ITI Diesel Mechanic Interview Questions and Answers

How does a diesel engine ignite the fuel?

By compression. Only air is drawn in and compressed to a high compression ratio, which heats it above the ignition temperature of diesel. Fuel is then injected as a fine spray and ignites on its own. There is no spark plug.

How to say it: Use the words "compression ignition". It is the textbook term interviewers expect.

Explain the four-stroke cycle of a diesel engine.

Suction: only air enters. Compression: the air is compressed and heated. Power: fuel is injected near the end of compression, burns and pushes the piston down. Exhaust: burnt gases leave. One cycle takes two crankshaft revolutions.

Why does a diesel engine need a higher compression ratio than a petrol engine?

Because the heat of compression alone has to ignite the fuel. A petrol engine uses a spark, so it runs a lower compression ratio.

What does a glow plug or heater plug do?

It heats the combustion chamber or intake air for starting in cold conditions, when compressed air alone may not get hot enough to ignite the fuel.

What does a turbocharger do?

It uses exhaust gas to spin a turbine that drives a compressor, pushing more air into the cylinders so more fuel can be burned and the engine gives more power. An intercooler, where fitted, cools that compressed air.

Trace the fuel flow in a diesel engine.

Tank → primary filter / water separator → feed (lift) pump → secondary filter → fuel injection pump → high-pressure lines → injectors → combustion chamber, with excess fuel returning to the tank through the leak-off line.

Why and how do you bleed a diesel fuel system?

Air in the fuel lines stops the injection pump delivering fuel, so the engine will not start or will stall. After a filter change or running dry, operate the priming pump and open the bleed screws on the filter and pump in order until bubble-free fuel comes out, then tighten them, as the manual describes.

How to say it: This is asked in almost every diesel interview: describe it as a sequence.

What is a common rail (CRDi) system?

A high-pressure pump keeps fuel at very high pressure in a rail shared by all injectors, and the engine control unit opens each electronically controlled injector. It allows finer control of injection timing and quantity than a mechanical pump.

How do you test an injector?

On an injector tester: check the opening pressure against the manual, the spray pattern, and that it does not dribble or leak after closing. Keep hands well away from the spray: high-pressure fuel can penetrate skin.

Why is water in diesel a problem, and how is it removed?

Water damages the injection pump and injectors and causes rough running. The water separator or sedimenter collects it; drain it regularly from the drain valve.

What do black, white and blue smoke mean?

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

How to say it: Give all three colours with one cause each. It is the most common diesel question.

A diesel engine cranks but does not start. What will you check?

Fuel in the tank, air in the system (bleed it), choked filters, the shut-off solenoid, the injection pump and injectors, and on a cold engine the glow plugs. Check compression if fuel supply is fine.

An engine is losing power. What are the likely causes?

Choked air filter, dirty fuel filter, faulty injectors, wrong injection timing, turbocharger problem, exhaust restriction, or low compression from worn rings or valves.

How do you check compression?

With a compression tester fitted in place of the injector or glow plug, cranking the engine and comparing each cylinder's reading with the manual and with the other cylinders. A low cylinder points to rings, valves or the head gasket.

Why does a diesel engine overheat?

Low coolant, a stuck thermostat, a failed water pump or slipping belt, a blocked radiator, a fan fault, or overloading. Never open a hot radiator cap.

What daily checks do you do on a DG set?

Engine oil level, coolant level, fuel level and leaks, battery and its terminals, air filter indicator, belt tension, and on running: oil pressure, temperature, voltage and frequency on the panel, as the maintenance schedule lists.

What safety do you follow in a diesel workshop?

Isolate the battery before electrical work, never put hands near high-pressure fuel spray, support vehicles on stands and chock wheels, keep the area free of fuel spills, and use the correct lifting equipment for heavy components.

What is the usual practical test for a diesel mechanic?

Commonly: bleeding a fuel system and starting the engine, finding a fault the examiner has set, setting valve clearance, or dismantling and assembling a component to the manual. Talk through each step and the safety check.

How to say it: Ask for the service manual if one is available: supervisors notice.