ASE H2 Diesel Engines (Transit Bus) cheat sheet

Diesel compression ignition requires cylinder pressure high enough to raise intake air above its autoignition temperature (roughly 700–900°F / 370–480°C at full load).

Wet compression test: add a small oil charge to a low cylinder; if pressure rises substantially, the loss is ring/piston related, not valve related.

Cylinder balance/power contribution test identifies a weak cylinder by measuring speed change when each injector is cut out; compare against the engine's programmed limits.

Black smoke = over-fueling or insufficient air; blue/white smoke = unburned fuel or lube oil; coolant loss with white smoke = combustion gas in coolant.

Blowby measured at the crankcase breather rises with worn rings, glazed liners, or failed turbo seals; correlate with leak-down and oil consumption data.

Leak-down test on a diesel: listen at intake (intake valve), exhaust (exhaust valve), oil fill (rings), and radiator (head gasket/cracked head).

Cylinder cutout misfire codes on electronically controlled engines are often caused by injector trim, harness resistance, or valve lash before mechanical failure.

Baseline first: verify battery voltage, fuel pressure, air filter restriction, and fault codes before condemning mechanical components.

Valve lash is set with the engine cold (per OEM spec) using the overlap or firing-order method; exhaust lash typically set slightly looser than intake due to thermal growth.

Cylinder head flatness is checked with a straightedge and feeler gauge across diagonals and edges; OEM limit is typically 0.002–0.006 in depending on engine.

Valve recession (sunk valves) reduces valve stem protrusion; measure stem height and compare to OEM to determine if seats need replacement.

Cylinder liner protrusion above the block deck is critical for head gasket sealing; measure at multiple points and compare to OEM tolerance.

Torque-to-yield head bolts are single-use; always replace and torque in the OEM sequence in stages (typically 3 stages plus a final angle).

Valve guide clearance is measured with a dial bore gauge and stem micrometer; excessive clearance causes oil consumption and seat pounding.

Camshaft lobe lift is checked with a dial indicator on the pushrod or valve spring retainer; compare intake vs exhaust and to OEM spec.

Cracked heads are detected with magnetic particle, dye penetrant, or pressure testing; combustion chamber cracks between valves are common on high-hour engines.

Oil pressure spec is typically 40–70 psi at rated speed and 10–20 psi at idle; low hot-idle pressure with normal high-speed pressure can indicate worn bearings.

Oil analysis: high iron = cylinder/valve train wear; high copper = bearings; high silicon = dirt ingestion; high coolant markers (K, Na) = coolant leak.

Coolant SCA (supplemental coolant additive) or OAT chemistry must be maintained; depleted SCA causes liner pitting (cavitation erosion) on wet-sleeve engines.

Cavitation liner pitting is driven by vibration-induced vapor bubbles; SCA/DCA additives form a protective film on the liner OD.

Thermostat opening temperature and full-open temperature are specified by OEM; verify with a thermometer in heated water before installation.

Pressure-test the cooling system at the radiator cap rating; a slow pressure drop with no external leak suggests head gasket, EGR cooler, or oil cooler.

Coolant flow restriction is diagnosed by measuring temperature differential across the radiator; large delta indicates a plugged core or stuck thermostat.

Oil cooler failures cross-contaminate oil and coolant; look for oil in coolant (sludge) or coolant in oil (milky) and pressure-test the cooler separately.

Air filter restriction is measured with a water manometer or Magnehelic gauge; OEM maximum restriction is typically 20–30 in H2O at full load.

Turbo boost pressure is checked at the intake manifold under load; low boost with high exhaust backpressure suggests a restricted exhaust or failed turbo.

Turbo shaft end play and radial play are measured with a dial indicator; excessive play causes oil seal leakage and compressor wheel contact.

Wastegate or VGT actuator calibration must match the ECM command; a stuck VGT causes overboost, underboost, or driveability codes.

Charge-air cooler (CAC) leaks are found by pressure testing and looking for boost loss or oil mist at connections; a leaking CAC causes low power and black smoke.

Exhaust backpressure is measured upstream of the turbo or DPF; high backpressure reduces boost and increases exhaust temperature.

Aftercooler/intake manifold leaks cause low boost, black smoke, and low power; check clamps and boots before condemning the turbo.

Turbo bearing failure often sends debris into the intake; always inspect the CAC and intake tract after a turbo failure before restarting.

Diesel fuel pressure at the injector inlet is checked against OEM spec; low pressure causes hard start, low power, and misfire codes.

Common-rail systems: rail pressure is commanded by the ECM; low rail pressure at crank indicates a leaking injector, pressure-relief valve, or pump.

Injector return (leak-off) test compares return volume per injector; excessive return on one injector indicates internal wear or stuck needle.

Injector trim codes (IMA/ISA) must be programmed into the ECM after replacement; incorrect trim causes rough idle and imbalance codes.

Air in the fuel system causes hard starting and white smoke; bleed the system per OEM procedure after filter or line service.

Throttle pedal (APPS) and engine speed sensor signals are compared by the ECM; mismatch sets a fault and defaults to limp or idle.

Crank and cam sensor correlation: a missing or out-of-time signal prevents start or causes injection timing codes; verify with a scope or OEM diagnostic.

Fuel quality matters: water contamination, low cetane, or gasoline contamination cause hard start, misfire, and injector damage.

Battery state of charge is estimated by open-circuit voltage: ~12.6 V = 100%, ~12.4 V = 75%, ~12.2 V = 50% for a 12 V battery at 77°F.

Cranking voltage drop test: measure across each cable and connection; more than 0.5 V drop on a cable or 0.1 V per connection indicates excessive resistance.

Starter draw test: measure current during crank; excessive draw with slow crank indicates a dragging starter, seized bearing, or high compression load.

Alternator output should be checked at idle and at 2000 rpm with loads on; typical charging voltage is 13.8–14.4 V regulated.

Parasitic draw: with all loads off and modules asleep, current should be below OEM spec (often <50 mA); isolate by fuse-pull method.

Diesel starting aids (grid heater, glow plugs, intake heater) must be verified; a failed grid heater causes hard cold start and white smoke.

Ground strap integrity is critical on diesel chassis; a bad ground causes sensor misreads, no-crank, and communication faults.

CAN/J1939 data bus faults cause multiple unrelated codes; verify terminating resistor resistance (~60 ohms across the bus) before replacing modules.

Compression-release (Jacobs) brake operation depends on oil pressure and correct valve lash; verify lash and oil supply before condemning the brake.

Exhaust brake restriction is checked against OEM backpressure limits; excessive restriction can damage the turbo or engine.

EGR cooler leaks cause coolant loss, white smoke, and low coolant level with no external leak; pressure-test the cooler separately.

DPF regeneration requires sufficient exhaust temperature; frequent derates indicate excessive soot load, failed sensor, or short-duty cycle.

DPF soot load is estimated by differential pressure across the filter; high delta at low flow indicates a loaded or cracked filter.

SCR/DEF systems require correct DEF quality (32.5% urea) and tank level; contaminated or diluted DEF sets derate and inducement codes.

NOx sensor and exhaust temperature sensor faults cause incorrect dosing and inducement; verify sensor readings against known values before replacement.

Crankcase ventilation (CCV) filter restriction raises crankcase pressure and causes oil leaks and seal failure; check CCV delta pressure.

Diesel fuel is combustible; store and handle per local fire code, and never use gasoline as a cleaning solvent near ignition sources.

High-pressure common-rail fuel can exceed 20,000 psi; never crack a high-pressure line while cranking, and use rated test equipment.

Before working on a running engine, secure loose clothing, remove jewelry, and keep hands clear of fans, belts, and turbo inlets.

Lockout/tagout applies to bus electrical and air systems before service; verify zero energy state before disassembly.

Use proper lifting equipment for engine and transmission assemblies; never support a vehicle on jacks alone under a suspended load.

Wear eye protection and gloves when handling coolant, DEF, and solvents; DEF is corrosive to some metals and should be rinsed promptly.

Exhaust and turbo surfaces reach several hundred degrees; allow cool-down and use heat-resistant gloves before service.

Battery service requires removing the ground first and reconnecting it last; hydrogen gas near batteries is explosive.