Air Induction and Exhaust System Diagnosis and Repair
What the candidate must know
This domain covers the air cleaner, intake piping, turbocharger, charge-air cooler, intake and exhaust manifolds, EGR, and the exhaust system including the aftertreatment components on modern trucks. Know how to diagnose intake restrictions (air filter minder, vacuum gauge), boost leaks (low power, black smoke, hissing), turbo faults (shaft play, oil seals, wastegate or VGT actuator), exhaust restrictions (backpressure), and exhaust leaks (noise, soot tracking).
Traps in questions
- Low power with black smoke often means a boost leak or restricted intake, not a fuel problem. Check charge-air piping and the intercooler before the injection system.
- Turbo oil seals: blue smoke and oil in the intake or exhaust can be turbo seals, but can also be rings or guides. Confirm with shaft play and oil tracking at the turbo.
- Exhaust backpressure restriction causes low power and can be measured at the test port; a clogged aftertreatment or collapsed pipe is a common cause. Do not blame the turbo first.
- EGR faults can cause rough idle, surge, and black smoke; a stuck-open EGR leans the mixture at idle and can cause a rough or stalling condition.
- Air filter minder that stays in the red after reset indicates a real restriction; a minder that resets is not proof the filter is clean - inspect it.
- Charge-air cooler leaks can be internal (coolant) or external (boost); pressure-test the cooler and piping.
Worked example
A truck loses power under load and puffs black smoke. Boost gauge shows low boost. A charge-air pipe has a split boot. The black smoke is unburned fuel from low air, not a bad injector. Repair the boot and recheck boost before touching fuel.
Rule of thumb
Black smoke under load = too much fuel or too little air. Check air first: filter, piping, cooler, turbo, then exhaust backpressure.