NAVTTC Auto Mechanic cheat sheet

Compression test: healthy gasoline engine typically 120–180 psi (8–12 bar); variation between cylinders >10% indicates leakage (rings, valves, head gasket).

Cylinder leakage test: introduce regulated air at TDC; leakage >20% is excessive. Listen at oil filler (rings), intake (intake valve), exhaust (exhaust valve), radiator (head gasket).

Vacuum gauge: steady 17–22 inHg at idle = normal; fluctuating needle = vacuum leak or valve issue; low steady = intake restriction or late timing.

Timing belt/chain: replace at manufacturer interval (often 60,000–100,000 mi); interference engines risk valve/piston contact if belt fails.

Engine noise diagnosis: deep knock under load = rod bearing; light tick at top = lifter/valve clearance; rattle at cold start = timing chain tensioner.

Spark plug reading: tan/light brown = normal; black soot = rich mixture; white/blistered = lean or overheating; oil fouling = worn rings/valve guides.

PCV system: clogged PCV causes oil leaks, rough idle, and sludge; test by removing oil cap with engine running-strong vacuum indicates blockage.

Head gasket failure signs: coolant in oil (milky), exhaust gas in coolant (bubbles), white smoke, unexplained coolant loss, compression loss between adjacent cylinders.

Manual clutch: free play typically 10–25 mm at pedal; excessive free play = no release, too little = slippage and release bearing wear.

Clutch slip test: in high gear at low speed, accelerate sharply; engine RPM rises without vehicle speed increase = worn clutch disc.

Manual transmission oil: check level at fill plug (vehicle level); use specified GL-4 or GL-5; GL-5 in some synchro transmissions causes corrosion.

Automatic transmission fluid (ATF): check hot in Park or Neutral per dipstick; color red/pink = good; brown/burnt = overheated; milky = coolant contamination.

CV joint diagnosis: clicking on turns = outer joint; vibration under acceleration = inner joint or half-shaft; torn boot = grease loss and joint failure.

Differential: whine that changes with speed = ring/pinion wear; clunk on acceleration/deceleration = worn spider gears or excessive backlash.

Driveshaft: vibration at certain speeds = unbalanced shaft or worn U-joints; check for missing balance weights and loose center bearing.

4WD/AWD: mismatched tire diameters >3 mm can damage transfer case; use same brand, size, and tread depth on all four wheels.

Brake pad minimum thickness: replace at 2–3 mm (1/8 in); brake disc minimum thickness is stamped on the disc; below it, replace.

Brake fluid: DOT 3/4 glycol-based absorbs moisture; replace every 2 years or when moisture >3%; DOT 5 silicone is not compatible with DOT 3/4.

Brake pedal feel: spongy = air in hydraulic system (bleed); low/pedal to floor = fluid leak or master cylinder bypass; pulsation = warped rotor or uneven pad deposit.

Parking brake: adjust rear shoes or caliper mechanism so lever travels 3–6 clicks; dragging brake causes overheating and poor fuel economy.

Suspension check: bounce test-vehicle should settle within one rebound; continued bouncing = worn shock absorbers/struts.

Tire wear patterns: inner/outer edge wear = incorrect camber or toe; cupping = worn shocks or unbalanced wheel; center wear = overinflation.

Wheel bearing noise: growl that changes with vehicle speed and load (louder in turns) = worn bearing; check by raising wheel and rocking top-bottom.

ABS warning light: ABS disabled but normal braking remains; common causes: wheel speed sensor, tone ring, or low brake fluid.

Ohm's law: V = I × R; power P = V × I. Example: 12 V across 3 Ω draws 4 A and dissipates 48 W.

Battery: 12.6 V = 100% state of charge (rested); 12.4 V = 75%; 12.2 V = 50%; below 12.0 V = discharged. Load test at half CCA.

Charging system: alternator should produce 13.5–14.5 V at idle with accessories off; below 13.0 V = undercharge; above 15.0 V = overcharge (regulator).

Voltage drop test: measure across a connection under load; >0.1 V per connection or >0.5 V total on a circuit indicates excessive resistance.

Starter circuit: cranking voltage should stay above 9.6 V at 20°C; slow crank = weak battery, bad cables, or failing starter.

Fuses: blade fuse color codes-5 A tan, 7.5 A brown, 10 A red, 15 A blue, 20 A yellow, 25 A clear, 30 A green.

Relay test: 4-pin relay-85/86 coil, 30 common, 87 normally open; apply 12 V to 85/86 and listen for click; check continuity 30–87.

OBD-II: DTC P0xxx = powertrain, B0xxx = body, C0xxx = chassis, U0xxx = network; freeze frame captures conditions at fault.

Fuel pressure: typical port injection 35–45 psi (2.5–3 bar); direct injection 500–2,000 psi; low pressure causes lean condition and misfire.

Fuel injector balance test: pulse each injector and compare pressure drop; variation >10% indicates clogged or faulty injector.

Engine oil: check level with engine off and vehicle level; API/ACEA rating must meet manufacturer spec; oil pressure warning = stop engine immediately.

Oil change interval: follow severe service schedule for short trips, dusty conditions, or towing; typically 5,000–7,500 mi (8,000–12,000 km).

Coolant: 50/50 mix provides freeze protection to −37°C and boil protection to 129°C at 15 psi; test with hydrometer or refractometer.

Thermostat: stuck open = engine runs cold, poor heater; stuck closed = overheating; test by heating in water-should open at rated temperature (e.g., 82°C/180°F).

Radiator cap: pressure rating (e.g., 0.9 bar/13 psi) raises boiling point; weak cap causes overheating and coolant loss.

Cooling fan: electric fan should engage when coolant reaches ~95–105°C or A/C is on; test fan clutch (viscous) by spinning-should have resistance when hot.

Toe: total toe-in typically 0–3 mm; incorrect toe causes rapid tire scrub and feather-edge wear.

Camber: negative camber (top inward) improves cornering; excessive negative camber wears inner tire edge; positive wears outer edge.

Caster: positive caster (steering axis tilted rearward) improves straight-line stability and steering return; typically 3–8°.

Thrust angle: rear toe difference; non-zero thrust angle causes steering wheel off-center and dog-tracking.

Steering wheel free play: typically 10–30 mm (0.4–1.2 in) at rim; excessive play = worn tie rods, steering box, or column.

Power steering: whine/groan = low fluid or air in system; stiff steering = failed pump or belt; check fluid level and belt tension.

Tie rod inspection: play at wheel (9 and 3 o'clock) = tie rod end; play at 12 and 6 o'clock = wheel bearing or ball joint.

Pre-alignment checks: tire pressure, tire wear, ride height, and suspension play must be correct before measuring alignment.

Lifting: keep back straight, lift with legs, keep load close; team-lift items over 25 kg (55 lb).

Jack stands: never work under a vehicle supported only by a jack; place stands on flat, hard ground at manufacturer lift points.

Torque: always use a calibrated torque wrench; torque = force × distance; tighten in sequence (e.g., cylinder head spiral) to avoid warping.

Micrometer/vernier: read main scale plus thimble; zero-check before use; store with measuring faces apart.

Feeler gauge: used for valve clearance, spark plug gap, and brake pad clearance; select blade that slides with slight drag.

Multimeter: measure voltage in parallel, current in series, resistance with circuit off; never measure resistance on a live circuit.

Fire safety: Class B extinguisher for fuel/oil fires; disconnect battery before welding; keep oily rags in closed metal container.

Hand tool safety: never use a wrench as a hammer; pull a wrench toward you rather than push; wear eye protection when grinding or hammering.