Engine and related systems
The engine needs clean air, fuel, a spark (petrol) and good oil. The main jobs:
- Engine oil and filter: change at the maker's interval. Oil cuts friction and carries dirt to the filter.
- Cooling system: check coolant level, hoses, radiator cap, fan and belt. Never open a hot radiator.
- Air filter and fuel filter: a dirty filter chokes the engine and wastes fuel.
- Ignition: spark plugs wear. Worn plugs give hard starting and misfire.
- Belts and chain: the timing belt or chain keeps valves and pistons in step. Replace at the set interval; a snap can wreck the engine.
- Compression test: a gauge in the spark plug hole shows the pressure each cylinder makes. Low or uneven readings point to worn rings or leaking valves.
Chassis and related systems
The chassis carries the car and connects it to the road.
- Brakes: check pad (or lining) thickness, discs or drums, brake fluid level and colour, hoses and the handbrake. Squeal, a long pedal or a pull to one side is a warning.
- Tyres and wheels: pressure, tread depth, cracks, even wear, wheel nuts torqued. Rotate tyres at the set interval.
- Suspension: springs and shock absorbers. A bounce test: push down on a corner and release; it should settle after one bounce.
- Steering: free play, joints and steering fluid (if power-assisted).
- Wheel alignment: correct angles stop uneven tyre wear and pulling to one side.
- Transmission: gearbox oil level, clutch feel, drive shaft joints and boots.
Equipment for conservation and safety
Some parts protect the air, and others protect people.
- Exhaust cleaning: catalytic converter, particle filter, oxygen sensor and the exhaust gas recirculation valve. A gas analyser measures CO, hydrocarbons or smoke and compares them with the legal limit.
- Fuel vapour control: the charcoal canister and fuel cap seal stop petrol vapour escaping.
- Noise: the silencer keeps the sound inside the limit.
- Lights: headlamp aim, brightness, indicators, brake lights and horn.
- Restraints: seat belts, child seat anchors, head restraints and airbags. Airbag warning lamp must switch off after start. Never repair airbags without training.
- Driver aids: ABS and stability warning lamps, wipers and washers, mirrors.
How a test works
Every test follows the same four steps: (1) set up safely, (2) measure, (3) compare the reading with the limit in the service manual, (4) record and repair if needed. Use the right tool: pressure gauge, vernier caliper for pad thickness, gas analyser, beam setter. Always read the value on the tool, never guess.
Try it: PASS or FAIL
In the 3D, drag the wear slider. Note the value where a reading crosses the green limit mark. Then at home, use a coin to check the tyre tread of a bicycle or scooter and write whether the tread looks even.
Key formulas and definitions
- Pass if reading is on the good side of the limit
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume
- Wear % = (new thickness − measured) ÷ (new − minimum) × 100
- Key terms: compression, alignment, shock absorber, emission test, torque
Worked examples
1. A brake pad is 10 mm when new, the minimum is 3 mm and it measures 4 mm now. Is it fit? How much life is left?
4 mm is above 3 mm so it still passes. Life used = (10 − 4) ÷ (10 − 3) = 6/7 ≈ 86%. About 14% is left, so replace soon.
2. Four cylinders read 11, 11, 11 and 7 bar. The maker says each must be at least 10 bar. What does it show?
Three pass. The cylinder at 7 bar fails, so it may have worn rings or a leaking valve.
3. A tyre has a 1.6 mm legal minimum and now shows 1.2 mm. What do you do?
1.2 is below 1.6, so replace the tyre.
4. Swept volume of one cylinder is 450 cm³, clearance volume 50 cm³. Compression ratio?
(450 + 50) ÷ 50 = 10, so 10 : 1.
5. Emission limit CO is 0.5%. A car reads 0.2%. Result?
0.2% is below the limit, so the car passes.
Common mistakes
- Opening the radiator cap on a hot engine. Hot coolant can spray out.
- Judging brakes only by noise. Measure the pad thickness.
- Mixing up the limits: for tyre tread and pad thickness, a smaller number is worse; for emissions, a bigger number is worse.
- Working on airbags without training.