Why do Engines Need PCV Valves?

Although some newer engines do not have PCV (Positive Crankcase Ventilation) valves, all engines need a mechanism to relieve the pressure that results from combustion gases leaking into the crankcase past the piston rings, as well as the pressure in the crankcase caused by the evaporation of some volatile ingredients of the engine oil.

As a practical matter, the mechanism that evacuates gases from the engine is known as the [Positive] Crankcase Ventilation system, and on most engines, this system consists of a PCV valve that usually (but not always) connects to the valve cover, and dedicated rubber hose that connects the PCV valve to the intake system. The “driving force” behind the crankcase ventilation system is the vacuum caused by the engine “sucking” atmospheric air into the intake manifold when the engine is running.

The above describes the basic operating principles of PCV valves, but in practice, both PCV valves, and crankcase ventilation systems that do not use PCV valves fulfil two functions, so let us look at these functions in turn, starting with-

Emission control

All internal combustion engines produce harmful exhaust emissions as a result of combusting fuel, and while the bulk of exhaust emissions largely consists of a toxic “cocktail” of partially combusted fuel, carbon monoxide, carbon dioxide, and various sulphur-based compounds, a significant percentage of an internal engine’s emissions consist of volatile oil-based compounds.

Before 1961, all of the volatile gases that were created when some ingredients in engine oil evaporated were simply vented to the atmosphere through devices known as “draught tubes”, which was the primary cause of urban smog that caused respiratory problems for millions of people in large urban centres the world over.  

When PCV valves were introduced on light vehicles in 1961 and became mandatory in about 1967 in the American automotive market, the harmful volatile oil-based products were evacuated from the engine and conveyed to the engine to be combusted along with the air/fuel mixture.

Thus, PCV valves can be seen as the first-ever emissions control device to be fitted to light vehicles, and while their basic design and principles of operation have not changed much, if at all during the past several decades, PCV valves remain an integral, and crucially important part of the overall emission control systems on modern vehicles.

Engine damage control

The term “engine damage control” requires some explanation, mostly because various forms of engine damage stem directly from the harmful compounds and substances that form when engine oil degrades- even during the useful life of an oil fill.

We need not delve into the chemistry of degraded oil here, but suffice it to say that even the most advanced engine oil formulations form sulphuric acid when some ingredients of the oil evaporate, and then combine with moisture that occurs naturally in all engines. While there are currently no effective measures to prevent atmospheric moisture from entering internal combustion engines, continuously removing volatile gases and combustion products from an engine via the crankcase ventilation system/ PCV valve, also removes much of the moisture that is present in an engine.

In practice, high concentrations of sulphuric acid in an engine not only promote corrosion of internal engine surfaces, but the acid also actively attacks and damages oil seals and gaskets. So, by removing most of the moisture and the oil waste products that combine to form sulphuric acid from an engine, it becomes a lot easier for the corrosion-inhibiting additives in engine oil to combat/prevent corrosion that results from the presence of moisture alone, which brings us to-

The other effects of degrading oil

While the presence of sulphuric acid in any engine is exceedingly harmful, a failure of the PCV valve or crankcase ventilation system to remove all combustion and oil waste products from an engine could potentially cause serious engine damage, if not always-catastrophic engine failures even if no significant corrosion is present in the engine. Here is why-

As stated elsewhere, some ingredients in engine oil evaporate from the oil during engine operation but provided the engine oil is replaced at recommended or specified intervals, this process does not affect the lubricity of the oil in meaningful ways. However, when oil is not replaced for extended periods, some of the evaporated ingredients begin to condense and accumulate on internal engine surfaces in the form of sludge.

Because the detergents and cleaning agents in severely degraded engine oil are no longer able to dissolve accumulations of sludge, the build-up of sludge may begin to clog up or block oil passages, as well as prevent the oil pressure relief valve from operating as designed. In practice, this means that-

  • some moving parts of the engine may become starved of lubrication, and/or
  • that the oil may begin to bypass the oil filter as it circulates through the engine, meaning that some sludge will circulate through the engine along with the degraded engine oil, which if it happens, can destroy an engine in very short order  

While PCV valves and crankcase ventilation systems that work as intended cannot prevent the effects of seriously degraded engine oil, they can and do remove the combustion and oil waste products that form the basis of oil sludge during the useful life of an oil fill. However, if the PCV valve or crankcase ventilation system does not function optimally, combustion and oil waste products are not removed from the engine effectively and as a result, sludge can begin to form very quickly after an oil change.

So from an engine durability perspective, PCV valves and crankcase ventilation systems (that work as intended) are critically important components that actively protect engines against the effects of degrading oil by removing waste products from the engine.

Ready to get started?
Get an instant estimate, or find an expert mechanic to make a booking.