ASE S2 Diesel Engines (School Bus) flashcards
ASE S2 Diesel Engines (School Bus) · Build recall with a focused review session.
31 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Cylinder balance test purpose
- A diagnostic test that disables one cylinder at a time and compares RPM drop; a cylinder with little or no drop is contributing less power than the others.
- Wet compression test
- A compression test repeated after adding a small amount of oil to the cylinder; a significant rise in pressure indicates worn rings or cylinder walls rather than valve leakage.
- Cylinder leakage test
- Pressurizing a cylinder at top dead center and measuring the percentage of air escaping; listening at intake, exhaust, crankcase, or radiator locates the leak source.
- Blow-by measurement
- Measuring crankcase pressure or flow at the breather with a manometer; excessive blow-by indicates worn rings, scored liners, or piston damage.
- Blue exhaust smoke cause
- Blue or gray smoke from the exhaust indicates lube oil is being burned; common causes are worn valve guides, worn rings, or turbo seal leakage.
- Valve lash tightening trend
- Valve lash that progressively tightens indicates valve seat recession or valve face wear; lash that loosens suggests wear in the rocker, pushrod, cam, or follower.
- Crosshead adjustment order
- On engines with a single rocker actuating two valves, the crosshead (bridge) must be adjusted before setting valve lash, or lash will be incorrect.
- Head gasket failure signs
- Coolant in oil, combustion gas in coolant, adjacent-cylinder compression loss, or external leakage at the head-to-block joint all indicate a failed head gasket.
- Liner protrusion importance
- Cylinder liner protrusion above the block deck must be within OEM specification; incorrect protrusion causes head gasket failure and coolant leaks.
- Valve guide clearance check
- Measuring stem-to-guide clearance with a dial bore gauge and micrometer; excessive clearance causes oil consumption, smoke, and valve seat distortion.
- Oil pressure cross-check
- Comparing a mechanical gauge reading at the main oil gallery to the electronic gauge or sensor verifies whether low pressure is real or a sensor fault.
- Fuel dilution of engine oil
- Fuel in the oil lowers viscosity and raises oil level; causes include a leaking injector, failed injector cup seal, or prolonged excessive idling.
- Coolant aeration symptoms
- Bubbles in the surge tank with overheating indicate combustion gas entering the cooling system from a head gasket, liner seal, or low coolant level.
- SCA over-concentration risk
- Excess supplemental coolant additive causes silicate gel drop-out, which plugs the cooling system and damages water pump seals; maintain OEM concentration.
- Intake restriction measurement
- A water manometer or vacuum gauge at the air cleaner outlet measures intake restriction; compare the reading to the OEM maximum allowable specification.
- Exhaust backpressure test
- Measuring pressure at a port ahead of the turbo or aftertreatment; excessive backpressure reduces power and raises exhaust temperature and DPF soot loading.
- Boost leak detection
- Pressurizing the intake system and applying soap solution or listening for escaping air locates leaks that cause low boost and power loss.
- Turbo seal failure signs
- Oil in the intake or exhaust piping, excessive shaft play, or wheel-to-housing contact indicates turbo bearing or seal failure requiring turbo replacement.
- Injector leak-off test
- Comparing fuel return volume across injectors identifies an injector with excessive return, indicating internal wear or a stuck needle and causing low power.
- Injector trim code purpose
- A calibration code unique to each injector that must be programmed into the ECM so fuel delivery is accurate; incorrect trim causes rough running.
- Low rail pressure at crank
- Insufficient rail pressure during cranking indicates a high-pressure leak, a faulty pressure control valve, or a weak high-pressure pump.
- High-pressure fuel leak safety
- High-pressure diesel leaks can inject fuel through skin; locate leaks with cardboard or OEM-approved methods, never with hands, and use eye protection.
- Engine brake enable conditions
- A compression release brake requires the enable switch on, clutch engaged, throttle at idle, and engine speed above the activation threshold for the ECM to command it.
- Slave piston lash setting
- The clearance between the engine brake slave piston and valve crosshead is set per OEM specification; incorrect lash causes weak braking or valve contact.
- 5V reference circuit fault
- A short to ground on a 5V reference circuit pulls the voltage down and causes multiple sensors on that circuit to read incorrectly or set codes.
- Voltage drop test
- Measuring voltage lost across a cable or connection while current flows; excessive drop indicates high resistance, such as a corroded terminal or ground.
- Battery open-circuit voltage
- After removing surface charge, a fully charged 12V battery reads about 12.6V at 70F; each 0.1V below indicates roughly 10 percent less state of charge.
- Parasitic draw test
- Measuring current flow with the key off after modules sleep; compare to OEM specification to find a circuit draining the battery overnight.
- Alternator AC ripple
- Excessive AC voltage at the alternator output indicates one or more failed rectifier diodes and can cause electrical noise and charging faults.
- DPF regeneration temperature
- A diesel particulate filter regenerates when exhaust temperature is high enough to oxidize soot; frequent failed regens suggest a faulty DOC or excessive oil consumption.
- DEF concentration check
- Diesel exhaust fluid is typically 32.5 percent urea; diluted or contaminated DEF reduces SCR efficiency and sets NOx conversion fault codes.