Positive Crankcase Ventilation (PCV)

Positive Crankcase Ventilation (PCV) was fitted on relatively few single carburetter Minis between 1964 and 1969.

In fact, probably only those destined for emissions-conscious export markets.

By contrast, virtually all of the 1964-69 twin carburetter Cooper and Cooper S models were PCV fitted.

The PCV system is easily recognised by the mushroom shaped PCV valve positioned abo ve and attached to the inlet manifold. Instead of venting to atmosphere as in open crankcase ventilation, the fumes passed from the tappet chest cover through an oil separator (trap) and then via the PCV valve into the inlet manifold. The rocker cover to air filter hose used in open crankcase breathing was not fitted on PCV models.

Essential Maintenance of Breathing Systems

It is of vital importance to ensure that the breathing system is kept in good working order. A blockage in the system or a PCV valve (if fitted) that is jammed closed will at best result in over-pressurisation of the engine crankcase, leading to failed engine oil seals or gaskets: if you have persistant problems keeping your engine oil-tight, this could be the reason why. At the opposite end of the scale, leaks in a closed circuit breather system (PCV models, 1964-69, plus all models from 1969 onwards) may effectively amount to being air leaks in the induction system, resulting in poor running or even lack of throttle control - if left unchecked too long, the exhaust valves in the engine may be damaged.

The best time to check the breathing system is when you carry out a major service on the car - either every 12000 miles or annually, which ever comes sooner. Check that the hoses are not perished, split or blocked and that the hose clips are in good working order and the correct size to clamp the hoses securely. Hoses and pipes can become blocked with oily sludge or even hard carbon deposits; in extreme cases they cannot be cleared and new ones will have to be purchased (from Somerford Mini, naturally!).

The same can be said for the oil separator cannister on closed circuit breathing models; remove from the engine and wash through with petrol to clean the gauze inside if an oily sludge blockage is suspected. Renew the separator (unfortunately some earlier versions are no longer available) if it is completely blocked with hard carbon. Don’t forget when refitting to use a new gasket or seal, as appropriate.

The engine oil filler cap on closed circuit cars is also a vital part of the breathing system and similarly contains a gauze filter. The cap assembly (part no. 13H2296) should be renewed every 12000 miles.

Finally, on PCV fitted engines, unclip the cover of the PCV valve and inspect the inter nal components. The diaphragm should be free from splits or other damage, the plunger should move freely on its spigot and the spring should resist light finger touch and not be broken. New PCV valve assemblies and repair parts are available from Somerford Mini.


As an engine runs, gases from the cylinders leak past the piston\'s sealing rings into the crankcase (containing the crankshaft and other parts). This leaked gas is sometimes referred to as "blow by" because the pressure within the cylinders "blows" them "by" the piston rings. These gases include compounds harmful to an engine, particularly hydrocarbons (unburned fuel), as well as carbon dioxide and water vapor. If allowed to remain in the crankcase, or become too concentrated, the harmful compounds will condense out of the air within the crankcase and form corrosive acids and sludge on the engine's interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. To keep the crankcase air as clean as possible, some sort of ventilation system must be present.

PCV System

The PCV valve is only one part of the PCV system, which is essentially a variable and calibrated air leak, whereby the engine returns its crankcase combustion gases. Instead of the gases being vented to the atmosphere, gases are fed back into the intake manifold, to re-enter the combustion chamber as part of a fresh charge of air and fuel. The PCV system is not a classical "vacuum leak." Remember that all the air collected by the air cleaner (and metered by the mass air flow sensor, on a fuel injected engine) goes through the intake manifold anyway. The PCV system just diverts a small percentage of this air via the breather to the crankcase before allowing it to be drawn back in to the intake tract again. It is an "open system" in that fresh exterior air is continuously used to flush contaminants from the crankcase and into the combustion chamber.

The system relies on the fact that, while the engine is running, the intake manifold's air pressure is always less than crankcase air pressure. The lower pressure of the intake manifold draws air towards it, pulling air from the breather through the crankcase (where it dilutes and mixes with combustion gases), through the PCV valve, and into the intake manifold.

The PCV system consists of the breather tube and the PCV valve. The breather tube connects the crankcase to a clean source of fresh air, such as the air cleaner body. Usually, clean air from the air cleaner flows in to this tube and in to the engine after passing through a screen, baffle, or other simple system to arrest a flame front, to prevent a potentially explosive atmosphere within the engine crank case from being ignited from a back-fire in to the intake manifold. The baffle, filter, or screen also traps oil mist, and keeps it inside the engine.

Once inside the engine, the air circulates around the interior of the engine, picking up and clearing away combustion byproduct gases, including a large amount of water vapor, then exits through a simple baffle, screen or mesh to trap oil droplets before being drawn out through the PCV valve, and into the intake manifold.

PCV Valve

The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. Sensing a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway.


Should the intake manifold's pressure be higher than that of the crankcase (which can happen in a turbo charged engine or under certain conditions, such as an intake backfire), the PCV valve closes to prevent reversal of the exhausted air back into the crankcase again. This is where the positive comes from in the name. Positive is basically a synonym for one-way.

It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will be insufficient. A plugged or malfunctioning PCV system will eventually damage an engine. PCV problems are primarily due to neglect or poor maintenance, typically engine oil change intervals that are inadequate for the engine's driving conditions. A poorly-maintained engine's PCV system will eventually become contaminated with sludge, causing serious problems. If the engine's lubricating oil is changed with adequate frequency, the PCV system will remain clear practically for the life of the engine. However, since the valve is operating continuously as one operates the vehicle, it will fail over time. Typical maintenance schedules for gasoline engines include PCV valve replacement whenever spark plugs are replaced. The long life of the valve despite the harsh operating environment is due to the trace amount of oil droplets suspended in the air that flows through the valve that keep it lubricated.

Not all gasoline engines have PCV valves. Engines not subject to emission controls, such as certain off-road engines, retain road draft tubes. Dragsters use a scavenger system and venturi tube in the exhaust to draw out combustion gases and maintain a small amount of vacuum in the crankcase to prevent oil leaks on to the race track. Small gasoline two cycle engines use the crank case to compress incoming air. All blow by in these engines is burned in the regular flow of air and fuel through the engine. Many small four-cycle engines such as lawn mower engines and small gasoline generators, simply use a draft tube connected to the intake, between the air filter and carburetor, to route all blow by back into the intake mixture. The higher operating temperature of these small engines has a side effect of preventing large amounts of water vapor and light hydrocarbons from condensing in the engine oil.

Reference: Your Auto Network