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Why Do Some Exhausts Turn Blue? (Explained)

Updated: Jan 17

If you own a motorcycle, you may have observed that the exhaust may become blue over time; this also occurs in some cars.

Heat from the exhaust gas can cause the exhaust pipe to become blue. The exhaust pipes are designed to withstand heat, but as temperatures increase, the metal of the exhaust starts to interact with the surrounding air and becomes blue, purple, or yellow.

The colour of the exhaust may increase as the exhaust system gets hotter, especially if the exhaust pops and bangs or backfires.

two exhaust pipes with blue, purple and yellow colours on them

Why Does an Exhaust Turn Blue?

Exhaust pipes becoming blue is a common occurrence. They go a blue colour due to the intense heat that travels through them.

  • Driving hard. When you increase the throttle, the engine burns more air and fuel, which means more heat is emitted which must all pass through the exhaust system, causing the exhaust to grow hotter.

  • Lean air/fuel ratio. Another possible reason of blue exhaust pipes is an engine with a lean air/fuel ratio. A lean air/fuel mixture with too little fuel will burn much hotter than a rich air/fuel ratio.