Which exhaust smoke colour would indicate a leaking dosing valve?

• A. Blue
• B. Black
• C. White
• D. Brown

Answer: (B)

Exhaust smoke color is a quick clue about the fuel-air balance and what’s happening inside the engine. Black smoke means there’s more fuel than air can burn, producing soot. If a dosing valve leaks, it can introduce extra fuel into the system in an uncontrolled way, making the exhaust richer than it should be. That excess fuel doesn’t burn completely and leaves behind visible soot, which shows up as black smoke from the exhaust.

Blue smoke points to burning oil, white smoke to coolant or cold-start conditions, and brown smoke isn’t a typical telltale for a dosing valve leak, so the best match for a leaking dosing valve is black smoke.