ITI Mechanic Interview Questions and Answers: Motor Vehicle and Diesel

Explain the four-stroke cycle.

Suction (intake), compression, power and exhaust. One cycle takes two revolutions of the crankshaft, and the camshaft turns at half the crankshaft speed.

How to say it: Name the four strokes in order, then add the two-revolutions point.

What is the difference between a petrol and a diesel engine?

A petrol engine is spark ignition: the air-fuel mixture is ignited by a spark plug. A diesel engine is compression ignition: air alone is compressed until it is hot enough to ignite fuel injected into it, so it needs a much higher compression ratio and has no spark plug.

What is the firing order of a four-cylinder engine?

Commonly 1-3-4-2 for an inline four; some engines use 1-2-4-3. The firing order is on the engine data or in the service manual.

Why is valve clearance kept, and how do you check it?

To allow for expansion when the engine is hot. Too little clearance and the valve may not seat fully and can burn; too much and the valve train is noisy. Check it with a feeler gauge, on the cylinder at the right position and engine temperature given in the manual.

What does the thermostat do?

It keeps the coolant circulating inside the engine until it reaches operating temperature, then opens to let coolant flow through the radiator. A thermostat stuck shut causes overheating; stuck open, the engine warms up slowly.

An engine is overheating. What will you check?

Coolant level and leaks, the radiator fan, the thermostat, the water pump and its belt, a blocked radiator, and the radiator cap. Never open the cap while the engine is hot.

What are the main parts of a clutch?

Flywheel, clutch plate (friction disc), pressure plate and release (thrust) bearing, with the linkage or hydraulic system. The clutch plate is clamped between flywheel and pressure plate; pressing the pedal releases it.

Why do you bleed brakes, and how?

To remove air from hydraulic brake lines: air compresses, so the pedal feels spongy. Keep the master cylinder reservoir topped up, open the bleed screw while a helper presses the pedal, and close it before the pedal is released. The usual order is from the wheel farthest from the master cylinder to the nearest, unless the manual gives another order.

What does ABS do?

It stops the wheels locking under hard braking, so the driver can still steer while braking.

What are camber, caster and toe?

Wheel alignment angles. Camber is the inward or outward tilt of the wheel seen from the front; caster is the tilt of the steering axis seen from the side; toe is whether the fronts of the wheels point inward (toe-in) or outward (toe-out). Wrong alignment causes uneven tyre wear and pulling to one side.

Why do you bleed the fuel system on a diesel engine?

After a filter change or running out of fuel, air in the lines stops the injection system from delivering fuel. Bleeding removes the air so the engine can start, using the bleed screws and the priming pump as the manual describes.

What do black, white and blue smoke mean?

Black smoke: too much fuel for the air: a choked air filter, a faulty injector or overloading. White smoke: unburnt fuel, often on a cold start, or coolant entering the cylinder. Blue smoke: engine oil burning, from worn rings, valve guides or seals.

How to say it: This is one of the most common diesel questions: give all three colours with a cause each.

What is a common rail (CRDi) system?

A high-pressure pump keeps fuel at very high pressure in a common rail shared by all injectors, and the engine control unit opens each electronically controlled injector at the right moment. It gives finer control over injection than a mechanical pump.

How do you use an OBD scanner?

Connect it to the vehicle's OBD port with the ignition on, read the stored fault codes and live data, find the cause of the code (the code points to a circuit or system, not always the failed part), repair it, clear the codes and road-test to confirm.

How do you check a car battery?

Check the terminals for corrosion and tightness, then the voltage with a multimeter: a fully charged 12 V lead-acid battery at rest reads about 12.6 V. A battery tester or a load test shows whether it can hold voltage while cranking.

Which tools do you use most?

Ring, open-end and combination spanners, socket set, torque wrench, feeler gauge, screwdrivers and pliers, multimeter, compression gauge and an OBD scanner. Name each with one job you used it for.

Why tighten cylinder-head bolts in a sequence?

To clamp the head evenly so it does not warp and the gasket seals. Follow the sequence and the torque stages in the service manual (usually from the centre outward, in steps) using a torque wrench.

What safety do you follow when working under a vehicle?

Lift it on firm level ground at the jacking points, then support it on axle stands or a proper lift. Never work under a vehicle held only by a jack. Chock the wheels, disconnect the battery negative terminal before electrical work, and do not open a hot radiator cap.

How to say it: "Never under a vehicle on a jack alone". Say it first.

A vehicle does not start. How will you find the fault?

Work step by step: does the engine crank? If not, check the battery, terminals, starter and its circuit. If it cranks but does not start, check fuel supply, then ignition (petrol) or injection (diesel), then any fault codes. Confirm each check before moving to the next.

How to say it: The practical test is often exactly this: talk through your steps aloud as you work.