Volkswagen Air Cooled 1949-1969 Repair Guide

Crankcase Emission Controls



To vent the crankcase vapors from the engine, 1949-69 Volkswagen vehicles utilize a simple road-draft tube. The road-draft tube is connected to the oil filler/generator stand and allows the engine crankcase vapors to vent out below the engine as the vehicle drives down the road.

This system is designed only to vent the vapors; the road-draft tube does nothing to prevent the release of these crankcase vapors into the atmosphere. Newer Volkswagen models still use a road-draft tube, but also have a hose attaching the upper end of the road-draft tube to the air cleaner. The incoming air charge passing through the air cleaner creates a vacuum in the road-draft tube so that the crankcase vapors are routed up and into the air cleaner. Once in the air cleaner, the crankcase vapors pass through the carburetor to be burned along with the incoming air/fuel mixture.

Most newer vehicles utilize a Positive Crankcase Ventilation (PCV) system to combat the release of crankcase vapors into the atmosphere. This system is described here to further general automotive understanding of emission systems.

Positive Crankcase Ventilation (PCV) System

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Fig. Fig. 1 Schematic of a typical crankcase ventilation system used on upright Volkswagen engines beginning in 1970

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Fig. Fig. 2 Schematic of a typical crankcase ventilation system used on suitcase Volkswagen engines beginning in 1970

The PCV system used on most newer vehicles is designed to prevent the crankcase vapors from entering the atmosphere by burning them along with the incoming air/fuel charge. The PCV system is usually composed of a PCV valve, vacuum hoses and a fresh air breather hose. The PCV system vents the crankcase vapors in the following manner:

Fresh air is introduced into the engine crankcase through a fresh air breather hose, which is connected to the air cleaner or a similar air filter element. This prevents dirt and other contaminants from entering the engine, which would accelerate engine wear. The fresh air enters the crankcase and sweeps the blow-by gases along with it. The gases pass through the cylinder head cover through the PCV valve, which is controlled by engine vacuum. Once through the PCV valve, the gases are routed through a vacuum hose to the carburetor. The gases enter the incoming air/fuel charge to be burned in the combustion chambers.

The PCV system only functions when engine vacuum is high (idle and high-way cruising). Carbureted vehicles with PCV systems utilize carburetors that are specially calibrated.

None of the 1949-69 Volkswagen vehicles are equipped with this system; this description is only for general information on crankshaft emission controls used on newer vehicles.