8 Symptoms of a Bad PCV Valve
The PCV valve is an inexpensive but often overlooked part of your engine. If ignored, a faulty PCV valve can cause a lot of problems and cost a lot of money to repair.
What is a PCV valve?
“PCV” stands for Positive Crankcase Ventilation. When your engine is running, some of the combustion gases are forced past the piston rings and into the crankcase (the part of the engine block that surrounds the crankshaft). This leakage into the crankcase is called blow-by.
The combustion gases in the crankcase include air, atomized fuel, and oil. The pressure of these gases has to go somewhere. Older PCV systems vented this pressure into the atmosphere. To reduce emissions, modern PCV systems recirculate crankcase gases into the intake manifold, where the gases eventually return to the combustion chamber.
PCV Valve
PCV valves are usually mounted on the intake manifold or valve cover, but may also be bolted to the engine block. Some PCV valves are visible when you open the hood, while others are hidden in the engine bay.
Why do PCV valves fail?
PCV valves are simple mechanical components, but they will fail after many miles of driving. Most PCV valves fail because they become clogged and are no longer able to open and close freely. The PCV valve can become clogged if oil, sludge, or carbon buildup occurs. This limits the range of movement of the internal valve.
Symptoms of a Bad PCV Valve
The PCV valve may get stuck in the open or closed position. If the PCV valve is stuck, symptoms may vary depending on the position of the valve. Here are some example symptoms that may be caused by a faulty PCV valve:
1) Rough Idling
If the PCV valve gets stuck open, it can cause a rough idle due to misfires or lean running. This is because the PCV system allows more air into the intake than would normally be allowed.
This is similar to what you might experience if you have a vacuum leak. If the PCV valve is defective, the excess air comes from the crankcase instead of from the atmosphere.
2) Fouled Spark Plugs
Too much oil flowing through the PCV system can cause oil and other carbon deposits on the spark plugs. A dirty spark plug may not be able to ignite the mixture cleanly.
A dirty spark plug can cause misfire and poor engine operation. If your vehicle has this issue, you may need to clean or replace your spark plugs.
3) Black smoke coming from the exhaust
When the PCV valve is open, a lot of oil enters the intake. When this oil burns in the combustion chamber, black or blue smoke can come from the exhaust. You’ll probably notice more black smoke when you step on the accelerator than when you’re just cruising down the highway.
4) Oil in the intake or air filter (clogged air filter)
Have you ever noticed oil on the intake or air filter itself when changing the air filter on your engine? A faulty PCV valve could be the cause.
You can try flushing the intake to get rid of the excess oil, but if the oil comes back after a short period of time, be sure to find and fix the source of the problem.
Motor oil doesn’t burn as much as fuel. Vehicles with air-oil separators have more reliable and complete combustion, reduce knocking, and improve efficiency. This can also result in more consistent vehicle performance. This is why air-oil separators are popular in high-performance vehicles.
5) Increased oil consumption (low oil pressure symptom)
If your PCV valve is faulty, you may need to add engine oil more frequently. This is because the PCV valve is stuck open, allowing more oil to flow through the engine than necessary.
It is a good idea to check your oil every time you go to the gas station. This will allow you to catch oil consumption problems early, before they become a more serious problem: oil starvation.
6) Oil leaks
If your PCV valve is stuck closed, pressure in the engine crankcase can build up. In some cases, this pressure can become great enough that oil can pass through the rubber seals throughout the engine. This can cause an oil leak.
7) Faulty Engine Seal (Rear Main Seal)
Excessive pressure in the PCV system can not only cause an oil leak, but it can also push the oil seal out. If the engine oil seal is already weakened or not properly installed, the excess pressure from a closed PCV valve can force the seal out of place. Such a failure can cause a very large oil leak and should be addressed as soon as possible.
8) Moisture Under the Oil Cap
If there is moisture under the oil cap, it could be that the PCV valve is stuck closed. Excess pressure from the crankcase will try to take the path of least resistance. This can leave oil and moisture trapped under the oil cap.
The PCV valve should be replaced whenever it is defective, but most manufacturers do not specify a specific interval for this small component. Many sources recommend cleaning or replacing the valve every 30,000 to 60,000 miles.
How to Test the PCV Valve
The PCV valve is a simple device, so it’s very easy to tell if it’s a good valve. After you remove the PCV valve, shake it back and forth a little. If you hear a rattle, your PCV valve is likely fine. If you don’t hear any rattle, or if the rattle is very quiet, it’s time to clean or replace the valve.
Cleaning and Replacement
In many cases, the PCV valve can be saved by cleaning it. If you hear a slight rattle, try spraying the PCV valve with brake cleaner and shaking the valve back and forth. Repeat this procedure several times and see if there is any improvement. If the rattle becomes louder, you may be able to reuse the PCV valve.
If the PCV valve is stuck or looks dirty, it does not need to be cleaned. PCV valves are incredibly cheap, so it’s often better to just buy a new one and replace it.
