A hunting idle combined with oil film all over the engine bay usually points at the crankcase ventilation system, not the ignition or fuel system. Perished hoses and seals let unmetered air into the intake and oil vapour into the engine bay, while a stuck PCV valve either destabilises the idle or pressurises the crankcase. The fix is often a set of rubber seals, not a new valve cover.
The crankcase ventilation system is one of the most commonly overlooked items when an engine is serviced. A fault here leads to several serious consequences for the engine as a whole. Here is what those consequences are and how to avoid them.
What does the crankcase ventilation system do?
The crankcase ventilation system removes combustion gases that leak past the piston rings, keeping crankcase pressure low and protecting the engine oil. In any internal combustion engine a small proportion of exhaust gas inevitably blows past the rings into the crankcase. Leave those gases in there and the raised crankcase pressure will push out oil seals, while the higher temperature and exhaust gas concentration degrade the engine oil. The system that removes them is sometimes also called the crankcase gas recirculation system.
What parts make up the system?
The design and application are specific to each engine model, but the general principles are the same across the board.
Routing crankcase gases straight into the intake causes real damage, because those gases carry a mist of oil. First, oil consumption rises, since some of that oil simply burns. Second, you get carbon deposits on the spark plugs and the problems that follow, up to and including ignition failure.
It is far better for the engine, and for the environment, to separate the suspended oil droplets from the gas and return the lubricant to the crankcase. That is the job of the oil separator, normally a labyrinth design. It condenses the oil mist and drains the oil back, which prevents high oil consumption and stops oil reaching the cylinders. The separator may be a discrete part or built into the valve cover, on engines where the crankcase gases pass through the cover as well.
The second key element is the crankcase gas recirculation valve, known in Western terminology as the PCV valve. It controls the pressure at which air leaves the crankcase and enters the intake. It is necessary because the intake vacuum that drives the whole system changes with engine speed and load, and can be either excessive or insufficient. It is often mounted inside the valve cover.
What are the symptoms of a faulty PCV system?
The main problems come down to valve failure and loss of sealing. Here are the faults you will actually meet.
Loss of sealing through perished hoses and seals. This lets unmetered air into the intake, which causes hard starting and a hunting idle. It also lets oil vapour escape into the engine bay, so the engine gets progressively filthier. Heat cycling in 40 °C summers and long highway runs hardens the rubber faster, so perished ventilation hoses are common on any high-kilometre engine here.
PCV valve malfunction. If the valve jams open, too much air enters the intake from the crankcase, which brings unstable idle and higher fuel consumption. If it jams shut, crankcase pressure rises excessively, which pushes out crankshaft seals or the valve cover gasket.
Blocked oil separator. This is typical of high-kilometre engines and of engines that have not had oil changes at the recommended interval. Thickened oil sludge blocks the separator drain, so some of the oil ends up in the intake. The result is high oil consumption, carbon in the combustion chamber and on the plugs, and unstable starting.
Safety note: let the engine cool before working on ventilation hoses, since oil and the surrounding components stay hot long after shutdown. Catch any oil that drains from a disconnected hose rather than letting it reach the exhaust, and clear engine fault codes and check the idle after reassembly before driving.
How is the system repaired?
Servicing the crankcase ventilation system usually comes down to replacing the ventilation valve or the damaged hoses and sealing rings. Unfortunately for owners, the system is often non-serviceable, particularly when it is built into the valve cover. Replacing a complete cover is expensive, which is galling when the only failed part is a rubber gasket.
FEBEST offers an alternative: a crankcase ventilation system repair kit. Kit 18402-004, for example, suits GM engines, meaning Opel engines in a range of capacities and models from Z18 to Z32, and the related Chevrolet F16. These engines were fitted across various models in the group, and locally many of them arrived in Holden-badged versions of the same cars, so always confirm the application against your engine code rather than the badge. Because the ventilation system in the valve cover is identical across those engines, a single kit covers a large number of applications. The kit contains all three rubber sealing rings that cause the ventilation system to leak on these engines. One kit, and the cause of the hunting idle and the oil-covered engine is gone.
A repair may also call for an oil filler cap gasket and a valve cover gasket, which are frequently damaged by high crankcase pressure when the ventilation system fails. Those items are in our catalogue too. The FEBEST gasket range is compact but versatile, with broad engine coverage across the wider rubber components catalogue, and it deals with the irritating weeps and oil misting that make an engine bay look neglected.
Frequently asked questions
Can a bad PCV valve cause a hunting idle?
Yes, and it is one of the most common causes. A valve stuck open lets extra unmetered air into the intake, which the engine management then has to correct, producing an unstable or surging idle and higher fuel consumption. A hunting idle combined with an oily engine bay is a strong indicator that the ventilation system, rather than the throttle body, is the problem.
What happens if the PCV valve jams shut?
Crankcase pressure builds with nowhere to escape, and it forces oil past the weakest seal in the engine. That usually means the valve cover gasket, the crankshaft seals or the dipstick tube. Repeated seal failure shortly after a seal replacement is a classic sign that the ventilation system was never checked.
How do I test the crankcase ventilation system myself?
Start with a visual check of the hoses and their connections for hardening, cracks and oil weeping, then look for oil misting around the valve cover. A blocked system often shows as pressure at the oil filler when the cap is loosened with the engine running. Anything beyond that is best confirmed with a scan tool and a smoke test.
Is it worth replacing the whole valve cover?
Not if only the rubber seals have failed, which is the usual case. A repair kit that replaces the sealing rings restores the system at a fraction of the cost of a complete cover. Replace the cover only when the plastic body itself is cracked, warped at the sealing face, or the integrated separator is damaged.
How often should the PCV system be checked?
Look at it at every major service, and always when investigating oil consumption, oil leaks or an unstable idle. Engines running long intervals between oil changes, or short trips that never fully warm the oil, sludge up the separator faster. Vehicles that tow or work in dusty conditions deserve a closer look.
Will a faulty crankcase ventilation system trigger a check engine light?
Often, yes. Unmetered air entering through a split hose skews the fuel trims, which sets lean running codes, and some engines monitor crankcase pressure directly. The absence of a warning light does not clear the system though, since seal and separator faults can develop well before any sensor threshold is crossed.
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