Hi Guys,
after spending some time figuring out why my mini reeked of oil, I decided to write this up to shed some light on this mysterious system.
What is PCV?
It is a system for venting crankcase pressure. If not vented properly, the crankcase becomes pressurized due to blow-by past the piston rings during combustion. Piston rings never seal 100% even when new, and the problem only gets worse with age. The word positive is used to mean active rather than passive; in fact, it's purpose is to keep crankcase pressure slightly negative (equal to intake vacuum). Historically, crankcases have been passively vented.
Why do I need it?
It is the difference in pressure between the cylinder and crankcase which causes the piston to move and do work. If the crankcase is poorly vented, the pistons cannot move as freely and you are wasting engine power to overcome crankcase pressure. Proper crankcase ventilation:
- Improves idling and revability
- Increases engine power
- Reduces incidence of oil leaks as oil isn't being forced out of seals under pressure
- Reduces oil sludge-ification by removing hydrocarbons from the crankcase
- Reduces hydrocarbon emissions by burning the crankcase vapours
How does it work?
The PCV system is an open system. It is not recirculating gases, but maintaining a constant stream of fresh air through the crankcase. In practice, this is done by using intake vacuum suction.
Yes, this implies that outside air is entering your crankcase from a source other than your air filter! It either comes from your charcoal canister, or a vent filter.
The reason the PCV system is so confusing to understand is because it is mixed up with the emissions control system (charcoal canister) and carburetion/intake system. The only reason it's connected up that way is because it's convenient.
What is a PCV valve?
The PCV valve's job is to make sure that air can flow out of the crankcase, but not back in. Not all mini configurations use one because
a) they have a flame trap to catch backfires
b) intake pressure on that car is always lower than crankcase pressure.
It's secondary function is to control the rate of air flow, by restricting the orifice and/or opening a breather orifice.
Wikipedia Article
OK, how is it supposed to be connected up?
Based on the number of different configurations I've seen on this site and 2 different workshop manuals, I'd say there are only a bazillion different ways. I'd say just try to understand how your mini is setup.
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1978 Leyland mini
Charcoal canister: 3 inputs at the top, and one vent at the base. Big centre input at the top connects to rocker cover input. This is where fresh air is fed to the crankcase. The charcoal is effectively the filter. The two small inputs on the sides are 1. Fuel tank vapours and 2. Vent (Is this supposed to go somewhere??)
There is a flame trap just next to the radiator-side engine steady bar. It is a cylindrical thing mounted to an engine cover plate. It contains a gauze to trap oil leaving the crankcase, and prevent a backfire from the intake getting into it. The outlet at the top should be connected to a vacuum port on the carby.
That's it! I can't believe the complicated plumbing I pulled out which the PO rigged up to fumigate the cabin!
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Feel free to contribute your setup! Please be as descriptive as possible because it is confusing as hell! Also, let me know if there are any mistakes/omissions.
hg.
Good on ya headgasket 
