Practice question

A school bus diesel produces excessive black smoke under acceleration but idles cleanly. The MOST likely cause is:

  1. Coolant entering the combustion chamber
  2. Worn valve stem seals
  3. A failed cabin heater core
  4. Insufficient air for the fuel being delivered (over-fueling/low boost)

Answer: D — Insufficient air for the fuel being delivered (over-fueling/low boost)

The correct answer is shown above. Try the full quiz for additional practice.

Practice more: ASE S2 Diesel Engines (School Bus) Practice Questions - ASE S2 - Diesel Engines (School Bus) Certification Exam →