ASE T2 Diesel Engines (Medium/Heavy Truck) cheat sheet

Valve lash is measured between rocker and valve stem tip (or at pushrod) with the lifter on the base circle of the cam; engine at TDC compression for that cylinder.

Intake valve recession limit is typically about 0.060 in (1.5 mm) over the original stem height; exhaust recession limit is smaller, often ~0.030 in (0.8 mm).

Valve stem-to-guide clearance for a typical heavy-duty diesel intake is about 0.0015-0.0035 in; worn guides cause oil consumption and seat burning.

Cylinder head flatness limit is usually 0.002-0.006 in across the full length; resurfacing is limited by valve recession and injector protrusion.

Valve seat width on a diesel intake is typically 0.060-0.090 in and exhaust 0.070-0.110 in; too narrow seats overheat, too wide seats leak carbon.

Head bolt torque-to-yield fasteners are single-use; measure bolt length or diameter before reuse and discard if beyond spec.

A leaking head gasket between cylinders shows as adjacent low compression readings and coolant pressurization; combustion gas in coolant is confirmed with a gas detector.

Valve spring free length and installed height are checked against spec; weak springs cause misfire at high rpm and valve float.

Cylinder taper limit is typically 0.002-0.004 in and out-of-round limit 0.001-0.002 in; excessive taper requires boring/honing or sleeve.

Cylinder liner protrusion on wet-sleeve engines is usually 0.002-0.005 in above the deck; too little causes gasket leakage, too much causes head cracking.

Main bearing bore alignment is checked with a straightedge and feeler gauge or by line-boring; misalignment shows as uneven bearing wear.

Crankshaft journal out-of-round limit is about 0.001 in; taper limit about 0.001 in per inch of journal length.

Crankshaft end play is typically 0.004-0.012 in on heavy-duty diesels; excessive end play indicates worn thrust washers.

Cylinder block deck flatness limit is usually 0.002-0.004 in in any 6 in span and 0.008 in overall.

Camshaft bearing bore wear causes low oil pressure at idle and is diagnosed by measuring bore diameter against cam journal diameter.

A cracked block is confirmed by pressure testing the water jacket; dye penetrant and magnetic particle methods locate surface cracks.

Oil pressure spec at operating temperature is typically 30-60 psi at rated rpm and a minimum of ~10-15 psi at idle.

Oil pump relief valve stuck open causes low pressure at all speeds; a worn pump causes low pressure that drops further as oil warms.

Coolant thermostat opening temperature is marked on the thermostat; a stuck-open thermostat causes low operating temperature and poor heater output.

Pressure-test the cooling system at 1.5 times the radiator cap rating, not above, to locate external leaks.

Combustion gas in coolant is detected with a chemical block tester or by bubbles in the radiator with the cap off during cranking.

Silicone hoses and ELC coolant require periodic testing for pH and additive depletion; pH below ~7 indicates need for service.

Oil analysis flags coolant contamination by elevated sodium and potassium; fuel dilution shows as reduced viscosity and flash point.

A restricted oil filter bypass opens at about 15-20 psi differential; a clogged cooler causes high oil temperature and low pressure.

Maximum intake restriction at full load is typically 25 in H2O for a clean filter and 30 in H2O for a dirty filter indicator.

Maximum exhaust backpressure at rated speed is typically 3 in Hg (about 40 in H2O); higher values indicate a restricted muffler or DPF.

Boost pressure is measured at the intake manifold; low boost with normal turbo speed indicates a charge-air cooler or intake leak.

Charge-air cooler leaks are found with a pressure test at 30 psi and soap solution; a leaking cooler causes low power and black smoke.

Turbocharger shaft end play limit is about 0.002-0.004 in and radial play about 0.004-0.008 in; excessive play indicates bearing wear.

A stuck-open wastegate or VGT vane causes low boost; a stuck-closed wastegate causes overboost and possible engine damage.

DPF restriction is measured by differential pressure; high delta-P at low flow indicates ash loading or soot overload requiring regeneration.

Exhaust manifold leaks show as black soot tracks and a ticking noise; they reduce turbo efficiency and can melt nearby components.

Diesel injection pressure on common-rail systems can exceed 30,000 psi; never crack a high-pressure line to bleed air.

Low-pressure fuel supply to the injection pump is typically 5-15 psi; low supply causes hard starting, low power, and injector damage.

Fuel return flow test detects excessive injector return; a high-return injector causes rough running and hard starting.

Air in the fuel system is bled at the low-pressure side or with a hand primer; never loosen high-pressure lines on a running common-rail engine.

Injector balance test compares cylinder contribution; a weak injector shows as a cylinder with low contribution and high return flow.

Fuel contamination by water is detected by the water-in-fuel sensor and by draining the separator; water causes injector tip erosion.

Fuel pressure decay after shutdown indicates a leaking injector or check valve; rapid decay causes long crank times.

Injection timing advanced too far causes knock and high NOx; retarded timing causes white smoke, poor power, and high exhaust temperature.

A fully charged 12V battery rests at 12.6V; 12.4V is about 75% state of charge and 12.0V is about 25%.

Alternator charging voltage should be 13.8-14.4V at the battery with the engine running and loads off.

Cranking voltage should stay above 9.6V at the battery during a load test; lower indicates a weak battery or excessive cable drop.

Voltage drop on the positive or negative cable should be less than 0.5V at 200-300A cranking current.

Starter draw test: excessive current with slow cranking indicates a shorted starter or seized engine; low current with no crank indicates an open circuit.

Battery capacity is rated in CCA and reserve capacity; a load test at half the CCA rating for 15 seconds should hold above 9.6V.

Parasitic draw above 50 mA after modules sleep indicates a fault; measure with an ammeter in series after the sleep cycle.

Alternator ripple voltage above 0.5V AC indicates a failed diode; this causes dim lights and battery discharge.

A 5V reference sensor circuit reading 0V indicates a short to ground; reading 5V with the sensor disconnected indicates an open circuit.

J1939 CAN bus has 120-ohm terminating resistors at each end; measured resistance with power off should be about 60 ohms.

CAN high-to-low voltage difference of about 2V indicates a healthy bus; both at 2.5V indicates a shorted bus.

Sensor signal voltage outside the expected range sets a fault code; use a scan tool to view live data and freeze-frame data.

ECM power and ground must be verified at the ECM connector under load; a poor ground causes multiple unrelated codes.

Crank and cam sensor signals are checked with a scope; loss of cam sync causes long crank and a sync fault code.

Injector solenoid resistance is checked against spec; an open coil causes a cylinder cutout and a circuit code.

Aftermarket electrical loads must be connected to the battery, not the ECM ground, to avoid ground offset faults.

Compression-release (Jake) brakes open the exhaust valve near TDC; they require correct valve lash and oil pressure to operate.

Slave piston clearance is typically 0.015-0.025 in; incorrect clearance causes weak braking or valve contact.

Jake brake oil supply must be 15-30 psi at operating temperature; low pressure causes weak or no braking.

Brake disable at low engine rpm (typically below 800-1000 rpm) is normal to prevent engine stall.

Exhaust-brake butterfly valves must fully open when off; a stuck-closed valve causes high backpressure and power loss.

Retarder controls are interlocked with the throttle and clutch; a misadjusted switch causes the brake to stay on or not engage.

Wiring to the brake solenoids is checked for voltage at the solenoid with the brake switch on; no voltage indicates a switch or harness fault.

Compression brake power is limited by exhaust backpressure; a restricted exhaust reduces braking effort and can overheat valves.