Your internal combustion engine obviously has the PCV system to balance the pressure in the crankcase. The system helps the engine get rid of excessive fuel vapors and other gases that are formed in the crankcase or get there through the problematic piston rings. But when something goes wrong and the PCV valve gets clogged or stuck, can this cause an engine knock?
In short, a bad PCV valve can cause an engine knock and also several other bad failures, so you better immediately react if you locate this problem. The PCV valve is a relatively inexpensive part and it’s not that hard to replace, so it’s important to solve problems with it once the valve malfunctions.
Here’s what I will cover in this article:
- Where is this PCV valve located and why can it be clogged?
- What are the symptoms of a bad PCV valve?
- Why can the engine knock when the PCV system doesn’t work well?
- How often should you clean or replace PCV valves?
- Can you drive with a bad PCV valve?
A short intro into the PCV valve system
So, PCV stands for positive crankcase ventilation. This is the system that helps the engine press out some gases and vapors that may gather in the crankcase. Usually, this happens with older engines with high mileage. Some vapors from unburnt fuel may get to the crankcase through piston rings and other engine imperfections.
If there was no PCV system, these gases just couldn’t find the way out and would start building up the pressure that’s not needed.
So, the PCV system helps these gases and vapors leave the crankcase by following directly to the intake manifold and then to the combustion chambers to be burnt.
One of the components of the PCV system is the PCV valve. The work of the PCV valve is to open or close according to the RPM of the engine and also to the pressure inside the crankcase. At high RPM, the valve opens completely and ensures high flow while at low RPM it is almost closed and helps the engine keep the compression high.
I won’t get deeper into the technical side of the problem. I will just tell you more about the PCV valve problems and what they can cause in your engine. Hope this short technical guide is enough for you to understand how the system works.
What are the common symptoms of a bad PCV valve?
- Your engine whistling or hissing. Very often, a blocked valve will hiss and this sound will become even louder at a higher RPM. This may mean that the PCV valve isn’t opening enough and the pressure in your crankcase is building up.
- Check engine light. It may be triggered because of numerous codes connected with emission system problems. You may even think that the catalytic converter’s days are over, but no, this may be just the valve that changes the vehicle’s emissions from normal.
- Unstable idling. One more very common symptom of a bad PCV valve is bad idling. You will see the RPM needle dancing all around the 1000-RPM point. This is not something you can ignore, so just find out what the reason is for this issue.
- Lower power and lower MPG. Because of the PCV valve failure, the engine will not be able to prepare the air-fuel mixture correctly. It may be too lean or too rich depending on the actual problem with the valve. This will lead to bad combustion and low power. As a result, the gas mileage will also deteriorate significantly.
- Unexpected oil leaks. Oil may start leaking from the dipstick hole, head gasket, PCV system, and in all kinds of other places. This means that the pressure is too high in the crankcase and oil tried to find the way out. If the pressure keeps building up, it can affect other parts of the engine, too.
- Black smoke from the tailpipe. One more problem is that the oil may be forced directly into the combustion chamber through old and inefficient piston rings and this will cause it to burn and create lots of black smoke.
- Oil level falling quickly. You may notice that all these problems lead to quick oil level drops because it leaks and burns heavily. This may also lead to fatal engine failure if you miss the point when you have to add some oil quickly.
Also, some engines may have a huge problem with oil level checks when the PCV valve is clogged. The pressure is building up and is sending the oil to all remote parts from the oil pan. When you take the dipstick out, you can find out that the level is very low. Naturally, you will want to add some oil, and will successfully do that.
But once you open the oil cap, the pressure will be relieved and the oil will quickly go down. In 5 minutes, you will see that now there is more oil than needed. Anyway, when the PCV valve is clogged, you may have problems with engine lubrication because the oil won’t naturally go down to the oil pan to be then pumped up by the oil pump.
Can your engine knock when the PCV valve is bad?
Sure, your engine may start knocking when the PCV valve is stuck or just broken. High pressure in the crankcase will lead to issues with the pistons. Also, the engine may misfire and knock because of the poor air-fuel mixture ratio. It may knock because the unburnt fuel will explode somewhere in the engine creating even more problems for you.
So, there are a lot of conditions when your engine may start knocking because of a clogged PCV valve. And I wouldn’t say that this knocking is safe to drive with. You should address this problem immediately because a long period of driving with a bad PCV valve may cause fatal issues with your engine.
What are the consequences of the problems with the PCV valve?
Well, the consequences are quite natural and they are caused by the common problems described above. I will focus on the most common and quite unpleasant consequences of a bad PCV valve:
- Low oil pressure in the engine. As said above, extra pressure in the crankcase may lead to problems with oil circulation, so the pump will not be able to create optimal pressure of oil to lubricate all its parts.
- Head gasket problems. In worst cases, abnormal pressure in your engine may lead to a broken head gasket and even to issues with cracks in the cylinder block.
- Emission system issues. When oil burns and the air-fuel mixture is incorrect, your EGR system (if present in your vehicle) and also catalytic converter suffer heavily. They may fail and lead to quite expensive repairs.
- Overall engine wear. You shouldn’t think that you can drive like this for half a year and then just replace the PCV valve and the engine will run like new. Most likely, it won’t because it will be quite worn out.
It’s important to remember the symptoms of the bad PCV valve and address this problem once it shows up. Otherwise, the engine may get great damages which will obviously lead to shorter engine life and issues with its performance.
Can you still drive when the PCV valve is bad?
Yes, you can carefully drive, but you better plan your visit to professionals to have the issue repaired before it killed your engine. If you noticed some of the symptoms described in the first part of the article, you should stop the vehicle and do the following right away:
- check the engine oil level first;
- open the oil cap and wait for 5 minutes, check the oil level again;
- look at the engine and try to locate any oil leaks;
- if the oil leaks are extensive, driving your car is not recommended;
- let the engine cool down, start it and look how it works;
- if it doesn’t go crazy, you can carefully drive it to the place where you can leave your car safely;
- if the engine is going crazy, it’s better not to risk and call a tow truck.
If the PCV valve was stuck five minutes ago, you will not damage your engine if you drive it another mile or two. But if the problem has been there for some time and is now torturing the engine so that it’s impossible to drive it, better stop and try to find a solution on how to deliver your car to the repair shop.
If you keep driving when the PCV valve is totally blocked, the engine may have a fatal problem that will maybe even kill it.
Final thoughts
PCV valves are not expensive yet extremely important parts of automobile engines. They relieve pressure in the crankcase and allow the engine to work in optimal conditions. Their main task is to help the engine get rid of blowby gases that get into the crankcase in the process of combustion without losing the optimal pressure inside the crankcase.
I recommend going to professionals once you experience several of the symptoms that I told you about in this article. Although you can replace the PCV valve on your own, you better do this with the help of the mechanic who will also provide your car with the proper diagnosis. Without this, the repair may not be as helpful as it could be.