Motor Vehicle Mechanics common mistakes
Motor Vehicle Mechanics · Understand the traps and how to avoid them.
Confusing the function of a turbocharger with that of a supercharger when asked which one suffers from turbo lag and why, giving 'both are driven by exhaust gas' as the explanation.
Both are forced-induction devices and candidates memorise 'turbocharger' and 'supercharger' as a single lump, so the drive mechanism (exhaust-gas turbine versus belt/chain from the crankshaft) is never separated in memory.
What to do instead
Write a two-column table for every forced-induction question: energy source, lag behaviour, boost control method, and typical mounting. State explicitly that only the turbocharger uses exhaust enthalpy and therefore has lag, then practise three short answers explaining lag in terms of exhaust pressure build-up.
Describing the four-stroke cycle in the wrong order, or placing ignition in a petrol engine at the start of the compression stroke rather than near the end of it.
Candidates recite 'suck, squeeze, bang, blow' as a slogan and never link each stroke to piston direction and valve state, so sequence errors and ignition-timing errors survive unnoticed.
What to do instead
Draw the cycle for each cylinder event with piston direction, both valve positions, and the crank angle at which the spark occurs. Rehearse the answer aloud naming intake, compression, power and exhaust with valve states, and state that the spark is timed before top dead centre on the compression stroke.
Mixing up camshaft and crankshaft speed when calculating valve or ignition timing, for example saying the camshaft turns at twice crankshaft speed.
The 2:1 relationship is memorised as a bare number without attaching which shaft is faster, and candidates rarely convert a crank-angle figure into cam degrees.
What to do instead
Anchor the ratio to the four-stroke cycle: one camshaft revolution per two crankshaft revolutions, so cam speed is half crank speed. Practise converting 30 degrees of crank rotation into 15 degrees of cam rotation and back until the direction of conversion is automatic.
Diagnosing a no-start petrol engine by replacing components in sequence (plugs, then coils, then pump) without first confirming whether the fault is spark, fuel or compression.
Under time pressure candidates reach for the most familiar part rather than running a structured check, and they have not internalised that diagnosis is a process of elimination with tests at each stage.
What to do instead
Adopt a fixed triage: confirm cranking speed and battery voltage, check for spark at a known-good plug, check fuel pressure and injector pulse, then run a compression or cylinder-leakage test. Write the triage order on a card and apply it to every no-start scenario, justifying each test before naming a component.
Treating brake fluid as incompressible and ignoring hygroscopic contamination, so the candidate says a spongy pedal can only be caused by air and never considers fluid boiling or a failing master cylinder seal.
Brake hydraulics are learned as a simple pressure-transmission story, and the service interval and moisture-absorption property of DOT fluid are treated as trivia rather than as diagnostic causes.
What to do instead
Build a cause list for a spongy or long pedal: air in the system, fluid with absorbed moisture boiling under heat, internal master-cylinder bypass, flexible hose expansion, seized caliper. For each, state the confirming test. Memorise that glycol-based fluid absorbs water and that this lowers its boiling point.
Bleeding brakes in the wrong sequence or allowing the reservoir to run dry, and being unable to explain why the sequence matters.
The procedure is copied as a list of steps without the reasoning that air is being pushed to the lowest bleed point and that an empty reservoir reintroduces air at the master cylinder.
What to do instead
Learn the rule rather than the list: start at the wheel farthest from the master cylinder and work closer, keep the reservoir topped up, and close the bleed nipple before releasing the pedal. Explain each rule in one sentence linking it to where air travels.
Confusing toe, camber and caster, or claiming that adjusting toe changes camber directly.
The three alignment angles are learned as a set of words with no mental picture of the axis each one rotates about, so they collapse into each other under recall.
What to do instead
Attach each angle to a viewing plane: toe is seen from above, camber from the front, caster from the side. For each, state the handling symptom of excess (inner shoulder wear for negative camber, outer shoulder wear for positive, pull for caster imbalance, feathering for toe) and the tyre-wear signature that confirms it.
Answering cooling-system questions by naming the thermostat as the cause of every overheating complaint, without distinguishing a stuck-closed thermostat from a failed water pump, blocked radiator or faulty cooling fan.
The thermostat is the only cooling component with a simple on/off story, so it becomes the default answer; candidates have not linked each component to the temperature pattern it produces.
What to do instead
Map symptom to cause: rapid overheating from cold suggests a stuck-closed thermostat; overheating only at idle or in traffic suggests fan or airflow; overheating under load at speed suggests radiator restriction or a slipping impeller. Practise stating the pattern first, then the component, then the confirming test such as feeling both radiator hoses.
Ignoring safety and environmental procedure in practical answers, such as describing how to drain fuel or open an air-conditioning circuit without mentioning refrigerant recovery, fire precautions or PPE.
Candidates focus on the mechanical outcome and treat procedure as padding, often because they have learned the task from informal workshop habit rather than from documented method.
What to do instead
For every practical answer add a short 'before you start' clause: isolate the battery, recover refrigerant with approved equipment, contain and label drained fluids, wear eye and hand protection, and dispose of waste through the licensed route. Keep the clause to one or two sentences so it stays specific rather than generic.
Quoting a torque or clearance figure from memory without units, or omitting the tightening sequence and lubricant condition that make the figure valid.
Specifications are memorised as bare numbers, and candidates do not realise that a torque value depends on thread condition, fastener coating and whether the reading is wet or dry.
What to do instead
Never state a number without its unit and its conditions. In answers, write the value with units, note whether threads are dry or lubricated, give the tightening sequence for multi-fastener joints such as a cylinder head, and say that the manufacturer's data must be consulted for the specific engine.