Thread: Positive crankcase ventilation

The crankcase will become pressurised due to a few reasons, blow by one you don't want. The general actions inside the case of crank motion will agitate the oil and stir the air up into quite a frenzy. Old engines will just vent these to atmosphere, sometimes or in a racing application, via a catchcan which collects the oil and moisture vapour.

The other option is the PCV Valve system. The valve is simple, 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 by manifold vacuum, 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. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, vacuum is much reduced, 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.

Yep.

Most EFI engines of European heritage, and some others, run a -ve atmosphere in the crankcase, for the reasons you stated.

Removing the oil cap is seen as the same as a vac hose being disconnected, and since it is esentially a BIG vac hose, it sees a BIG vac leak and raises the idle accordingly, then the ECU recognises the throttle as still being at idle position, so slows it to a stall (nearly), and so it goes.

If its a MAF EFI system, then any air entering the system after the MAF is seen as unmetered air, and havoc is the usual result.

If you are talking about your RRC, it doesn't have a PCV system of breathers. A PCV system directs all of the fumes in to the manifold after the throttle plate, and has a variable orifice depending on engine vacuum. "Positive ventilation" means more fresh air is drawn into the engine than fumes drawn out, keeping the engine inside cleaner of fumes. The RRC has a simple tee with a small orifice to the vacuum and a big hose straight to the inlet side of the throttle for the main fumes loading.

Also I recall that you have an Impco gas system, and they generate considerable inlet restriction. This means that if you open a better path to the throttle inlet (eg opening the oil filler), air will pass through that instead and less will travel through the mixer head. The net result is the mixture going very lean and the engine running like crap. It is one of the reasons I don't have any liking for Impco systems. Your car is behaving as expected.