Torque Shop

Recently, my car lost power while accelerating. It also emitted white and blue smoke. My mechanic told me that I had to replace the "PCV valve". What does this part do and is the problem serious?

A small amount of combusted gases will inevitably escape past the piston rings into the crankcase where the engine oil is "stored".

If allowed to build up, the gases will create a high pressure in the crankcase and cause deterioration of the content of the oil sump in the crankcase. Eventually, sludge will form and the engine oil will lose its lubricating properties.

In the early days, the blow-by gases were vented out of the crankcase and into the atmosphere. Eliminating this air pollutant was one of the first emissions-control processes. Theses gases are now channelled back into the combustion chamber via the intake manifold while the crankcase is provided with fresh-air ventilation via the air-filter.

All cars have what is called the positive crankcase ventilation (PCV) valve. This simple but effective component lies in the plumbing between the crankcase and intake manifold, and facilitates the recycling of the blow-by. It is designed to open at a specific pressure differential between the intake manifold and crankcase. This is so that sufficient ventilation is provided while preventing a condition of vacuum in the crankcase that would cause excessive oil to be drawn by the intake manifold.

A PCV valve that is stuck "open" will lead to oil build-up in the intake manifold, leading to a plume of bluish white smoke when accelerating.

On the other hand, a PCV valve that is jammed "shut" will cause pressure build-up to a point where air-oil fumes from the crankcase will be diverted to the air filter. This also results in bluish white smoke when accelerating.

The condition is not serious, nor is it expensive to fix. But it should be fixed. Prolonged running of the car with a faulty PCV valve will lead to formation of sludge in the engine oil or contamination of the air filter, intake manifold as well as other air and vacuum lines.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on April 14, 2018, with the headline 'Torque Shop'. Print Edition | Subscribe