OBJECTIVES

OBJECTIVES After studying Chapter 26, the reader will be able to: • Prepare for ASE Engine Performance (A8) certification test content area “A” (General Engine Diagnosis) and ASE Advanced Level (L1) certification test content area “A” (General Powertrain Diagnosis). • List the possible causes of an engine performance problem based on its symptoms. • List the possible causes of a rich air-fuel mixture. • List the possible causes of a lean air-fuel mixture. • Describe what symptoms may occur if a particular sensor is defective. • List the possible causes of excessive HC, CO, and NOX exhaust emissions.

ENGINE HESITATES, SAGS, OR STUMBLES DURING ACCELERATION • Hesitation means a delay in the operation of the engine when the accelerator pedal is depressed. • Sometimes hesitation is described as sag or “lack of response” as the accelerator is pushed down. • The most common cause of hesitation is a too lean air-fuel mixture being delivered to the engine during the time the accelerator is depressed.

ENGINE HESITATES, SAGS, OR STUMBLES DURING ACCELERATION FIGURE 26-1 Valve deposits on the intake valves can cause hesitation during acceleration, especially if the engine is cold.

ENGINE HESITATES, SAGS, OR STUMBLES DURING ACCELERATION FIGURE 26-2 Typical throttle-position (TP) sensor.

ENGINE HESITATES, SAGS, OR STUMBLES DURING ACCELERATION FIGURE 26-3 Many areas of the country use gasoline that is blended with up to 10% ethanol (ethyl alcohol). Sometimes too much alcohol can cause driveability problems.

ROUGH IDLE OR STALLING • Rough (or unstable) idle is a common occurrence because many different systems have a direct effect on idle quality. If the engine stalls (stops running), most customers are very concerned because it can be a safety-related malfunction. FIGURE 26-4 This throttle body needs to be cleaned. The deposits on the back (engine) side of the throttle plate can cause rough idle or stalling due to lack of proper air flow into the engine.

SPARK KNOCK (PING OR DETONATION) • Spark knock (also called detonation or ping) is most noticeable during acceleration. • Spark knock is usually due to excessively lean air-fuel mixtures or excessively hot engine operation. • When this secondary flame front hits the primary spark-ignited flame front, two situations occur: • Temperature greatly increases at the instant the two flame fronts collide. • Pressure greatly increases at the same time due to the temperature rise.

ENGINE CRANKS OKAY, BUT IS HARD TO START • In order to start engines, the correct air-fuel mixture must be delivered to the cylinders. • A strong spark delivered to the cylinder at the proper time is also required for an engine to start. • Worn engine parts can also cause an engine to not have enough compression or to have the valves open and close at the proper time and duration.

ENGINE CRANKS OKAY, BUT IS HARD TO START FIGURE 26-5 This engine does not have any vacuum while cranking. This newly overhauled engine had been assembled and the valve had not been properly adjusted. Cranking vacuum should be greater than 2.5 in. Hg.

ENGINE DOES NOT CRANK OR CRANKS SLOWLY • The starter motor is designed to crank the engine between 80 and 250 revolutions per minute (RPM) to permit proper intake of a combustible air-fuel mixture to start. • If the engine does not crank, then the fault is in the cranking circuit, which consists of the following components: • Battery • Starter motor • Starter solenoid • Ignition switch • Park-neutral or clutch safety switch • Cables, wires, and connectors

ENGINE DOES NOT CRANK OR CRANKS SLOWLY FIGURE 26-6 (a) This is a voltmeter reading of the battery voltage on a vehicle that has a defective starter. The 7.84 volts reading on the meter is much lower than the specification of greater than 9.6 volts. (b) This meter indicates a cranking voltage of 10.32 volts, which is within specifications.

POOR FUEL ECONOMY • Poor fuel economy means lower-than-usual miles per gallon (or liters per 100 kilometers in the metric system) as determined by an actual road test. • The test procedure should include the following steps: • Fill the fuel tank (DO NOT overfill) and record the mileage (for example, 52,168 miles). • Drive the vehicle normally for 100 to 200 miles or more. • Fill the tank again. Record the gallons of fuel used and the ending mileage (for example, 10.6 gallons and 52,406 miles). • Calculate the miles per gallon

POOR FUEL ECONOMY FIGURE 26-7 This stuck-open thermostat caused the engine to fail to reach normal operating temperature. As a result, the fuel economy was much lower than normal and it failed a state vehicle exhaust emission test due to excessive hydrocarbons (HC).

DIESELING OR RUN-ON • Dieseling or run-on is a term used to describe the engine continuing to run after the ignition is turned off. • A diesel engine operates by ignition of the fuel by heat of compression without the need for a spark to occur.

BACKFIRE • A backfire is the burning of fuel in the intake manifold or in the exhaust system. It is accompanied by a loud popping noise.

LACK OF POWER • A lack of power may also be noticeable as sluggish or spongy performance. • This means that the engine delivers less-than-expected power and the vehicle speed does not increase as desired when the accelerator pedal is depressed.

SURGES • A surge is a change in engine power under steady throttle conditions. • A driver may feel a surge as if the vehicle was speeding up and slowing down with no change in the accelerator pedal. • A lean air-fuel mixture is the most common cause of this condition.

CUTS OUT OR MISFIRES • When an engine misfires, it jerks or pulsates and is usually more noticeable when the engine is accelerated. • Because the engine is not running smoothly, the cause of the misfire is usually due to faults in one or more cylinders, either with a lack of spark, fuel, or compression.

RICH EXHAUST • A rich exhaust can be determined by a variety of methods, including: • High CO exhaust readings (over 0.5%) • Exhaust smell • Poor fuel economy • High O2S readings (consistently over 700 mV) • Low block learn numbers (less than 115 or more than a 20% long-term fuel trim correction factor) • Black exhaust smoke

LEAN EXHAUST • A lean exhaust can be determined by a variety of methods including: • High O2 exhaust readings (over 2%). • Engine hesitates, bucks, jerks, or backfires through the air inlet. • Low O2S reading (consistently less than 200 mV). • High block lean numbers (more than 150 or more than a 20% long-term fuel-trim correction factor).

SYMPTOMS OF A DEFECTIVE COMPONENT • It is a good idea to know what symptoms a particular part or component will cause if defective.

EXCESSIVE CO EXHAUST EMISSIONS • The chemical abbreviation CO stands for carbon monoxide, which is formed during the combustion process inside the engine by combining the carbon (C) from the gasoline (HC) and the oxygen (O) from the air. • An efficient engine should produce very little CO if there is enough oxygen in the cylinder to create CO2. • However, if the air-fuel mixture is too rich, an excessive amount of CO emissions will be created.

EXCESSIVE HC EXHAUST EMISSIONS • The chemical abbreviation for hydrocarbons (gasoline) is HC. • Excessive HC exhaust emissions mean that the gasoline is not being properly burned inside the engine. • Because the ignition system is used to ignite the air-fuel mixture, any malfunction in this system can result in higher-than-normal HC exhaust emissions.

EXCESSIVE HC EXHAUST EMISSIONS FIGURE 26-8 This distributor cap came off an engine that seemed to run okay. The only problem was that the vehicle failed an exhaust emission test for excessive hydrocarbon (HC) emissions.

EXCESSIVE NOX EXHAUST EMISSIONS • The chemical abbreviation for oxides of nitrogen is NOX. • Both nitrogen (N) and oxygen (O2) are normally part of our atmosphere. • It requires heat and/or pressure to combine them to form oxides of nitrogen. • Excessive NOX emissions, therefore, mean that the engine combustion chamber temperatures are too high or the chamber has excessive compression.

EXCESSIVE NOX EXHAUST EMISSIONS FIGURE 26-9 This badly eroded water (coolant) pump caused the engine to overheat.

SUMMARY • A lean air-fuel mixture is the usual cause of hesitation or stumble during acceleration. • A vacuum leak and lean air-fuel mixture can cause a rough idle or stalling. • Spark knock (ping or detonation) is often caused by a too lean air-fuel mixture, or if the engine operating temperature is too high. • A hard-start problem is often due to a lack of fuel.

SUMMARY • A slowly cranking engine is usually due to low battery voltage. A no-crank condition is usually due to an open circuit in the cranking circuit. • Poor fuel economy is usually due to an excessively rich air-fuel mixture. • A lean air-fuel mixture is usually due to a vacuum leak. • A rich air-fuel mixture can be caused by a defective oxygen sensor or the engine getting fuel from another source not controlled by the computer or fuel system.

REVIEW QUESTIONS • List five engine performance faults that can occur if a vacuum (air) leak occurs. • What symptom(s) may occur if the EGR valve is inoperative (never opens)? • List four items that can cause excessive CO exhaust emissions. • List four items that can cause excessive HC exhaust emissions. • List four items that can cause excessive NOX exhaust emissions.

CHAPTER QUIZ • Technician A says that a partially stuck-open EGR valve can cause ping (spark knock) during wide-open throttle engine operation. Technician B says that the partially stuck-open EGR valve could cause the engine to stall while operating at idle speed. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 2. Technician A says that a too rich air-fuel mixture can be caused by a defective fuel-pressure regulator. Technician B says that a defective pressure regulator can cause a too lean air-fuel mixture. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 3. Technician A says that excessive NOX exhaust emissions can be due to a defective PCV valve. Technician B says that a too lean air-fuel mixture can cause excessive NOX exhaust emissions. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 4. Technician A says a defective TP sensor can cause the engine to hesitate during acceleration. Technician B says that dirty throttle plate(s) on a port-injected engine could cause a hesitation during acceleration. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 5. Technician A says that defective spark plug wires can cause the engine to misfire. Technician B says a fouled spark plug can cause the engine to misfire. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 6. Technician A says a rough idle on a fuel-injected engine can be caused by dirty throttle plates. Technician B says the wrong PCV valve could cause the engine to idle roughly. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 7. Technician A says that spark knock (ping or detonation) can be caused by a lean air-fuel mixture. Technician B says an inoperative cooling fan could cause the engine to spark knock. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 8. Technician A says that a stretched (worn) timing chain can cause the engine to lack power at slow speeds. Technician B says a clogged exhaust system can cause the engine to lack power at high speeds. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 9. Technician A says that poor fuel economy can be caused by a defective thermostat. Technician B says a defective evaporative charcoal canister can cause poor or reduced fuel economy. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

CHAPTER QUIZ 10. Technician A says a rich exhaust could be caused by a hole in the rubber diaphragm of the fuel-pressure regulator on a portinjected engine. Technician B says that a defective spark plug wire can cause the engine computer to supply a too rich air-fuel mixture. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

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