5 Leaking Exhaust Valve Symptoms – Here Is How To Spot a Leaking Exhaust Valve

Exhaust valves are important components of a car’s engine. They regulate the release of exhaust gases from the engine cylinders as they burn fuel. This release has to be done in a pre-calculated timely manner. When the vehicle has a leaking exhaust valve, the release of exhaust gasses is either all the time or not at all. In both scenarios, the engine suffers and this manifests in an out-of-the-ordinary behavior of the engine.

In this article, I will be talking about the 5 most common leaking exhaust valve symptoms. By the end of this article, you will be able to stop a leaking exhaust valve by yourself.

The most common symptoms of a leaking exhaust valve include hard starting, the engine running rough, the exhaust gasses being sucked back into the exhaust, carbon deposits on spark plugs, and no or low cylinder compression.

Key Takeaway

  • A leaking exhaust valve can lead to hard starting because the air/fuel mixture can escape the combustion chamber before getting ignited by the spark plugs.
  • Engine running rough due to the pressure created by combustion not having a controlled escape route.
  • Air is getting sucked back in through the exhaust pipe.
  • Carbon deposits on spark plugs due to a lot of unburnt fuel in the combustion chamber.
  • Lack of cylinder compression due to a leaky exhaust valve that is unable to hold compression.

5 Leaking Exhaust Valve Symptoms – Here Is How To Spot a Leaking Exhaust Valve

5 Leaking Exhaust Valve Symptoms - Here Is How To Spot a Leaking Exhaust Valve

The exhaust valves are located in the cylinders of a vehicle. They work together with the inlet valves and regulate the amount of fresh air that is being fed to the engine, as well as the exhaust gas that leaves the cylinder once combustion is complete. When the exhaust valves start to leak, you will notice that the engine is behaving out of the ordinary. Here are 5 leaking exhaust valve symptoms that can help you spot a leaking exhaust valve.

1. Hard starting

5 Leaking Exhaust Valve Symptoms - Here Is How To Spot a Leaking Exhaust Valve

When one or more exhaust valves are leaking, starting the car can be quite hard. Each component of the engine works at a specific time. The exhaust valves are designed to allow built-up gas fumes to escape from the engine block at a certain time. When the combustion process is complete, the computer orders the exhaust valves to open. This allows the exhaust gasses to escape the combustion chamber.

When the exhaust valve is leaking, it will allow some of the air and fuel mixture that is supposed to be burnt to escape the combustion chamber and out of the exhaust. Depending on the leaking, the spark plug may not be able to ignite the air/fuel mixture in order for the car to start because all of this mixture is just escaping the combustion system through the leaking valve. This is why it is hard to start an engine when the exhaust valve is leaking.

The hard starting is actually a pretty good way to tell how many exhaust valves are actually leaking. Most vehicles have two exhaust valves in each cylinder, but there are cars with just one exhaust valve in each cylinder. So, if your car starts hard, but it takes only like one or two times to start it, it could be a sign that maybe just one exhaust valve is leaking.

However, if your car takes five or six tries, it could be a sign that more than one exhaust valve is leaking. This issue has to be fixed. Unfortunately, replacing the exhaust valves requires strong mechanical skills. This being said, it is best if you take your car to the mechanic shop to get it fixed.

2. Engine running rough

When the exhaust valve of an engine is leaking, it can cause rough running as the pressure created by combustion within the engine does not have a controlled escape route. This forces air to either escape around the leaking valve or back through the intake causing misfiring and decreased performance due to the extra air needed for each cycle.

In order to understand how a leaking valve can cause the engine to run rough, you need to understand how combustion works. The combustion process starts with the fresh air being sucked into the engine by the inlet valve. Before the air gets inside the combustion chamber, it gets mixed with an appropriate amount of fuel.

At this point, there is air and fuel inside the combustion chamber. What happens next is the spark plugs ignite this mixture and cause it to ”detonate”. The pistons move the air/fuel mixture up to the spark plug and get ignited. This is how combustion works in engines. However, for the combustion to create compression, it has to be a closed system.

This means that before the combustion occurs, there should not be leaks. The exact amount of air and fuel needs to be present inside the combustion chamber in order for controlled combustion to occur. If the exhaust valve is leaking, it allows the air/fuel mixture to escape which results in poor combustion, therefore the engine starts to run rough.

3. Exhaust gas being sucked back into the exhaust

5 Leaking Exhaust Valve Symptoms - Here Is How To Spot a Leaking Exhaust Valve

The vehicle engine relies on the exhaust system to safely escort the harmful gases out of the engine through the tailpipe. When one combustion process is complete inside the combustion chamber, the exhaust valves open and allow the exhaust gases to travel through the catalytic converter, muffler, and out of the tailpipe.

When the engine is running, the exhaust tailpipe should be constantly blowing out the exhaust gas. However, when the exhaust valves are leaking, they tend to both blow the exhaust gases out of the tailpipe and suck them right back in. This is very harmful to the engine because these exhaust gases are hot and full of carbon monoxide and nitrogen oxide.

I would say that the biggest threat is the heat. The exhaust gases normally discharge at 788 degrees Fahrenheit. They lose some of this heat as they travel through the exhaust system. But still, when the leaking exhaust valves allow these gases to be sucked back into the combustion chamber, they bring the heat with them. Adding additional heat to the combustion chamber can lead to engine overheating.

An easy way to check for a leaking exhaust valve is to start the vehicle and place a paper towel near the exhaust tailpipe. If the exhaust valves are leaking, they will blow the paper towel, but they will also try to suck it inside the exhaust tailpipe. If this is the case, then you got a leaking exhaust valve.

4. Carbon deposits on spark plugs

Leaking exhaust valves can be a serious and costly problem for car owners. If there is a leak in the system, unburned fuel passes out of the exhaust valve and then gets deposited on the spark plugs as hard carbon residues, which can reduce engine efficiency and cause misfires.

The combustion process is very straightforward. Fresh air and fuel enter the combustion chamber. The piston then compresses this mixture up to the spark plug. The spark plug then ignites the air/fuel mixture. This results in a ”detonation” which pushes the piston down and that’s how power is created. The combustion has a byproduct which is exhaust gases.

When the combustion process is over, the exhaust gases should escape the chamber through the exhaust valves and out of the exhaust tailpipe. However, when the exhaust valve is leaking, the unburnt fuel passes out of the exhaust valve and gets to the body of the spark plug as carbon deposits.

To check for a leaking exhaust valve, you will need to remove the spark plugs. Make sure that you are labeling which spark plug came from which cylinder. This is because if the exhaust valve is leaking on a particular cylinder, that spark plug will have much more carbon deposit buildup on it. This is an indicator that the exhaust valve in that particular cylinder is leaking.

5. No or low cylinder compression

Cylinders are basically sealed chambers that have controlled openings: intake and exhaust valves. When everything is working properly, each cylinder should produce around 125 PSI of compression. If the cylinder can’t produce compression, it can’t really produce the power that the engine needs in order to run.

If your exhaust valve is leaking, you may be dealing with a decreased or total lack of cylinder compression. This means that this particular cylinder is unable to produce compression, therefore it isn’t able to produce power.

To test which cylinder has low or no compression, you will need a compression tester. It is a simple tool that requires you to remove the spark plugs and then screw one end of the compression tester in the spark plug thread. On the other end, there is a gauge with PSI readings. When you hook up the compression tester, crank the vehicle for about five seconds. The vehicle will not start because there are no spark plugs to ignite the air/fuel mixture.

However, when you crank the car, the cylinder will create compression and the compression tester will be able to pick it up. Each vehicle will have a different PSI reading. But, most of the vehicles will have about 125 PSI and above. If one of your cylinders has low compression, for example, 0 or 50,60 PSI, it is an indicator that something is going on inside the cylinder. It is most likely that the exhaust valve is leaking.

Vide Polowenski

General Mechanic with over 20 years of experience specializing in General mechanics, exhaust systems, fuel systems, and fuse-related problems.

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