Air Induction and Exhaust Systems Diagnosis and Repair
What the candidate must understand
Air induction and exhaust control how much air enters and how cleanly it leaves. Know the path: air cleaner, intake piping, turbocharger (compressor, turbine, wastegate or VGT), charge-air cooler, intake manifold, then exhaust manifold, turbo, DOC/DPF, SCR, and tailpipe. Understand turbo basics: boost is driven by exhaust energy, a stuck VGT or wastegate causes overboost or underboost, and oil seals can leak into intake or exhaust. Know that a restricted air filter or collapsed intake hose causes low power and black smoke, while a boost leak causes low power, whistling, and sometimes over-fueling codes.
For exhaust, understand backpressure and its effect on power and EGT, DPF restriction symptoms, and exhaust leaks (soot streaks, noise). Know that a cracked exhaust manifold or leaking gasket can cause a whistle or low boost. Understand charge-air cooler leaks: they cause low boost, high intake temperature, and sometimes white smoke.
Traps
Questions often mix boost leak (hiss, low power, high intake temp) with exhaust restriction (high backpressure, derate). Another trap is blaming the turbo for low power when the air filter or CAC is restricted. Watch for answers that ignore checking intake piping and clamps before condemning the turbo.
Worked example / rule of thumb
Rule of thumb: measure pressure before and after the restriction. A vacuum gauge at the intake or a backpressure gauge at the DPF tells you whether the restriction is upstream or downstream. If intake vacuum is high, the filter or piping is the problem; if exhaust backpressure is high, the DPF or exhaust is restricted.
Key points
- Turbo boost comes from exhaust energy; VGT/wastegate faults cause over/underboost.
- Boost leaks cause hiss, low power, and high intake temperature.
- Restricted intake causes low power and black smoke.
- High exhaust backpressure indicates DPF or exhaust restriction.
- Check piping, clamps, and filters before condemning the turbo.