Blue or gray smoke and a pungent, bitter odor from the tailpipe signal the engine is burning oil. Possible causes are leaking valve seals, worn or seized piston rings, a plugged PCV valve, overfilling the crankcase, failing to change the oil regularly or using the wrong type of oil.

The most likely reason for blue smoke from the tailpipe is engine oil being burned up. The oil that's meant to be used to lubricate the car's powertrain components could be leaking through the pipes it travels through and into the engine or fuel tank, resulting in blue smoke leaving the exhaust.