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Diesel & Heavy Duty
Duramax, Powerstroke, Cummins — plus semi trucks.
21 skill areas
60 key concepts
$0 to learn
Diesel & Heavy Duty covers diesel-powered light-duty trucks (Duramax, Powerstroke, Cummins) and Class 7-8 heavy-duty operations. Premium-value certification. Everything below is free — no login, no paywall. Work through the skill areas, drill them in Study Mode, and when you're ready, prove it with the certification exam.
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Drill all 60 concepts in Study Mode. Mark each one “Got it” once you know it cold. When every concept is cleared, you're ready for the DSL exam.
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What you'll be able to do
- Diesel fuel systems: high-pressure common rail, HEUI, mechanical injection
- Turbocharger diagnosis and service (VGT, twin-turbo)
- DPF (diesel particulate filter) service and regeneration
- EGR system diagnosis and service
- DEF (diesel exhaust fluid) system diagnosis
- Fuel injector testing (electronic and mechanical)
- Boost leak testing methodology
- Class 7-8 introductions: air brake, PTO, heavy-duty electrical
Skill areas
Jump to any area — each one distills the concepts you need to master it.
Fundamentals
2 concepts- Diesels compress air to a temperature high enough to ignite fuel injected at the top of the compression stroke — no spark plugs needed.
- Diesel compression ratios range from about 16:1 to 22:1, needed to raise intake air temperature to auto-ignition temperature.
Fuel
3 concepts- ULSD is 15 ppm or lower. Required for all on-road diesel vehicles in the US since 2010 to protect emissions equipment (DPF, SCR).
- Red-dyed diesel is untaxed and only for off-road use. Using it in on-road vehicles is federal and state tax evasion. Fines are severe.
- Cetane number (typically 40-55 for on-road diesel) measures how readily the fuel ignites. Higher = smoother combustion, easier cold starts.
Injection Systems
4 concepts- Modern common-rail diesels use pressures from 20,000 to 40,000+ psi for fine atomization and precise multi-shot injection events.
- High-pressure fluid injection injuries look minor but require immediate emergency care. Never search for leaks with your hand — use cardboard.
- HEUI uses a high-pressure oil pump (HPOP) at up to 3000 psi to trigger injectors mechanically. Common on older Powerstroke engines.
- Electronic control allows pilot, main, and post injections in a single cycle — reducing NOx, particulates, and combustion noise.
Turbocharger
2 concepts- VGT vanes close at low RPM (small A/R for quick spool) and open at high RPM (large A/R). Vane sticking from soot is a common failure.
- Soot fouls VGT mechanisms. Stuck-closed vanes overboost. Stuck-open underboost. Regens or cleaning are the fix; severe cases require replacement.
Emissions
8 concepts- DPF traps soot (particulate matter). When full, active or passive regeneration burns it off — raising exhaust temp to convert soot to ash.
- Active regen fires when DPF loading exceeds a threshold. Fuel injected late (or in a 9th injector into the exhaust) raises DPF temp to burn soot to ash.
- Regen burns soot to ash, but ash accumulates and must be manually cleaned (via professional service) or the DPF replaced. Typical life: 100k-200k mi.
- SCR mixes DEF (32.5% urea, 67.5% deionized water) with hot exhaust over a catalyst, converting NOx to harmless N2 and H2O.
- DEF freezes below 12°F. Vehicles have heated tanks and lines. If DEF freezes solid, the SCR won't inject and the vehicle may derate power.
- EGR routes cooled exhaust back into intake, lowering peak combustion temp and reducing NOx formation. Trade-off: increased soot production.
- EGR cooler failures are common on many diesels. Coolant into intake = white steam smoke, coolant loss, and potentially hydro-lock (bent rods, cracked heads).
- Deleting emissions equipment on on-road diesels violates federal law. EPA has aggressively pursued shops and owners with major fines.
Powerstroke
2 concepts- 6.0L Powerstroke has known failure of EGR coolers and stacked oil cooler restrictions. Common upgrades include improved oil cooler, EGR cooler, head studs, and injectors.
- 6.7L Powerstroke has had radiator, EGR cooler, and (on 2011-2014) turbo bearing failures. Fuel system contamination is also common if the CP4 pump fails.
Duramax
2 concepts- LB7 injectors were placed under the valve covers — replacement requires significant labor. Common warranty extension and known issue.
- CP4 pumps can fail catastrophically, sending metal shards through the injectors. Repair cost: $10-15k+ including tank, lines, pump, injectors, rails.
Cummins
2 concepts- Original in-tank lift pumps couldn't reliably supply fuel to the frame-mounted VP44. Upgrade to FASS or AirDog aftermarket lift pump is common and extends injection pump life.
- Aborted regens leave soot in the DPF. Complete cycles when possible. Frequent short trips or idling worsen DPF loading.
Cold Start
2 concepts- Cold intake air can prevent auto-ignition. Grid heaters warm intake charge; glow plugs heat the combustion chamber; block heaters keep coolant warm.
- Ether ignites at lower temp than the glow plug/grid heater surface, causing pre-ignition and detonation. Never use ether on engines with intake heating systems.
Injector
3 concepts- Stuck injectors dump fuel into cylinders when they shouldn't. Combustion pushes fuel past rings into the crankcase, diluting oil and creating a fire risk.
- Balance/contribution tests compare each injector's power output. Weak or dead injectors show as outliers, guiding diagnosis without disassembly.
- Modern piezo and solenoid injectors have serial numbers with calibration data. Replacements must be coded to the ECM for correct fuel metering.
Water in Fuel
2 concepts- Diesel systems need lubricity — water rusts internals and destroys HPFP pumps and injectors. Drain the fuel-water separator immediately when the light comes on.
- Regular draining prevents water accumulation. Some vehicles have manual drain valves; others have automated systems that alert.
Filters
2 concepts- Diesel filters have very tight microns (2-4 micron on modern common-rail). Clogged filters starve HPFP and injectors, causing severe damage.
- Restricted filters reduce air, cause smoke, and increase fuel dilution. Replace at the indicator threshold, not on a fixed interval.
Oil
2 concepts- CK-4 is for older and current engines. FA-4 is for newer, fuel-economy-optimized engines. Wrong oil can plug DPFs (from wrong ash characteristics) or wear engines.
- Late-cycle regen fuel can wash past rings into the crankcase. Extended short-trip use amplifies dilution. Monitor oil level — rising oil indicates fuel dilution.
Air System
4 concepts- Boost leaks let air escape before the engine, reducing charge and creating rich conditions. Pressure test intake at operating boost with smoke or pressure to locate.
- Systematic pressure testing isolates the leak location. Intercooler-only tests confirm whether the core itself is cracked.
- Closed EGR = no recirculation = higher peak combustion temp = more NOx. PCM sees insufficient flow and codes it.
- EGR soot builds over miles. Cleaning restores function; severely fouled valves are replaced. Modern engines often diagnose and set specific codes.
Diagnostics
6 concepts- White steam-like smoke smells sweet from ethylene glycol. Pressure test cooling system and pull EGR cooler for inspection. On some engines, cracked heads or gaskets are common.
- Black smoke is unburned soot. Excess fuel or insufficient air causes it. Diagnostic path: air first (filter, boost, VGT), then fuel.
- Blue smoke = oil in combustion. Sources: piston rings, valve seals, or turbo seals letting oil into intake/exhaust.
- Diesel runaway: engine runs on fuel it can't control. Shut off by blocking the intake with a rag or covering the air intake. Damage is often severe if not stopped fast.
- Diesel no-start diagnostic: verify fuel pressure at rail, verify injector command, check compression, check for crank/cam sync loss.
- HPFP failure progresses from performance loss to catastrophic failure. Catch it early via rail pressure logs; late failure contaminates the entire system.
Heavy Duty
5 concepts- CVSA inspection criteria have specific pushrod stroke, drum lining, and adjustment requirements. Failing brakes puts a truck out of service on the spot.
- Air brakes use compressed air to actuate S-cam or disc foundation brakes. Spring brakes provide fail-safe parking — spring applied, air released.
- Compressors create moisture that collects in the wet tank. Draining daily (or via automatic drain valves) prevents freeze-related brake failure.
- CDL classes are based on weight rating and vehicle type. Mechanics don't always need one, but road-testing may require it depending on GVWR.
- Jake brake dumps compression through the exhaust, using the engine as an air compressor. Very effective braking; loud (banned in some jurisdictions).
Injection
2 concepts- High-pressure rail sensors are inside the fuel path. A leak sprays diesel at high pressure and causes rail pressure faults. Replace at first sign of leak.
- Momentary run after start means fuel is in the rail but not being replaced. Check lift pump operation, tank pickup, and injector return check valves.
Cold Weather
2 concepts- Diesel wax content precipitates at cold temps. Winter blends and anti-gel additives lower CFPP for cold-climate operation.
- Fuel stations blend #1 diesel (kerosene) with #2 to lower gel point. Blending ratios depend on ambient temperature.
Regulation
2 concepts- CARB Truck & Bus Rule and related programs mandate emission compliance for CA-registered and CA-operated diesel vehicles by age. Non-compliance blocks registration.
- California requires smog inspection for most diesels. OBD readiness, visual emission equipment inspection, and opacity may all be checked.
Safety
2 concepts- Common rails hold high pressure for time after shutdown. Follow manufacturer depressurization procedures before disconnecting lines to avoid injection injuries.
- Diesel injection components have micron-level tolerances. Clean-room conditions and lint-free wipes are essential. Contamination causes premature failure.
Documentation
1 concept- Consumer protection rules require detailed invoicing. Fleet/DOT records must be kept longer for driver/vehicle qualification files and roadside inspection defense.
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