Audi Cars 1999-05

Positive Crankcase Ventilation (PCV) System


Description & Operation

The system consists of a tube from the air filter housing to the camshaft cover and a second tube from the camshaft cover to the intake manifold. The 2.0L 16v (9A) engines use this same system, except the ventilation hose attaches to a plastic baffle on the cylinder block, instead of the cylinder head cover.

Under normal operating conditions, clean airflows from the air filter into the camshaft cover where it mixes with crankcase oil vapors. These vapors are drawn through the PCV valve and into the intake manifold to be burned with the air/fuel mixture. The flow to the intake manifold is metered by the PCV valve. When manifold vacuum is high, the valve is pulled closed and flow is restricted to maintain a smooth idle. If crankcase pressure is very high, vapors can flow directly into the air filter housing.

The small hose connecting to the intake manifold has a built-in restrictor orifice, which essentially creates a controlled vacuum leak to the intake manifold. When the throttle opening is small and manifold vacuum is high, crankcase oil fumes are drawn directly into the intake manifold. When the throttle opening is large and manifold vacuum low, some of the oil fumes flow through the large hose to the airbox.

A plugged PCV system will cause oil leaks or a build up of sludge in the engine. An air filter coated with engine oil indicates excessive crankcase pressure. A leaking valve or hose might cause rough or high idle, engine stalling and/or Engine Control Module (ECM) trouble codes.

On models equipped with a diaphragm control valve, the control valve takes the place of the previously described system. The diaphragm control valve is mounted to the cylinder head cover, and controls the ventilation of the crankcase and cylinder head into the intake tract. The diaphragm control valve maintains a constant balance against manifold vacuum, keeping blow-by vapor flow at a constant percentage of the total intake air volume.

The 2.8L V6 and VR6 engines use a simple crankcase ventilation system, similar to the 4-cylinder engines. A single hose connects the cylinder head cover to the intake tract, ahead of the throttle body. A heater element is contained within the hose, which prevents the vapors from icing over in cold weather. As with the 4-cylinder engines, the cylinder head cover contains a metal mesh flame trap that contains any ignited vapors during the event of an engine backfire.

On all systems, there is no maintenance required other than to check for vacuum leaks and clogged hoses. When removing hoses for inspection, take note of the direction of the restriction valves. Replace any clogged or cracked hoses.

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Fig. Diaphragm-type crankcase ventilation valve as used on the 2.0L (ABA) engines

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Fig. A typical PCV system flow diagram

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Fig. Cutaway view of a typical inline PCV valve, showing the flow of mixed air into the intake manifold

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Fig. The PCV valve mounts into the intake air bellows on 1.8L engines

Diagnosis & Testing

  1. Visually inspect the PCV valve hose, the fresh air supply hose and their attaching nipples or grommets for splits, cuts, damage, clogging, or restrictions. Repair or replace, as necessary.
  3. If the hoses pass inspection, start the engine and allow it to warm until normal operating temperature is reached.
  5. Remove the PCV valve from the rocker arm cover, but leave it connected to the hose. With the engine at idle, feel the end of the valve for manifold vacuum. If there is no vacuum, check for a plugged or leaking hose, PCV valve or manifold port. Replace a plugged or damaged hose.
  7. On models equipped with an internal check valve, stop the engine and remove the PCV valve. It should rattle when shaken. If the valve does not rattle or it is plugged, replace the valve.

Removal & Installation

  1. Loosen the clamp(s) securing the valve.
  3. Carefully remove the valve taking care to not damage the rubber fitting.
  5. Installation is in reverse order of removal.