Bad PCV Valve Symptoms and How to Test the PCV Valve Yourself
Signs of a Bad PCV Valve
You probably don't realize how important the PCV system—the Positive Crankcase Ventilation (PCV) valve and its related components—is to the well-being of your engine.
A bad PCV valve or related component can produce a number of symptoms. For example, if the valve clogs, or gets stuck in the closed position, you'll notice one of these symptoms.
Symptoms of a Bad PVC
- Increase in internal engine pressure
- Failure of one or more oil seals or gaskets
- Engine oil leaks
- Moisture and sludge buildup inside the engine
- Engine surges and possibly black smoke
If the PCV valve gets stuck open, or a system hose gets disconnected or ruptured—producing a vacuum leak—you'll notice one or more of these symptoms.
Symptoms of a Stuck PCV
- Engine misfires at idle
- Lean air-fuel mixture
- Presence of engine oil in PCV valve or hose
- Increased oil consumption
- Hard engine start
- Rough engine idle
In addition, a PCV valve stuck open can trigger the "check engine" light due to increased air flow. And a diagnostic computer may erroneously blame this light on a mass air flow sensor or oxygen sensor instead, making it harder for you to detect the real source of the problem.
Why the PCV Valve Is Important
Bad PCV valves can cause engine oil contamination, sludge build-up, oil leaks, high fuel consumption, and other engine-damaging problems, depending on the type of failure.
Although you can detect some of these problems before they escalate with simple inspections, a failure of the PCV valve or related components often results in expensive repairs. That's because most car owners don't include the PCV system in their maintenance routine. Even though some car manufacturers suggest replacing the valve at regular intervals, car owners still forget to replace it. To add to the problem, not all manufacturers stress the importance of regular system inspections.
Below in this article, we will discuss how car owners can test their own PCV valves.
But before we get to that, here's this whole article in a nutshell: What the PCV valve does, what happens when it fails, and how to test it.
PCV Valve Function in a Nutshell
What the PCV valve does:
• Uses the engine vacuum to pull blow-by gases out of the crankcase.
• Pushes the gases down the intake manifold and back into the combustion chambers where they are re-burned.
Some signs it's failing:
• One or more oil seals or gaskets fail.
• The engine surges.
• The engine may produce black smoke.
• Internal engine pressure increases.
• Moisture and sludge build up inside the engine.
How to test it:
• Inspect rubber parts.
• Replace mesh filter beneath valve.
• Disconnect hoses and carefully inspect them.
• Remove valve and shake. If it does not rattle, it needs to be replaced.
Understanding the PCV Valve
First, let's discuss the valve's function so that you understand better the reasons behind the symptoms. Understanding this will help you make better sense of the system when you inspect and test it.
Up until the late 1950s, car engines released "blow-by" gases—unburned fuel—to prevent engine damage. Problem was, these gases were harming the environment. Real bad.
When your car engine is running, an air-fuel mixture enters each cylinder. Hundreds of powerful explosions take place to release the fuel's energy, producing highly toxic and harmful gases. After each combustion process, the exhaust valve routes these gases into the exhaust system where the catalytic converter turns them into much less toxic fumes before releasing them into the atmosphere.
Still, a small quantity of the gas in the combustion chambers finds its way into the crankcase (engine block) by way of pressure leakage between the piston rings and the cylinder wall.
Left on their own, these vapors and fumes will play havoc with your engine. Blow-by gases contain hydrocarbons (unburned fuel), carbon monoxide (partially burned fuel), particulates, water, sulfur, and acid. Together, these substances will corrode any engine metal component they touch, dilute engine oil, build up harmful sludge that accelerates parts wear, and plug small passages and hoses.
In 1961, the PCV system was introduced to deal with this problem. This simple emission control system uses the engine's vacuum to pull blow-by gases out of the crankcase, pushing them down the intake manifold and back into the combustion chambers where they are reburned.
Yet, the PCV system will fail with poor system or engine maintenance.
How often do you check the PCV system?
How the PCV Valve Works
Inspecting Your PCV Valve
Unfortunately, many car manufactures are not strict about PCV system maintenance. Some suggest servicing the system every 20,000 or 50,000 miles. However, a more frequent system inspection helps prevent costly repairs and keep the engine running smoothly.
To start checking the PCV system in your vehicle, first locate the PCV valve and its related components. Depending on your particular model, you may find the valve on a rubber grommet on the valve cover; on a breather opening around the intake manifold; or to one side of the engine block.
Keep in mind that some new models don't have a PCV valve at all; instead, you'll find a simple vacuum hose going from the valve cover to an air inlet duct. Others may have a simple restrictor in place. Still, you can check the restrictor, hoses and other components.
If you are not familiar with the PCV system in your vehicle, or can't find the valve, buy the service manual for your particular vehicle make and model from a local auto parts store. The aftermarket manual costs around $20 dollars, and it contains instructions for many simple maintenance tasks and repairs. If you don't want to buy a copy right now, check the reference section of your local public library for the manual, or your library's website for access to an online shop manual.
Luckily, it doesn't take much to check the system.
- Check PCV system parts. Rubber components like grommets, O-rings, and hoses swell and turn hard and brittle after constant exposure to high temperatures. They begin to leak. Replace one or more of these components as necessary.
- Carefully disconnect the valve and any system hoses and visually inspect them. If you find the hoses filled with slime, clean them with PCV solvent or lacquer thinner and replace the valve. Or, simply replace those components along with the PCV valve.
- Many engine models use a simple, inexpensive valve, and many car owners just replace it every service interval. Other valves incorporate heating elements and cost more. Regardless of the type of PCV valve your engine uses, always buy a quality brand valve, since it's more likely to have a more precise calibration for your specific engine model.
- On some engines, you'll find a mesh filter underneath the valve. Some car manufacturers recommend replacing the filter every 30,000 miles or so.
- Most PCV valves contain a spring-loaded device. Once you remove the valve, shake it with your hand. You'll hear a rattle. If you don't, it is time to replace the valve. Even if the valve rattles, if your engine is experiencing one or more of the bad PCV valve symptoms described above, it's a good idea to replace the valve.
Some vehicles—including some old Ford Escort models—come equipped with a small, hollow, plastic block with no moving parts. If you have this type of valve, just clean with lacquer thinner, if necessary, and reinstall.
Servicing the PCV Valve
Besides visually inspecting the condition of the different PCV valve and related components, test the system during engine operation.
1. Testing for Vacuum
- Start the engine and let it idle for about twenty minutes to warm it up to operating temperature.
- Then, open the hood and disconnect the valve from the valve cover and block the end of the valve with your finger. You'll feel vacuum from the system sucking at your fingertip and notice a momentary idle speed drop of about 40 to 80 rpm.
- If you notice a bigger rpm drop and the engine idle smooths out, your PCV valve might be stuck open.
- If you don't feel vacuum at your fingertip, check the valve and hoses for gunk obstructing air flow. Clean the PCV valve and hoses with lacquer thinner and a thin hose brush, if necessary.
2. Alternative Tests
- Another way to test for vacuum is to pinch or block the vacuum hose connected to the PCV valve. Idle speed will drop between 40 to 80 rpm, and then rise back to normal. If not, look for a blocked or restricted vacuum hose or valve.
- On some engines, access to the PCV valve is difficult. In these models, you can remove the engine oil dipstick and seal the dipstick tube opening with a piece of tape. With the engine at idle, remove the cap from the oil filler on the valve cover. Then place a thin piece of cardboard over the opening. Wait for about one minute. You'll notice vacuum suctioning and holding the paper against the opening. Otherwise, there's a leak in the system, or the system is clogged. Check the condition of the hoses, hose connections, and grommet.
Maintaining the PCV System
Sometimes, bad PCV valve symptoms are misrecorded as coming from a bad sensor. That's why it's important to check the PCV valve and related components regularly. It just takes a few minutes. If your engine lacks a PCV valve, or you can't reach it without removing one or more components, consult your workshop manual for the best way to check your particular system. Also, check the service schedule for your PCV system and replace the valve at intervals even if it seems to be in good condition. Most PCV valves and related components are inexpensive and will save you money in costly repairs if you replace them at the suggested interval.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Could a bad PCV affect engine start?
It can certainly make it hard to start; if it sticks open it may lead to a no start.Helpful 52
Will a bad PCV cause my truck to stall when running and accelerating?
If it sticks open it's possible.Helpful 47
Is it true that if you suck on PCV valve and air comes in, it's defective?
Most are designed as a check valve to prevent the valve from flowing back, for example, during a back-fire.Helpful 36
Should you be able to pass air flow in both directions in a PCV valve for testing purposes?
Usually, a PCV valve is normally closed when the engine is not running; as the engine accelerates, the valve opens to allow blowby gases. So you shouldn't be able to pass air flow in both directions. However, some models like the ‘87 Escort, use a fully open valve design.Helpful 21
Will a check engine light come on for a misfire?
The P0300 (random misfire) and the P030x series codes (specific-cylinder misfire codes) point to this type of issues.Helpful 3