Home » Engine » What Is Piston Slap? | What Causes It & What To Do About It

What Is Piston Slap? | What Causes It & What To Do About It

Most car owners know that a knocking sound from their engine is generally not good, but many probably don’t know what causes it. While a few different problems can cause knocking, piston slap is one thing that can cause the sound. Unless you’re a mechanic or a serious car enthusiast, you probably don’t know what piston slap is. That’s OK!

We are going to give you all the details you need to know about the dreaded piston slap. Keep reading as we tell you exactly what it is, what causes it, and how you can fix it.

What Is Piston Slap & What Causes It?

To fully understand piston slap, you need to think for a moment about how an internal combustion engine operates. Think about the inner workings of your car’s engine for a moment. The engine block has cylinders inside it where the pistons sit. Your engine might have 4, 6, 8, or even more cylinders. The pistons move up and down inside the cylinder walls compressing the air and fuel mixture in the combustion chamber. When this mixture ignites, it forces the piston back down into the cylinder. The piston rings help provide a seal along the cylinder wall to prevent oil and gas from bypassing the piston.

The cylinders are made to fit tightly inside the cylinder, and they are designed to move up and down but not side to side. As your engine ages, the pistons and cylinder walls begin to wear. This wear creates a wider gap between the piston and the cylinder wall. Once the cylinder wall clearance gets too wide, the piston may wobble or move side to side as it travels in the cylinder. This side-to-side movement causes the piston skirt to slap against the side of the cylinder wall. When that happens, you will hear a slapping noise from the engine.

Piston slap is most likely to occur in older engines that have some wear and tear. Likewise, it is more likely to occur on cold starts. Once the engine warms to operating temperature, the pistons may expand slightly and eliminate the piston slap. You will probably notice the slapping sound more often at idle or on overrun. Overrun is when you release the accelerator and the engine RPMs drop. Now that you know what causes piston slap, let’s look at how you can properly diagnose the condition.

Properly Diagnosing Piston Slap

As we previously mentioned, several different things could cause your engine to make a knocking sound. Some of these include low-quality fuel, fouled spark plugs, or improper ignition timing. So, how can you know for sure that piston slap is responsible for the noise coming from your engine? Here are some of the signs you can watch for that can help you diagnose piston slap.

— Check Engine Light

Your check engine light could be an early warning sign that your engine has developed a piston slap. The check engine light will illuminate when there is an engine misfire. Misfires can occur for a number of reasons, not just due to a bad piston. Some other causes of misfires include bad spark plugs, clogged fuel injectors, or bad engine timing. A local auto repair shop can use a scan tool to read the trouble codes from your car’s computer. The trouble code should alert the mechanic to the specific cylinder in which the misfire occurred. The mechanic can then further diagnose the problem to determine whether the piston slap was the cause of the check engine light.

— Burned Pistons

A burnt piston will almost always lead to piston slap. So, what causes a burnt piston? The most common cause of a burnt piston is dirty fuel injectors. When the fuel injectors are dirty, they spray dirt and other contaminants into the combustion chamber. These contaminants and debris begin to accumulate on the top of the piston. Over time, the dirt will create a hole in the top of the piston. Using the wrong spark plugs can also cause a burnt piston. The wrong spark plugs for your vehicle might create a spark that is too hot. This will eventually lead to a hole in the top of the piston as well. To properly diagnose a burnt piston, you will need to open the engine and examine the pistons.

— Broken Timing Belt

The timing belt on your vehicle ensures that the pistons and valves remain in sync. The valves need to open and close at just the right times as the piston moves up and down in the cylinder. The timing belt keeps everything in sync. When the timing belt breaks, the piston and valves can slam into each other and cause catastrophic engine damage. Piston slap can create a great deal of stress on the timing belt, and it might lead to a broken belt. The cost to replace a timing belt is not cheap, but it is much less than the cost of an engine rebuild. If you hear the piston slap noise, you should address it before it leads to additional damage.

— Worn Piston Rings

The piston rings in your engine are designed to maintain a tight clearance between the cylinder and piston. The piston rings are made to fit tightly, but they will become looser over time. As the rings become loose, you might start to hear the knocking noise in your engine. The sound will usually be more apparent at startup because the cold clearance of the engine is typically higher. As the engine warms, the clearance gets tighter due to the slight expansion of the pistons and rings from the heat. If your piston rings are worn, you might also see blue smoke coming from the exhaust because oil can make its way into the combustion chamber. If this happens, you likely need to get new piston rings before the problem worsens and causes more damage.

How To Fix Piston Slap

If you hear the knocking engine noise, you might suspect you have piston slap. However, once you confirm that piston slap is the issue, how do you fix it? Unfortunately, fixing piston slap is not an easy task. You will need an experienced mechanic to repair the problem. First, it is a good idea to perform a compression test to determine how bad the problem is. If one or more cylinders have lost compression, you will probably need a complete engine rebuild. At a minimum, you will need to perform an engine teardown so that you can see the pistons. Once you have access to the pistons, you should assess them for damage. In addition, you will need to assess the cylinder walls and other internal engine components for damage.

If there is no damage inside the engine, the replacement of the pistons and piston rings should stop the piston slap. In some cases, the cylinder bore may have increased due to wear, so the rings may need to be replaced with slightly larger ones. Replacement with larger rings can help get the engine tolerances back to the proper specs. Many mechanics prefer to go ahead and perform an engine rebuild at this point. Instead of repairing only a couple of pistons, it makes sense to rebuild the engine while it is torn apart. This may include new connecting rods, new pistons, new piston rings, and inspection of the lifters and valves. Your engine might not need all new parts. You may be able to simply remove the wrist pin, replace the piston skirt, and reinsert the piston pin.

Cost To Repair Piston Slap

The cost to repair piston slap is not cheap! Since the repair requires at least a partial teardown of the engine, the labor costs will be quite high. Repairing piston slap will usually cost you $1,000 or more. The cost to rebuild an engine is usually around $3,000 to $4,000. You might get lucky and only need to replace the piston rings to fix the problem. However, that still requires tearing your engine apart. An experienced mechanic is needed for that job, and it will require many hours of labor. In some cases, your car might not even be worth repairing! You will want to assess the value of your car against the cost of the repair and determine whether it makes sense to proceed with fixing it. On the plus side, most mechanics provide a warranty with their engine rebuilds, so you shouldn’t have to worry about paying for any problems related to the fix for a year or so.

Other Causes Of Engine Knocking Noises

Piston slap is sometimes called a false knock. Some vehicles are especially susceptible to it, like the LS1 Corvette. A cold engine will sometimes encounter piston slap until the engine is allowed to warm up, and aluminum pistons are even more prone to piston slap. However, there are other things that can cause a knocking sound from your engine. Some are not that serious, while others might be detrimental to your engine. Here are a few other things that can cause a knocking sound.

— Low-Quality Gas

Low-quality gas will cause a pinging or knocking sound in your engine. The knocking is usually more apparent when the engine is under a heavy load. This sound is caused by premature ignition of the air/fuel mixture in the combustion chamber. Instead of igniting when the spark plug fires, the mixture ignites early due to the compression in the chamber. A diesel engine operates this way, but the fuel in a gasoline engine should not ignite until the spark plug fires. When the fuel ignites early, it attempts to reverse the motion of the piston by pushing it back into the cylinder before it is time. Severe pinging can lead to engine damage, so you should always use high-quality gas in your vehicle.

— Bad Spark Plugs

Bad spark plugs will lead to engine misfires and might cause a knocking sound from your engine. In the case of a bad spark plug, the fuel mixture in the combustion chamber might not ignite on time. If the mixture is igniting too late or does not ignite at all, you might notice a knocking sound coming from your engine. Unburned fuel can also wreak havoc on your catalytic converter and exhaust system, so you should always replace bad spark plugs as soon as possible.

— Carbon Buildup

Carbon buildup in your engine can lead to many problems. Carbon can cause engine misfires, fouled spark plugs, clogged fuel injectors, and many other problems. Ultimately, many of these issues can result in a knocking sound coming from your engine. If you hear a knocking noise, carbon buildup could be the root cause. It is crucial that you always use high-quality gas in your vehicle and make sure that you use the proper viscosity of oil recommended in your owner’s manual. You might also consider an oil with special detergent additives at your next oil change to help remove sludge and buildup in your engine.

— Worn Rod Bearings

The connecting rods attach to the piston on one end and the crankshaft on the other. Your rod bearings keep the rods tightly attached to the crankshaft but allow the crankshaft to spin freely in order to push the pistons up and down in the cylinders. When the rod bearings start to wear, the connecting rods can wiggle slightly on the crankshaft. This motion can cause a knocking sound, and it is not likely to go away as your engine warms up.

— Improper Ignition Timing

We have already discussed the importance of the proper ignition of the fuel mixture in your engine. The ignition system is responsible for sending the spark through the spark plug at just the right time. When the ignition timing is off, the spark arrives either too early or too late. Either of these conditions can result in a knocking sound from the engine. Instead of firing at just the right time to push the piston back down into the cylinder, the fuel mixture may fire as the piston is still on its way up. This puts a lot of stress on the engine and causes a knocking or pinging sound from the motor.

The Bottom Line

Piston slap is caused when the pistons in your engine become loose in the cylinder walls. Instead of only moving up and down in the cylinder, the piston has enough room for a slight side-to-side motion as well. This wobbling movement causes a knocking sound, although it usually goes away as the engine gets hot. You might also hear piston slap called false rod knock. It can be caused by improper lubrication from the wrong viscosity of oil in your engine, but it usually results from wear over time. If you hear a knocking noise from your engine, you should head to your dealership soon to have things repaired.

Frequently Asked Questions

Is piston slap serious?

Yes, piston slap is serious. However, continuing to drive with piston slap is not likely to result in any immediate damage to your engine. If you do have piston slap, you should allow your engine to warm up before accelerating too quickly. Piston slap tends to go away as the engine gets hot, so allowing your engine to warm before driving can help prevent further damage. Some new cars even have slight piston slap at startup, such as the LS1 Corvette. The design of the aluminum engine block and aluminum pistons made that engine more prone to the condition.

Can you fix piston slap?

Yes, you can usually fix piston slap. However, it usually requires a teardown of the engine and replacement of the pistons. In some cases, you might only need to replace the piston rings. However, in severe cases, you might need to replace the piston skirts, rings, and other parts. An experienced mechanic will need to assist you with the diagnosis of the problem and the proper fix.

Can you drive with a piston slap?

Most of the time, you can drive for a while with piston slap without causing any severe damage to the engine. However, you should get the problem fixed as soon as you can. Continuing to drive for long periods of time with piston slap will make the condition worse. You might also do further damage to your cylinder walls or other engine components.

Does piston slap occur in all cars?

No, piston slap does not occur in all cars. It is possible for any car to develop piston slap, but not all cars will experience it. You are more likely to have piston slap as your engine gets old. When the engine wears, the space between the piston and cylinder wall increases. This increase in space will eventually lead to piston slap. Many engines can exceed 200,000 miles without ever encountering piston slap as long as regular oil changes and other maintenance items are performed on the engine.

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