ASE H2 Diesel Engines (Transit Bus) flashcards
ASE H2 Diesel Engines (Transit Bus) · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Wet compression test
- A compression test repeated after adding a small amount of oil to a low cylinder; a large pressure rise indicates worn rings or cylinder walls rather than valve leakage.
- Cylinder balance test
- A diagnostic procedure that cuts out each injector in turn and measures the resulting change in engine speed to identify a weak or misfiring cylinder.
- Leak-down test
- Supplies regulated air to a cylinder at TDC and measures leakage; listening at intake, exhaust, oil fill, and radiator locates the leak path.
- Blowby measurement
- Crankcase gas flow measured at the breather; elevated blowby indicates worn rings, glazed liners, or turbo seal leakage and correlates with oil consumption.
- Valve lash adjustment
- Setting the clearance between rocker arm and valve stem to OEM specification with the engine cold, using the firing-order or overlap method for each cylinder.
- Cylinder liner protrusion
- The height the liner stands above the block deck, measured with a dial indicator; incorrect protrusion prevents proper head gasket sealing and causes coolant leaks.
- Torque-to-yield bolts
- Head bolts designed to stretch permanently during tightening; they are single-use and must be replaced, then torqued in stages plus a final angle.
- Valve recession check
- Measuring valve stem protrusion above the head to detect sunk valve seats; excessive recession requires seat replacement or head reconditioning.
- Oil analysis interpretation
- Lab report of wear metals and contaminants; iron indicates cylinder wear, copper indicates bearings, silicon indicates dirt ingestion, and potassium indicates coolant.
- SCA/DCA additives
- Supplemental coolant additives that form a protective film on wet-sleeve liners to prevent cavitation pitting caused by vibration-induced vapor bubbles.
- Cooling system pressure test
- Pressurizing the system to cap rating and observing pressure drop; a slow drop without external leak points to head gasket, EGR cooler, or oil cooler.
- Thermostat test
- Heating the thermostat in water with a thermometer to verify it opens at the specified temperature and reaches full open within the OEM range.
- Air filter restriction
- Measured with a manometer in inches of water at full load; exceeding OEM maximum restriction reduces boost, raises exhaust temperature, and causes black smoke.
- Turbo shaft play check
- Using a dial indicator to measure radial and axial play of the turbo shaft; excessive play causes wheel contact, oil seal leakage, and boost loss.
- Charge-air cooler leak
- A pressure-tested CAC that loses boost causes low power, black smoke, and oil mist at connections; test before condemning the turbocharger.
- Exhaust backpressure
- Pressure measured upstream of the turbo or DPF; high backpressure reduces boost, raises exhaust temperature, and can damage the engine.
- Injector leak-off test
- Measuring fuel return volume from each injector; excessive return from one injector indicates internal wear or a stuck needle and causes low rail pressure.
- Injector trim codes
- Calibration values unique to each injector that must be programmed into the ECM after replacement to maintain correct fueling and smooth idle.
- Rail pressure test
- Comparing commanded versus actual common-rail pressure during crank and run; low actual pressure indicates a leaking injector, relief valve, or pump.
- APPS signal check
- Verifying accelerator pedal position sensor voltage against OEM spec; a mismatch with engine speed sets a fault and defaults the engine to limp or idle.
- Cranking voltage drop
- Measuring voltage loss across battery cables and connections during crank; more than 0.5 V on a cable indicates excessive resistance and slow cranking.
- Parasitic draw test
- Measuring current with all loads off and modules asleep, typically under 50 mA; excess draw is isolated by pulling fuses one at a time.
- Starter draw test
- Measuring current during cranking with an inductive ammeter; excessive draw with slow crank indicates a dragging starter, seized bearing, or high compression load.
- Alternator output test
- Checking charging voltage at idle and 2000 rpm with electrical loads applied; regulated output is typically 13.8 to 14.4 volts on a 12-volt system.
- Compression release brake
- An engine brake that opens exhaust valves near TDC to release compressed air; operation depends on oil pressure and correct valve lash settings.
- EGR cooler leak test
- Pressure-testing the EGR cooler separately to find internal coolant leaks that cause white smoke, coolant loss, and low coolant with no external leak.
- DPF differential pressure
- Pressure drop across the diesel particulate filter used to estimate soot load; high delta at low flow indicates a loaded or cracked filter.
- DEF quality check
- Verifying urea concentration is about 32.5 percent with a refractometer; diluted or contaminated DEF causes SCR faults and engine derate.
- High-pressure fuel safety
- Common-rail pressure can exceed 20,000 psi; never crack a high-pressure line while cranking, and use only rated test equipment and procedures.
- Lockout/tagout
- Isolating electrical and air energy sources and applying locks and tags before service; verify zero energy state before disassembly of bus systems.
- Battery disconnect order
- Remove the negative ground cable first and reconnect it last to prevent arcing and sparks near explosive hydrogen gas during battery service.
- Lifting engine assemblies
- Using rated cranes or lifts and approved slings for engine and transmission removal; never support a suspended load with jacks alone.