All internal combustion engines produce gases that leak through the piston rings into the crankcase. The crankcase must be vented so that these gases do not create problems, such as moisture in the oil and excessive pressure. We have made for you a list with the common failures for a PCV valve, to help you know the problems that can occur and avoid them.
But the gases cannot be vented directly into the atmosphere because that would increase harmful emissions. That’s why automakers have been equipping vehicles with Positive Crankcase Ventilation (PCV) systems since the 1960s.
Many systems incorporate a PCV valve to regulate the flow of crankcase gases. Like any automotive part, the PCV valve can fail over time, leading to issues you’ll want to fix right away.
What does a PCV valve do?
No matter how well an engine is designed, there will always be some combustion gases that seep through the piston rings into the crankcase. These gases, which are often referred to as “blowby”, consist mainly of unburned fuel and water vapor.
The PCV system is designed to remove blow-by gases from the crankcase and then recirculate them back to the engine, where they are burned during normal operation. If it weren’t for the PCV system, crankcase pressure would build until oil was pushed past the seals and gaskets . The PCV system also prevents blown gases from mixing with the oil and forming harmful sludge for the engine.
All PCV systems must have some way of regulating the flow of exhaust gases into the engine. Some designs handle this task with a calibrated hole or spacer; others rely on a PCV valve to get the job done.
How does a PCV valve work?
The PCV valve is a one-way check valve that contains a spring-loaded plunger to regulate the flow of exhaust gases. One end of the valve connects to a hose connected to the manifold vacuum, while the other end usually fits into the engine’s valve cover or intake vale.
When the engine is off, the PCV valve’s internal spring forces the plunger closed . But once the engine is running, the manifold vacuum begins to pull the plunger open. The open PCV valve then draws fresh air, which enters the engine through a breather tube, through the crankcase. The resulting sweeping effect draws blown gases toward the PCV valve.
After leaving the valve, the gases enter a rubber hose that is usually connected to the engine’s intake manifold or throttle body. The gases then enter the engine, where they are burned as part of the normal combustion process.
However, the PCV valve cannot allow the same amount of blow-by gases to enter the engine at all times. Instead, the valve must use its internal plunger to regulate flow, as follows:
Blowby production is low when manifold vacuum is high. In this state, the vacuum pulls the ribbon-shaped plunger out of its seat, so that gases can only flow through small slits in the plunger body.
As the vacuum decreases and the blowout increases, the plunger moves further off its seat until it is at a maximum flow position. Crankcase gases can then flow freely into the engine’s intake system.
But wait, that’s not all. The PCV valve also provides backfire protection if needed . If an intake manifold backfires, the valve’s internal plunger will be forced into the crankcase, preventing the flame from igniting fuel vapors inside.
Common failures by a PCV valve
Think you may be dealing with a bad PCV valve? If you notice one or more of the following symptoms, you could be correct and it could be a common PCV valve failure.
Note : Other problems can mimic a bad PCV valve. You (or your mechanic) should perform a thorough diagnosis before carrying out any repairs.
1. Oil consumption
If the PCV valve is stuck open, oil will be bypassed from the crankcase under high vacuum conditions (when the valve would normally be closed). Consequently, a stuck open PCV valve can lead to oil burning inside the engine and increased consumption.
2. Slow down / hard shutdown
The PCV valve draws air and exhaust through the crankcase and into the engine . If the valve is stuck open, the extra air getting into the engine will upset the air-fuel mixture, causing problems such as rough running and stalling.
3. Check Engine Light Illuminated
In many modern vehicles, the engine management computer controls the operation of the PCV system. If the device detects a problem with the PCV system, it will turn on the check engine light and store a corresponding diagnostic trouble code in memory. A stuck open PCV valve can also upset the fuel/air mixture and cause misfiring that triggers the check engine light.
4. Oil in the air intake system
If the PCV valve becomes clogged or stuck, oily exhaust gases will begin to build up and eventually be pushed back into the air intake and/or air cleaner housing.
5. Oil leaks
A stuck closed PCV valve can also cause crankcase pressure to build to the point that oil is pushed past the engine’s gaskets and seals.
6. Accumulation of sludge
If a clogged PCV valve is left out, moisture will begin to collect in the crankcase oil , eventually leading to sludge formation inside the engine.
Questions and answers
In case you have any questions, we’ve taken a few questions that are frequently asked by the consuming public about common PCV valve failures. Here are the questions and answers for each one:
1. Can you drive with a bad PCV valve?
You shouldn’t keep driving with a bad PCV valve. A faulty PCV valve can cause additional (potentially costly) problems, such as broken gaskets and increased oil consumption.
2. How often should a PCV valve be changed?
Some (but not all) car manufacturers recommend PCV valve replacement as part of the vehicle’s regular maintenance schedule. You can find the maintenance schedule in your owner’s manual or supplemental service booklet.
If the schedule doesn’t list a service interval for the PCV valve (and you’re sure your car has one; remember some don’t) a good rule of thumb is to replace the valve every 30,000 to 50,000 miles.