Apart from preventing engine oil from leaking out of the joint between the rocker cover and the cylinder head, rocker cover gaskets are also critical components in the vacuum systems of many modern engines. Here is why-
It is interesting to note that modern vehicles do not only emit harmful emissions from their exhaust tailpipes, although exhaust emissions resulting from burnt fuel account for a large percentage of emissions. Nonetheless, harmful emissions also derive from fuel vapours escaping from the fuel tank/system, dust that results from brake pads wearing down, as well as from rubber and plastic residues that are generated as a result of normal tire wear.
Collectively, these sources of harmful emissions account for much of a modern vehicle’s total emissions, but apart from exhaust gas, the single biggest source of harmful emissions is a combination of volatile gases that are generated inside a modern engine’s crankcase.
During normal engine operation, the oil pump forces engine oil through narrow passages to lubricate all moving parts, such as the crankshaft, camshaft(s), timing chain(s), and other moving parts. However, some lubricating oil is also forced through openings in the connecting rods, so as the engine rotates, some lubricating oil is squirted against the underside of the pistons through these openings. Thus, the combination of pressurised oil colliding with the underside of fast-moving pistons, and oil being flung from the rotating crankshaft against the inside surfaces of the engine creates a cloud of small droplets of oil that greatly assists in the overall lubrication and cooling of the engine.
However, since the oil is hot when it is broken up into a fine mist, many of the constituents of the oil evaporate rather quickly, and while some of these volatile substances re-condense and is re-incorporated into the oil in the sump, the heat of the engine prevents some volatile gases from re-condensing. So during extended engine operation, many volatile gases remain in the engine, and unless some mechanism existed to evacuate these gases, the pressure inside the engine will eventually build up to the point where oil seals are either damaged or pushed out of their housings, resulting in extensive oil losses.
So, in practice, all modern engines are designed so that negative pressure (vacuum) "sucks" the volatile gases from the crankcase during engine operation. In all cases, the evacuated gases are fed into the intake tract where it mixes with the incoming intake air to be combusted with the regular air/fuel mixture. It should be noted though that while the crankcase is evacuated into the intake tract via a dedicated hose, a calibrated valve known as the (PCV) Positive Crankcase Ventilation Valve, which is located in the rocker cover, regulates the intensity of the vacuum that clears the volatile gases from the crankcase.
We need not delve into the complexities of modern engine vacuum systems here, but suffice to say that unless the PCV valve was calibrated to limit the vacuum in the crankcase, the airflow through the engine vacuum system would follow the path of least resistance, which would have the effect of starving the engine of intake air. Thus, the amount of intake vacuum that is diverted through the calibrated PCV valve is just strong enough to evacuate the crankcase, without affecting normal engine operation.
So what does this have to do with the rocker cover gasket? Simply this; along with the oil filler cap, which is also located in the rocker cover, the rocker cover gasket forms a critical part of the many seals and gaskets that a) prevent oil from leaking out of the engine, and b), prevent air under atmospheric pressure from entering the engine, which if it did, could prevent the crankcase ventilation system from working effectively.
As a practical matter, catastrophic rocker cover gasket leaks are not as common as they used to be. However, when such a leak does occur, it can cause serious driveability issues on many vehicles because engine management systems cannot compensate for large volumes of unmetered air that enter the engine through vacuum leak paths, i.e., leaking rocker cover gaskets and/or seals.
Moreover, if atmospheric air can enter an engine through a leaking rocker cover gasket, at least some oil can leak out onto the engine through the same leak path. While such an oil leak might not always be catastrophic, some of the volatile constituents of oil that escape through even small leaks evaporate rapidly when they encounter hot engine surfaces, which releases large volumes of hydrocarbon molecules into the atmosphere.
From an emissions perspective, then, rocker cover gaskets are more than seals that prevent oil from leaking out of the engine; they are in fact, critically important components in any modern engine’s emission control equipment.