge solenoid stuck open C. fuel pressure too high D. fuel injector(s) stuck open ✔✔A. An EGR valve that is stuck dosed could cause spark knock or other driveability problems, but would not cause rough idle and black smoke. An EVAP canister purge solenoid that was stuck open could be the cause because it would cause the canister to improperly purge at idle. High fuel pressure could be the problem because it would cause more fuel to be injected per injector opening. Fuel injectors that were stuck open would allow fuel flow when they were not energized and could also be the cause of the problem. A vehicle is towed in with a 'no start' condition. The technician disconnects the coil wire from the distributor cap, connects a suitable spark tester and checks for a good spark while cranking the engine. If there is no spark, all of the following could be the cause EXCEPT: A. a problem in the ignition primary circuit B. a problem in the ignition secondary circuit C. a bad coil D. a bad distributor pick-up coil ✔✔B. There would only be a problem in the ignition secondary circuit if there were a good spark. If there is no spark, the problem is in the primary circuit. Since the primary windings of the coil and the distributor pick-up coil are part of the primary circuit, they are possible causes of a no-spark condition. Which of the following diagnostic methods can be used to determine which cylinder is leaking combustion chamber gas into the cooling system? A. observing the radiator coolant for bubbles while the engine is running B. placing a vial of a chemical that is sensitive to combustion gases over the radiator filler neck while the engine is running C. holding an exhaust analyzer probe over the radiator filler neck while checking for a reading on the exhaust analyzer D. observing the radiator coolant for bubbles while performing a cylinder leakage test ✔✔D. While each of these methods can be used to determine whether there is a combustion chamber leak into the cooling system, only a cylinder leakage test will identify which cylinder is leaking. A vehicle's tailpipe emissions are being checked with a four-gas exhaust analyzer. The level of CO2 in the exhaust gas is lower than normal and the level of O2 is high. Technician A says that the engine has a lean condition. Tech B says that, if the analyzer had five-gas capabilities, the NOx level would be low. Who is right? A. Tech A only B. Tech B only C Both A and B D. Neither A or B ✔✔A. Any deviation, rich or lean, from the optimum air/fuel ratio will cause the C02 level in the exhaust to drop. Higher 02 levels indicate a lean mixture, while lower levels mean a richer mixture. Technician B is wrong because lean air/fuel mixtures can cause high levels of NOx in the exhaust. Under which of the following conditions is exhaust gas recirculation required? A. cold engine at part throttle B. warm engine at idle C. warm engine at part throttle D. warm engine at wide open throttle ✔✔C. When the engine is warm and above idle speed, EGR is required to prevent spark knock and reduce NOx emissions. Until the engine is warm, there is no need for exhaust gas recirculation because combustion is still sufficiently cool. Even when the engine is warm, there is no need for exhaust gas recirculation at idle because combustion pressures are relatively low and NOx is not formed, and exhaust gas recirculation would stall the engine because of too much dilution. At WOT, the need for power outweighs the need to control NOx emissions, and since WOT needs richer, and therefore slightly cooler mixtures, NOx formation is minimal anyway. An electrical circuit is being tested using a DMM (Digital Muitlmeter). When measuring voltage in the circuit, Technician A says that the meter should be connected in parallel. When measuring amperage, Technician B says that the meter should be connected in series. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔A voltmeter is always connected in parallel when making a voltage check of a circuit. An ammeter is connected in series with the circuit, so all the current passing through passes through the ammeter. When retrieving stored codes from a computerized engine control system, a hard fault is found regarding a fuel injector circuit. Technician A says that fuel pressure and volume tests should be performed. Technician B says that the injectors should be replaced on that circuit. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔D. Technician A is wrong because a problem with fuel pressure and volume would not set a fuel injector circuit code. Technician B is wrong because the code indicated a malfunction in the fuel injector circuit, and replacing the injectors would be jumping to the conclusion that the injectors are bad. The circuit should be tested in the order specified by the manufacturer's diagnostic procedures before any components are replaced. A technician Is testing the ignition on a vehicle with a 'no start' condition. He connects a test light between the negative side of the coil and ground and has an assistant crank the engine. A test light that flutters on and off indicates all of the following EXCEPT: A. The pickup coil signal is OK. B. The ignition module is OK. C. The module is triggering the ignition coil to fire. D. The coil is OK. ✔✔D. The test light fluttering on and off means that tile coil is being triggered to fire, it is not a test of the coil itself. Technician A says that a secondary air injection system forces air into the intake ports. Technician B says that the secondary air injection system reduces spark knock. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔D. The secondary air injection system injects air into the exhaust system. Since the secondary air injection system is used after the combustion event, it cannot reduce spark knock. All of the following are true statements regarding fuel pressure regulators EXCEPT: A. With the vacuum line connected to the fuel pressure regulator, fuel pressure should be higher than with the line disconnected. B. Most fuel pressure regulators are not adjustable. C. Most fuel pressure regulators are connected to manifold vacuum and vary the fuel pressure according to engine load. D. Some regulators can suffer from a ruptured diaphragm, allowing fuel to be sucked into the intake manifold through the vacuum line, which operates the regulator, causing a rich running condition. ✔✔A. With the vacuum line connected to the fuel pressure regulator, fuel pressure should be lower than with the line disconnected. Technician A says an oscilloscope can be used to read spark plug firing voltage. Technician B says an oscilloscope can be used to check the ignition coil. Who is right? A. Technician A only B. Technician B only C. Both A and B D, Neither A or B ✔✔C. The spark plug firing voltage can be determined as well as the condition of the ignition coil by using an oscilloscope. All of the following are indications of a charging system that is undercharging EXCEPT: A. slow cranking B. dim headlights C. short light bulb life D. low ammeter indication ✔✔C. Overcharging is indicated by short light bulb life and a battery that continually needs water. An engine has a light metallic knocking noise during light engine loads, however, when the cylinder with the noise is disabled during a cylinder balance test, the sound diminishes. Technician A says that the noise is caused by excessive connecting rod bearing clearance. Technician B says that the noise is caused by excessive piston-to-wall clearance. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔A. When you eliminate the ignition or injection co a cylinder with a rod knock, the sound diminishes. Unlike a connecting rod bearing noise, piston slap does not quiet down and may in fact grow louder when you eliminate ignition or fuel injection to that cylinder. When an EGR valve diaphragm is opened with a vacuum pump while the engine is idling, there is no effect on idle speed. Technician A says that the EGR valve is bad and should be replaced. Technician B says that the EGR passages are clogged with carbon. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔B. Manually raising the EGR diaphragm when the engine is idling should allow exhaust gas into the intake air stream, which should stall an engine at idle. Since this had no effect on the idle speed, we know that there is no exhaust gas flow, so replacing the EGR valve would have no effect. Technician B is correct because the lack of exhaust gas flow is most likely caused by carbon buildup in the EGR passages. The MIL is illuminated on a late-model vehicle that is brought in for service. A scan tool is connected to the DLC and the DTC PO107 is obtained. Technician A says that this code indicates a failure in the vehicle's powertrain. Technician B says that this code is manufacturer specific. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔A. Technician A is right because the alpha character indicates the area of the vehicle where the failure occurred: B for Body, C for Chassis, P for Powertrain, and U for Network. Technician B is wrong because Manufacturer specific codes have the number 1 as the first digit. DTC P0l07 has a zero as the first digit, so it is a generic code, common to all manufacturers. A vehicle with a normally closed canister purge solenoid is not purging the EVAP canister when engine operating conditions for purging are met. When the purge solenoid is removed from the vehicle for testing, it allows vacuum flow when voltage is applied. Technician A says the purge solenoid is defective. Technician B says the purge solenoid power circuit is at fault. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔B. A normally closed purge solenoid should allow vacuum flow when activated by the PCM/ECM, so the purge solenoid is functioning properly. If the purge solenoid is being commanded by the PCM/ECM, then the problem is in the power supply to the purge solenoid. When manifold vacuum is checked on a vehicle brought in for service it is found to be low but steady. Technician A says that an air/fuel mixture that is too rich can cause this condifion. Technician B says that an air/fuel mixture that is too lean can be the cause. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔C. When the idle vacuum is low but steady, suspect an air/fuel mixture that is too lean or too rich. Air leaks or vacuum leaks can cause a lean condition and are common causes of rough idle, hesitation, stalling and hard starting. If the mixture is artificially enriched by injecting propane into the induction system, an engine running lean should speed up and the vacuum reading should rise. If the air/fuel mixture is too rich, the engine will slow down and the vacuum reading will drop. If the engine Is running lean and you suspect a vacuum leak as the cause, try using a propane kit with a length of hose attached to find the leak. Pass the hose end around the suspected areas and listen for a change in idle. A vehicle with multlport fuel injection has a stalling problem. When the throttle body is inspected if is found to be contaminated with sludge buildup. Technician A says that this could be caused by worn rings. Technician B says that this could be caused by a clogged or restricted PCV system. Who is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔C. The PCM/ECM bases idle speed calculatlons on a set amount of air that bypasses the throttle plates. If this air flow is reduced by accumulated sludge deposits, these calculations will be incorrect and may result in stalling. Worn rings can cause excessive blowby into the crankcase. Excessive blowby can be more than the PCV system can handle, so instead of going into the intake manifold, these gases force their way through the fresh air intake and into the air cleaner. Oil deposits that are part of the crankcase gases can then accumulate on the throttle body when these gases mix with the air entering the engine, eventually causing the sludge buildup. A dogged or restricted PCV valve can cause the same results. All of the following are true of the EVAP canister vent solenoid EXCEPT: A. It is activated by the PCM/ECM to allow the flow of fuel vapor from the EVAP canister to the engine. B. It is activated by the PCM/ECM to block the entrance of outside air into the canister during the EVAP leak test. C. It is used only on vehicles with enhanced EVAP systems. D. It is located in the fresh air supply hose to the canister. ✔✔A. The canister purge solenoid, not the vent solenoid, is activated by the PCM to allow the flow of fuel vapor from the EVAP canister to the engine. A vehicle has been towed in with a no-crank condition. The starter solenoid is being tested using a jumper wire between the battery and the solenoid 'S' terminal. Technician A says that if the engine cranks, the problem is In the starter control circuit. Technician B says that if the solenoid makes a clicking sound, it is operating properly and the starter may be defective. Who Is right? A. Technician A only B. Technician B only C. Both A and B D. Neither A or B ✔✔C. The solenoid would be defective in this test if it made no noise and did not click or activate the starter. All of the following can cause air flow problems through a radiator core EXCEPT: A. broken shroud B. debris buildup C. bent fins D. excessive mineral deposits ✔✔D. Excessive mineral deposits can restrict coolant flow through the radiator, but would have no effect on air flow. A broken fan shroud would cause less than optimal air flow through the radiator core. Debris and bent fins would block air flow through the core. The short term fuel trim value on a scan tool is shown with a negative number. Which of the following will also be true? A. There is less oxygen in the exhaust B. The voltage signal from the oxygen sensor decreased. C. The injector pulse width was lengthened. D. The short term fuel trim value is greater than 1. ✔✔A. An SFT with a negative number means the injector pulse width was shortened, so answer C is wrong. A shortened injector pulse width means that the SFT value was decreased below 1, so answer D is wrong. The SFT will decrease in response to an increased voltage signal from the oxygen sensor, so answer B is wrong. A rich mixture, one with less oxygen, will increase the voltage signal from the oxygen sensor. Technician A says that a 'pending' code is recorded if monitor results indicate a failure after one drive cycle. Technician B says pending codes should be checked to verify a repair. Who is right? A. Technician A only B. Technican B only C. Both A and B D. Neither A or B ✔✔C. A 'trip' is a completed drive cycle. Although some monitors will set a DTC and turn on the MIL if a fault is found after one trip, for most DTCs the PCM/ECM must see the same fault occur during two trips. After the first trip, the fault is recorded as 'pending'. Once the PCM/ECM sees the same fault the next time the monitor runs, the PCM will turn on the MIL. If the same fault is not seen during the next monitor, the pending code will be cleared. Once repairs are completed, operate the vehicle in a manner necessary to satisfy the drive cycle requirements for the monitor that set the DTC. Recheck the monitor status and check for pending codes to see if the fault returned.