ASE S2 Diesel Engines (School Bus) cheat sheet

Baseline checks before diagnosis: fuel level and quality, air filter restriction, battery state of charge, and intake/exhaust restrictions.

Cylinder balance test: disable one cylinder at a time and compare RPM drop; a weak cylinder shows little or no drop versus the average.

Blow-by measured at the crankcase breather or oil fill with a manometer; excessive flow or pressure indicates worn rings, scored liners, or piston damage.

Oil analysis trends: elevated iron suggests cylinder/valve train wear; elevated silicon suggests dirt ingestion; coolant markers (sodium, potassium) suggest head gasket or liner seal failure.

Compression test interpretation: low on adjacent cylinders points to head gasket; low on one cylinder points to piston, ring, or valve sealing; wet test rise indicates ring sealing loss.

Cylinder leakage test thresholds: leakage under 10 percent acceptable; locate leak by listening at intake, exhaust, crankcase breather, or radiator.

Smoke color diagnosis: black is overfueling or air restriction; blue is lube oil; white is unburned fuel or coolant.

Electronic fault isolation: retrieve active and inactive fault codes first, then verify sensor values against mechanical gauges before replacing components.

Valve lash setting is performed with the engine cold or per OEM specification; measure with a feeler gauge at the specified point on the rocker or crosshead.

Crosshead (bridge) adjustment must be set before valve lash on engines using a single rocker actuating two valves.

Valve recession shows as lash that tightens over time; lash that loosens suggests wear at rocker, pushrod, cam, or cam follower.

Cylinder head flatness is checked with a straightedge and feeler gauge; maximum allowable out-of-flatness is OEM-specific and typically measured both longitudinally and diagonally.

Head gasket failure indicators: coolant in oil, combustion gas in coolant (bubbles or hydrocarbon test), adjacent-cylinder compression loss, or external leakage at the head-to-block joint.

Liner protrusion must be measured and within OEM specification; incorrect protrusion causes head gasket failure and coolant leakage.

Valve guide wear is checked by measuring stem-to-guide clearance; excessive clearance causes oil consumption, smoke, and seat distortion.

Camshaft inspection includes lobe lift measurement and bearing journal measurement; lobe wear reduces valve lift and alters timing.

Oil pressure diagnosis: compare mechanical gauge reading at the main gallery to the electronic gauge or sensor; low hot idle pressure indicates worn bearings, worn pump, or diluted oil.

Oil dilution by fuel is detected by viscosity drop and fuel odor; causes include leaking injector, failed injector cup seal, or excessive idle.

Coolant pressure test: pressurize the system cold and observe for pressure decay; a slow drop with no external leak suggests internal leakage.

Thermostat function is verified by temperature rise and by comparing top-tank and bottom-tank temperatures; a stuck-open thermostat causes low operating temperature and poor heater output.

Coolant conditioner (SCA) concentration is maintained per OEM; over-concentration causes silicate gel drop-out and water pump seal damage.

Fan drive diagnosis: air-actuated, viscous, or electronic fan clutches are tested for engagement at the specified temperature and for correct signal at the solenoid or control valve.

Aeration in coolant appears as bubbles in the surge tank and causes overheating; sources include head gasket leak, liner seal leak, or low coolant level.

Oil cooler failures show as oil in coolant or coolant in oil; pressure-test the cooler separately before condemning the engine.

Intake restriction is measured with a water manometer or vacuum gauge at the air cleaner outlet; compare to OEM maximum restriction specification.

Exhaust backpressure is measured at a port ahead of the turbo or aftertreatment; excessive backpressure reduces power and raises exhaust temperature.

Turbocharger boost is verified against OEM specification at rated load; low boost with high exhaust temperature indicates a leak, restriction, or turbo fault.

Boost leaks are found by pressurizing the intake system and listening or spraying with a soap solution.

Aftertreatment (DPF) restriction is indicated by high backpressure and by differential pressure sensor readings; soot load can be checked with the diagnostic tool.

Turbo shaft play and wheel-to-housing contact are checked by hand rotation and axial/radial movement; oil in the intake or exhaust indicates seal failure.

Charge-air cooler leaks cause low boost, high intake temperature, and power loss; test by pressure test or by comparing inlet and outlet temperatures.

Exhaust leaks ahead of the turbo or sensor reduce turbo performance and can skew oxygen or NOx sensor readings.

Low-pressure fuel supply is verified with a gauge at the OEM test port; measure both pressure and flow, since restriction can show normal pressure with low volume.

Air in the fuel system causes hard start, low power, and misfire; check for suction-side leaks and verify primer or lift pump function.

Injector return (leak-off) testing compares return volume across injectors; excessive return from one injector indicates internal wear or a stuck needle.

Cylinder cutout test on common rail engines identifies a weak or leaking injector by comparing contribution and by monitoring rail pressure behavior.

Rail pressure codes: low rail pressure at crank indicates a leak, a faulty pressure control valve, or a weak high-pressure pump; verify with a mechanical gauge if available.

Injector trim codes must be programmed into the ECM after injector replacement; incorrect trim causes rough running and imbalance codes.

Fuel contamination: water in fuel damages high-pressure components; check the water separator and sample fuel for water and microbial growth.

High-pressure fuel leaks are located using the OEM-recommended method; never use hands to search for high-pressure leaks.

Engine brake operation requires the ECM to receive the correct inputs: enable switch, clutch switch, throttle position at idle, and engine speed above the activation threshold.

Jake brake (compression release) diagnosis: verify oil supply pressure to the brake housings, solenoid function, and slave piston lash adjustment.

Slave piston lash is set per OEM specification with the engine at the specified temperature; incorrect lash causes poor braking or valve-to-piston contact.

Exhaust brake diagnosis: verify the butterfly valve closes fully and the actuator receives the correct air or electrical signal.

Sensor reference voltage checks: most engine sensors use a 5V reference; a shorted reference pulls down all sensors on that circuit.

Data link diagnosis: verify battery voltage and ground at the diagnostic connector before condemning the ECM or tool.

ECM power and ground circuits are verified under load with a voltage drop test; a poor ground causes multiple unrelated faults.

Actuator and solenoid tests use resistance and current draw compared to specification; open or shorted coils set circuit codes.

Cranking circuit voltage drop: measure from battery positive to starter positive post and from starter case to battery negative while cranking; each should be within OEM limits (commonly under 0.5V total per side).

Battery state of charge is estimated from open-circuit voltage after surface charge removal; a fully charged 12V battery reads about 12.6V at 70F.

Load test batteries at half the CCA rating for 15 seconds; a battery that falls below 9.6V at 70F is suspect.

Parasitic draw is measured with a clamp meter or by series ammeter after modules sleep; compare to OEM specification.

Alternator output is tested at the B+ terminal with a load applied; compare voltage and current to specification and check for AC ripple indicating diode failure.

Starter circuit includes the control circuit (key, neutral switch, relay) and the power circuit; isolate which is at fault before replacement.

Series-parallel battery switching (24V start, 12V run) requires verifying both the series and parallel solenoids; a failed solenoid causes no-crank or overvoltage.

Charging system overcharge (above about 14.8V) damages batteries and electronics; check voltage regulator control and sense wire.

Diesel aftertreatment typically includes DOC, DPF, and SCR; each has specific fault codes and sensor sets to verify before replacement.

DPF regeneration requires sufficient exhaust temperature; frequent failed regens indicate a faulty DOC, excessive oil consumption, or a boost/exhaust leak.

DEF quality and concentration (typically 32.5 percent urea) affect SCR efficiency; diluted or contaminated DEF sets efficiency codes.

NOx sensor readings are compared across the SCR: high inlet and high outlet NOx indicates low DEF dosing or SCR catalyst failure.

EGR system faults: excessive EGR flow causes black smoke and low power; insufficient flow causes high NOx and high combustion temperature.

Crankcase ventilation (CCV) system must be open; a plugged CCV filter causes seals to leak and oil to be pushed out.

OBD monitor readiness and fault code clearing: verify repairs with a drive cycle or OEM regen before releasing the vehicle.

Tampering or defeat devices are prohibited; emission control components must be restored to OEM configuration during repair.