ASE T1 Gasoline Engines (Medium/Heavy Truck) cheat sheet

Vacuum test: a steady gauge reading of 17-22 in-Hg at idle in good engines; a slowly dropping needle indicates an exhaust restriction.

Blow-by check: a steady flow from the crankcase breather with the engine running indicates worn rings or cylinder walls; a puffing, rhythmic flow indicates one damaged cylinder.

Compression test: cranking compression should be within 10 percent across cylinders and above the manufacturer's minimum; add oil to a low cylinder; if pressure rises, the fault is rings, not valves.

Cylinder leakage test: air escaping at the intake indicates an intake valve; at the exhaust an exhaust valve; at the oil filler or dipstick tube indicates rings or a piston.

Engine noise diagnosis: a deep knock under load that changes with load indicates a rod or main bearing; a light, sharp tick at half engine speed indicates a valve train fault.

Coolant in oil (milky oil) or combustion gases in the cooling system (bubbles at the radiator) indicates a failed head gasket or a cracked head.

White smoke with a sweet odor indicates coolant burning; blue-gray smoke indicates oil consumption; black smoke indicates an over-rich fuel mixture.

A rough idle with a misfire code isolated to one cylinder points to that cylinder's ignition, injector, or compression; swap components to isolate.

Valve lash: check and adjust with the engine cold unless the manufacturer specifies hot; set intake and exhaust to the published clearance specification.

Valve recession: measure installed height or valve stem height; a valve that has sunk into the seat reduces spring installed height and alters lash.

Valve spring: measure free length and installed height; a spring below the specified pressure or length must be replaced.

Cylinder head warpage: check with a straightedge and feeler gauge along the length, across the width, and diagonally; compare against the manufacturer's maximum flatness tolerance.

Camshaft: measure lobe lift and base circle; a worn lobe reduces lift and can cause a misfire that is not ignition related.

Cam bearing wear: check camshaft journal-to-bearing clearance with a micrometer and bore gauge; excessive clearance reduces oil pressure and valve lift.

Valve guide clearance: measure stem-to-guide clearance with a dial indicator; excessive clearance causes oil consumption and valve seal failure.

Pushrod and rocker: check for bent pushrods by rolling on a flat surface; check rocker arm wear at the valve tip and pivot.

Main bearing clearance: use plastigage or a micrometer and bore gauge; compare against the manufacturer's specification, typically 0.001-0.003 in.

Crankshaft end play: measure with a dial indicator against the thrust surface; excessive end play indicates thrust bearing wear.

Crankshaft journal taper and out-of-round: measure at two points 90 degrees apart; taper and out-of-round must fall within specification.

Bearing failure analysis: scoring indicates lack of lubrication; fatigue (cracking, flaking) indicates overload or detonation; wiping indicates excessive clearance or misassembly.

Cylinder bore: measure taper and out-of-round with a bore gauge at the top, middle, and bottom; compare against the maximum wear limit.

Piston ring end gap: check with the ring squared in the bore; too small a gap causes ring butting and breakage; too large causes blow-by.

Torque-to-yield bolts: measure bolt stretch or use the angle-torque method; replace bolts that exceed the specified length or stretch.

Main bearing bore alignment: check with a straightedge or dial bore gauge; misalignment causes rapid bearing failure and crankshaft breakage.

Oil pressure: check with a mechanical gauge at the specified rpm and temperature; low pressure at idle can be normal, but low pressure at speed indicates worn bearings or a weak pump.

Oil pump: measure gear or rotor clearance and end clearance against specification; excessive clearance reduces output.

Pressure relief valve: a stuck-open relief valve causes low oil pressure; a stuck-closed valve causes high pressure and filter or seal failure.

Oil consumption: check for external leaks, valve guide wear, worn rings, and a failed PCV system; blue exhaust smoke confirms oil burning.

Oil filter bypass: a clogged filter opens the bypass, allowing unfiltered oil to circulate; change the filter and check for debris.

Oil analysis: elevated iron indicates cylinder or valve train wear; elevated copper or lead indicates bearing wear; coolant in oil indicates a head gasket or cooler leak.

Oil cooler: a failed internal cooler leaks coolant into the oil or oil into the coolant; pressure test the cooler separately.

Oil viscosity: use the grade specified for the ambient temperature range; too thick an oil restricts cold flow; too thin an oil reduces hot pressure.

Pressure test: pressurize the system to the cap rating; a dropping gauge indicates an external or internal leak.

Combustion gas test: a chemical block tester that changes color confirms combustion gases in the coolant, indicating a head gasket or cracked head.

Thermostat: check opening temperature in a heated water bath; a thermostat stuck open causes slow warm-up and low heater output; stuck closed causes overheating.

Radiator cap: test to the rated pressure; a weak cap lowers boiling point and allows coolant loss.

Coolant flow: a restricted radiator shows a temperature difference across the core; check with an infrared thermometer.

Water pump: a leaking weep hole, loose or noisy bearing, or eroded impeller reduces flow; check belt tension and drive condition.

Fan clutch: a viscous fan clutch should engage when hot; a failed clutch causes overheating at low speed and idle.

Coolant concentration: check with a refractometer; a 50/50 mix is typical; too high a concentration reduces heat transfer.

Intake restriction: a vacuum gauge reading that drops at wide-open throttle or a restricted air filter reduces power and increases fuel consumption.

Turbocharger: check for shaft end play and radial play; excessive play indicates bearing wear; check for oil seal leakage into the intake or exhaust.

Boost pressure: low boost indicates a leaking charge-air cooler, loose clamps, or a failed wastegate or VGT actuator.

Exhaust backpressure: a restricted exhaust (plugged catalyst or muffler) raises backpressure and reduces power; measure with a backpressure gauge.

EGR system: a stuck-open EGR valve causes rough idle and black smoke; a stuck-closed valve increases NOx and can set a code.

Charge-air cooler: a leaking cooler causes low boost and can allow oil into the intake; pressure test the cooler.

Air filter restriction indicator: a gauge reading in the red zone indicates the filter must be replaced; do not clean paper elements.

Exhaust manifold: a cracked manifold or leaking gasket causes a ticking noise and a soot trail at the leak point.

Fuel pressure: check with a gauge at the specified test port; compare against specification at idle and under load; low pressure indicates a weak pump, clogged filter, or restricted line.

Fuel volume test: measure the volume delivered over a set time; adequate pressure with low volume indicates a restricted filter or failing pump.

Injector balance test: compare the pressure drop or rpm change per cylinder; a weak injector shows a smaller drop than the others.

Diesel injector return flow: excessive return flow indicates worn injector internals; measure return volume against specification.

Air in the fuel system: hard starting and rough running after a filter change indicate air; bleed the system per the manufacturer's procedure.

Fuel contamination: water or debris in the fuel causes poor running and injector damage; drain and flush the system and replace filters.

Common rail pressure: low rail pressure at cranking prevents starting; check the pressure sensor, regulator, and pump.

Gasoline injection: a leaking injector causes rich running and black smoke; a clogged injector causes a lean misfire.

Spark test: a strong blue spark indicates adequate secondary voltage; a weak or orange spark indicates a worn coil, cap, rotor, or plug.

Spark plug gap and condition: a fouled plug indicates rich mixture or oil; a worn plug indicates normal wear; set gap to specification.

Ignition coil: measure primary and secondary resistance; an open or shorted winding causes a misfire on the affected cylinder.

Distributorless ignition: a misfire on two paired cylinders often indicates a coil or ignition module fault; swap coils to isolate.

Ignition timing: set to specification with the timing connector or per the manufacturer's procedure; incorrect timing causes poor power and detonation.

Crankshaft position sensor: a failed CKP sensor prevents spark and fuel; check for a signal with a scope or scan tool.

Spark plug wire resistance: excessive resistance or an open wire causes a misfire under load; check with an ohmmeter.

Detonation: caused by over-advanced timing, low octane fuel, or excessive heat; it damages bearings and pistons.

Battery: a fully charged 12V battery reads about 12.6V at rest; a reading below 12.4V indicates a partial discharge; test with a load tester.

Charging system: alternator output should be 13.5-14.5V at the battery with the engine running; low output indicates a failed alternator or regulator.

Starting system: a voltage drop test across cables and connections should be under 0.5V; excessive drop indicates a bad cable or connection.

ECM codes: retrieve with a scan tool; a code indicates a circuit or component fault, not necessarily a failed part; verify with a test.

Sensor testing: check reference voltage (typically 5V), signal voltage, and ground; a signal out of range indicates a faulty sensor or wiring.

Actuator testing: use a scan tool to command the actuator and observe the response; no response indicates a wiring or actuator fault.

Wiring: check for opens, shorts, and high resistance with a multimeter; wiggle-test connectors while monitoring the meter.

Grounds: a poor engine ground causes many electrical faults; check ground straps and clean connections.

Before removal: label and photograph connections, disconnect the battery, and relieve fuel pressure; drain coolant and oil.

Lifting: use a load leveler and attach the sling to the manufacturer's lifting points; never lift by the intake manifold or accessory brackets.

Torque sequence: tighten cylinder head and main cap bolts in the specified sequence and to the specified torque or angle.

Cleanliness: clean all parts, passages, and bolt holes before assembly; debris causes bearing and oil passage failures.

Plastigage: use to verify bearing clearance at assembly; compare against specification before final torque.

Cam timing: align timing marks before installing the belt or chain; verify with the manufacturer's procedure and rotate by hand.

Break-in: change oil and filter after the initial run-in; check for leaks and re-torque as specified.

Installation: reconnect all grounds and hoses, refill fluids, and check for leaks before starting; verify no codes after start.