Town Car 1993

Positive Crankcase Ventilation System


Description & Operation

Your car is equipped with a closed Positive Crankcase Ventilation (PCV) system. The PCV system vents crankcase gases into the engine air intake where they are burned with the air/fuel mixture. The PCV system keeps pollutants from being released into the atmosphere, and also helps to keep the engine oil clean, by ridding the crankcase of moisture and corrosive fumes. The PCV system consists of the PCV valve, a closed oil fill cap and the various connecting hoses.

The PCV system recycles crankcase gases as follows: When the engine is running, clean filtered air is drawn into the crankcase through the intake air filter. As the air passes through the crankcase, it picks up the combustion gases and carries them out of the crankcase, up through the PCV valve and into the intake manifold. After they enter the intake manifold, they are drawn into the combustion chamber and burned.

The most critical component of the PCV system is the PCV valve. The PCV valve regulates the amount of ventilating air and blow-by gas to the intake manifold and also prevents backfire from traveling into the crankcase, avoiding the explosion of crankcase gases. At low engine speeds, the PCV valve is partially closed, limiting the flow of gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of gases into the intake manifold.

If the PCV valve becomes blocked or plugged, crankcase gases will not be able to escape by the normal route. Since these gases are under pressure, they will seek an alternate route, which is usually an oil seal or gasket. As the gases escape, an oil leak will be created.

Besides causing oil leaks, a clogged PCV valve will also allow gases to remain in the crankcase for an extended period, promoting the formation of sludge in the engine.

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Fig. PCV system functional diagram

Diagnosis & Testing

  1. Visually inspect the PCV valve hose and 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, remove the PCV valve from its mounting grommet. Shake the PCV valve and listen or feel for the rattle of the valve plunger within the valve body. If the valve plunger does not rattle, the PCV valve must be cleaned or replaced. If the valve plunger rattles, the PCV valve is okay; reinstall it.
  5. Start the engine and bring it to normal operating temperature. Remove the fresh air supply hose from the air cleaner or air outlet tube. Place a stiff piece of paper over the hose end and wait 1 minute. If vacuum holds the paper in place, the system is okay.
  7. On the 4.6L engine, the PCV system is connected with the evaporative emission system. If the paper is not held in place, disconnect the evaporative hose, cap the connector and retest. If vacuum now holds the paper in place, the problem is in the evaporative emission system.
  9. If the paper is not held by vacuum, check the fresh air and PCV hoses for leaks or loose connections. Also, check for a loose fitting oil fill cap or loose dipstick. Correct as required until vacuum can be felt at the end of the supply hose.

If air pressure and oil or sludge is present at the end of the fresh air supply hose, the engine has excessive blow-by and cylinder bore or piston ring wear.

Removal & Installation

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Fig. The PCV valve is located in the passenger side valve cover-4.6L engine

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Fig. Grasp the valve and gently remove it from the grommet in the valve cover

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Fig. Remove the valve from the hose by carefully twisting it out

  1. Locate the PCV valve -usually located either in the valve cover (3.8L or 4.6L engine) or in the intake manifold (5.0L engine).
  3. Disconnect the crankcase ventilation tube from the positive crankcase ventilation valve.
  5. Remove the PCV valve from the PCV valve grommet.

To install:

  1. Install the PCV valve into the grommet.
  3. Connect the ventilation tube to the PCV valve.