Toyota Celica, Corolla, ECHO & MR2 1999-05

Positive Crankcase Ventilation (PCV) System


The Positive Crankcase Ventilation (PCV) valve regulates the release of crankcase vapors during various engine operating conditions. As the engine operates, some combustion gas will escape from the cylinder by passing the piston rings. These gasses accumulate in the oil pan. Since a small amount of vapor is added on every piston stroke, the pressure within the oil pan quickly builds. If these vapors are not allowed to escape through a planned path they will quickly find their own exit, usually by forcing a hole in an engine gasket.

Since the gasses contain hydrocarbons and other pollutants, they cannot simply be vented to the atmosphere. The PCV valve allows the release of the vapors under controlled conditions back into the intake air stream. The vapors are then mixed with the incoming air, reintroduced to the combustion chamber and reburned. At high vacuum (idle speed and partial load range) the PCV will open slightly and at low vacuum (full throttle) it will open fully. This causes vapor to be removed from the crankcase by the engine vacuum and then sucked into the combustion chamber where it is dissipated.

Description & Operation

A Positive Crankcase Ventilation (PCV) system is used on all Toyota gasoline engine vehicles sold in the United States. Exhaust blow-by gasses are routed from the crankcase to the intake manifold, where are combined with the fuel/air mixture and burned during combustion. this reduces the amount of hydrocarbons emitted by the exhaust.

A valve (PCV) is used in the line to prevent the gases in the crankcase from being ignited in case of a backfire. The amount of blow-by gasses entering the mixture is also regulated by the PCV valve, which is spring loaded and has a variable orifice.

The important components of the PCV system are the following:

PCV valve
Valve cover
Charcoal canister
Hoses, connections and gaskets
Thermal Vacuum Valve (TVV)
Check valve

Diagnosis & Testing

Inspect the PCV system hoses and connections at each tune-up and replace any deteriorated hoses. Check the PCV valve at every tune-up and replace it at 30,000 mile (48,000 km) intervals.

The PCV valve is easily checked with the engine running at normal idle speed (warmed up).

  1. Remove the PCV valve from the valve cover or intake manifold, but leave it connected to its hose.
  3. Start the engine.

Click image to see an enlarged view

Fig. Blow air from the cylinder head side of the PCV valve, it should flow easily

  1. Place your thumb over the end of the valve to check for vacuum. If there is no vacuum, check for plugged hoses or ports. If these are open, the valve is faulty.
  3. With the engine OFF , remove the valve completely. Shake it end-to-end, listening for the rattle of the needle inside the valve. If no rattle is heard, the needle is jammed (probably due to oil sludge) and the valve should be replaced.

Click image to see an enlarged view

Fig. Blow air from the intake manifold side, check that the air passes with difficulty

Don't blow directly into the valve; petroleum deposits within the valve can be harmful.

An engine without crankcase ventilation is quickly damaged. It is important to check the PCV at regular intervals. When replacing a PCV valve you must use the correct one for the engine. Many valves look alike on the outside, but have different mechanical values. Putting the incorrect valve on a vehicle can cause a great deal of drivability problems.

Click image to see an enlarged view

Fig. Leaks from any of these areas can cause the PCV system to malfunction

Removal & Installation

  1. Pull the PCV valve from the valve cover.
  3. Remove the hose from the valve.
  5. Check the valve for proper operation. While the valve is removed, the hoses should be checked for splits, kinks and blockages. Check the vacuum port (that the hoses connect to) for any clogging.
  7. Inspect the rubber grommet the PCV valve fits into. If it is in any way deteriorated or oil soaked, replace it.

To install:

  1. Insert a new valve into the hose.
  3. Push the valve into the rubber grommet. Make sure the valve is firmly into place.