Automotive Repair

102 flashcards by MentisLibrary

Engine, brake, electrical, and drivetrain systems explained, plus how to read OBD-II codes, fuel trims, and live data instead of guessing at parts. Includes the shop safety practices that keep the job routine.

What are the four strokes of the four stroke cycle, in order?
Intake, compression, power, exhaust.
What is compression ratio, and why does a higher ratio demand higher octane fuel?
It is the ratio of cylinder volume with the piston at bottom dead center to the volume at top dead center. Squeezing the charge harder raises its temperature, so a low octane fuel can self ignite and knock.
How is engine displacement calculated?
It is the total volume swept by all pistons in one stroke: bore area times stroke length times the number of cylinders.
What is firing order and why is it not simply 1-2-3-4?
It is the sequence in which the cylinders deliver their power strokes. The sequence is chosen to space power pulses evenly and balance the crankshaft loading.
What is the difference between an OHV pushrod engine and an overhead cam engine?
OHV places the camshaft in the block and uses lifters, pushrods, and rocker arms to reach the valves. OHC places the camshaft or camshafts in the cylinder head, acting on the valves much more directly.
How does a timing belt differ from a timing chain, and what is an interference engine?
A timing belt is a reinforced rubber toothed belt run dry outside the engine and replaced on an interval. A timing chain runs inside the engine on pressurized oil and is generally designed to last the life of the engine.
What does variable valve timing do?
It changes valve timing, and on some systems valve lift, while the engine runs so the engine can favor low speed torque, high speed power, or emissions and economy as conditions change.
What is the fundamental difference between a turbocharger and a supercharger?
A turbocharger is driven by the energy in the exhaust stream. A supercharger is driven mechanically by a belt or gear off the crankshaft.
How is a cranking compression test performed, and what does a wet test add?
With the engine warm, remove all spark plugs, disable fuel and ignition, hold the throttle open, and crank each cylinder through about four to six compression strokes with the gauge installed.
What does a cylinder leak down test tell you that a compression test cannot?
It tells you where the cylinder is leaking. Regulated shop air is applied to a cylinder held at top dead center on compression, and you listen and look for where it escapes.
What is the difference between port fuel injection and direct injection?
Port injection sprays fuel into the intake port at relatively low pressure, in front of the intake valve. Direct injection sprays fuel straight into the combustion chamber at very high pressure.
What does a fuel injector do, and what happens when one is dirty or leaking?
It is a computer controlled solenoid valve that atomizes and meters a precise quantity of fuel. The ECM controls how long it stays open, which is the pulse width.
How do you test a fuel pump properly?
Measure pressure with a gauge against the manufacturer's spec, measure volume delivered in a set time, and watch how the pressure holds after shutdown.
Why must a fuel system be depressurized before a line is opened?
Because the system holds residual pressure even with the engine off, and a spray of fuel onto a hot exhaust component or an ignition source causes a fire.
What is the difference between a MAF sensor and a MAP sensor?
A mass air flow sensor directly measures the mass of air entering the engine, usually with a heated element. A manifold absolute pressure sensor measures pressure in the intake manifold, and the ECM calculates airflow from pressure, rpm, and air temperature.
What jobs do the throttle body and engine air filter perform, and how does each cause complaints?
The throttle body controls how much air enters the engine, and on nearly all modern vehicles it is an electronic unit with a motor and position sensors instead of a cable. The air filter keeps abrasive dirt out of the engine and the airflow sensor.
What is the difference between short term and long term fuel trim?
Short term fuel trim is the immediate correction the ECM makes in response to the oxygen or air fuel ratio sensor. Long term fuel trim is the learned correction stored in memory that takes over the persistent part of that adjustment.
What does a positive fuel trim mean, and what does a negative fuel trim mean?
Positive trim means the ECM is adding fuel to correct a lean condition. Negative trim means it is subtracting fuel to correct a rich condition.
What is a coil on plug ignition system?
An ignition system with an individual coil mounted directly on top of each spark plug, with no distributor and no plug wires. The ECM fires each coil individually.
How do copper, platinum, and iridium spark plugs differ, and why does gap matter?
They differ mainly in electrode material and service life. Copper conducts best but wears fastest, platinum lasts longer, and iridium uses a very fine center electrode for the longest service life and the lowest required firing voltage.
What are the major categories of misfire causes?
Ignition, fuel, mechanical, and everything that affects the air the cylinder receives.
What does a three way catalytic converter do?
It oxidizes hydrocarbons and carbon monoxide into water and carbon dioxide and reduces oxides of nitrogen back to nitrogen, using precious metal catalysts.
What is the difference between a conventional oxygen sensor and an air fuel ratio sensor?
A conventional narrowband oxygen sensor is a switching device. It produces roughly 0.1 to 0.9 volts and only tells the ECM whether the mixture is richer or leaner than stoichiometric. A wideband air fuel ratio sensor measures actual mixture across a broad range and reports it as lambda or an equivalent current.
What is the difference in job between the upstream and downstream oxygen sensors?
The upstream sensor, before the converter, is the fuel control sensor. The downstream sensor, after the converter, monitors converter efficiency.
What does the EGR system do, and what are the symptoms when it fails each way?
Exhaust gas recirculation routes a metered amount of inert exhaust back into the intake, which lowers peak combustion temperature and therefore reduces oxides of nitrogen.
How does the EVAP system work, and what do the purge and vent valves do?
It captures fuel vapor from the tank in a charcoal canister instead of letting it escape to atmosphere, then burns that vapor in the engine.
What does the PCV system do?
Positive crankcase ventilation draws blowby gases out of the crankcase and burns them in the engine, while metering fresh air in, so pressure and moisture do not build up inside the engine.
What do the DPF and DEF systems do on a modern diesel?
The diesel particulate filter traps soot from the exhaust and periodically burns it off, a process called regeneration. Diesel exhaust fluid is injected into the selective catalytic reduction catalyst, where it converts oxides of nitrogen into nitrogen and water.
What does the radiator do, and what part do the cooling fans play?
The radiator is a liquid to air heat exchanger that transfers heat from the coolant into the passing air. The fans, electric or belt driven with a clutch, supply that airflow when vehicle speed cannot.
What does the thermostat do, and what are the symptoms of each failure mode?
It is a temperature controlled valve, usually a wax pellet element, that blocks flow to the radiator until the engine reaches operating temperature, then modulates flow to hold that temperature.