Piston slap is when the piston rocks or rattles within the cylinder. Instead of smoothly moving up and down (reciprocally), it comes loose and starts making a terrible noise.
Piston slap is a sign that your engine is on its way out. It may still last for some time, but your best bet to repair it is an engine rebuild or getting a fresh engine or car altogether.
This guide will discuss how an internal combustion engine functions, what piston slap is, some indicators, and what you should do afterward.
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How Does An Internal Combustion Engine (ICE) Work?
An internal combustion engine, such as the one found in your car, is an extremely simple – yet complex – device.
Despite numerous advancements, primarily in the fields of power output, fuel efficiency, and emissions – the vehicular ICE remains essentially the same today as when it was first developed in the 1800s.
Car engines, known as four-cycle or four-stroke, function differently compared to two-cycle/stroke engines typically found on mopeds and garden equipment such as lawnmowers and leaf blowers.
As shown in the video example above, a typical four-cylinder engine is the simplest to visualize the process.
The Four-Stroke Engine Process
During the preliminary “intake stroke,” the piston travels downwards in the cylinder. Simultaneously, the intake valve opens, allowing air and fuel into the combustion chamber. (Gas may be introduced directly into the cylinder through the fuel injector as an alternative.)
The piston rises again, compressing the air/fuel mixture in the confined space above it. This is known as a compression stroke.
Next, a spark plug fires, igniting the compressed mixture. The force of the explosion drives the piston downwards again forcefully. During this power stroke, the engine generates its horsepower.
Finally, the piston rises from BDC (bottom dead center) to TDC (top dead center), pushing the burnt exhaust gases through the now-open exhaust valve. This last phase, the exhaust stroke, completes the “cycle,” and the process begins again.
If you’re traveling with an engine speed of 3,000 RPM, your motor is operating at 3,000 revolutions per minute. That is, it’s finishing that four-stroke cycle in every cylinder 50 times per second.
What Does A Piston Do In A Car Engine?
As you may have noticed, the piston is crucial to the whole process.
It creates the pressure differentials required for the intake and exhaust strokes while compressing the air and fuel and absorbing the energy of the burned mixture.
It carries out these actions thousands of times a minute when you drive. It is more extraordinary that an engine can last hundreds of thousands of miles now.
You might also notice the considerable strain a piston is put through.
Not only does it complete its tasks, as mentioned above, but it also rubs against the cylinder wall as it travels up and down. Over time, both it and the wall are bound to develop some form of deterioration.
This is typically what causes piston slap.
It’s an almost inevitable consequence of time and is simply a sign that, if left unchecked, your engine doesn’t have much longer for this world.
What Is Piston Slap? – Symptoms Of Piston Slap
In this section, you’ll find out how to diagnose piston clatter, although you can only conclusively confirm it by stripping down and rebuilding the engine.
The main – and only, really – symptom of piston slap is the sound. You’ll hear a troubling rattling noise, often worse at idle or overrun (when you lift off the throttle pedal).
Keep an eye on your exhaust gas, too. When piston slap occurs, the piston rings are also probable on the way out. This could allow oil to get past them and into the combustion chamber.
When oil burns, it produces a blue, dense smoke. You’ll see this pouring out the tailpipe at all times.
Oily exhaust smoke is only a possible by-product of piston slap. Your car might be experiencing this problem without any obvious issues with the exhaust gases. Treat any rattling or knocking sounds as the clear sign.
Your vehicle might (but might not) also light up the Check Engine lamp.
Piston slap is unlikely to happen on a newer car with more sophisticated technologies. Knock sensors also help manage unwanted engine noises (for better or worse).
Could The Knocking Sound Be Something Other Than Piston Slap?
A tapping sound from your engine could be caused by several distinct things. It’s not necessarily an indication of piston slap.
Take your vehicle to an auto shop for a thorough diagnosis before deciding it’s the final outcome!
- Low octane/poor quality gas
- Rod knock
- Misfires
- Ignition timing
- Poor-quality spark plugs
- Incorrect spark plug gaps
- Carbon deposits
- Lean/rich mixture
- Bad knock sensor
Before committing to an engine overhaul, consider a thorough maintenance. A new set of spark plugs, a cleaning fuel additive, and an OBD II check may solve it for you!
Can You Fix Piston Slap?
There’s only one way to remedy piston slap: with a complete engine overhaul.
First, a mechanic should do a compression test (which is absolutely essential and will come at an extra charge to you).
It may be identified that the cylinder walls are bored out beyond repair (more likely in aluminum than iron engines). This extreme worn-down space tends to develop near TDC. Therefore, the block is relatively useless, and you’ll need a completely new engine or car.
The problem of piston slap is often confined to the piston and piston rings. Piston rings fit around the piston to seal it against the cylinder wall. They also contain a meticulously crafted gap.
As the metal expands when the engine warms up, this gap closes, providing ideal sealing conditions when it’s at operating temperature. This is why piston slap often sounds worse when it’s cold, or the engine is just warming up.
Piston slap is often confined to one cylinder. To repair it, you’ll need an engine rebuild, including a replacement set of pistons and piston rings. If the block has been damaged, the engine should be condemned and sold for salvage.
Will Piston Slap Damage My Engine?
This isn’t quite the right question to ask. Yes, piston slap will damage your engine since everything will be wearing down much faster and thrown out of balance.
However, in reality, piston slap is engine damage. It won’t just lead to issues – it is a trouble. At the very least, it’s an early symptom of more to come.
While the piston slap sound isn’t very pleasant, you could choose to live with it for a while. However, know that by doing so, there may be no way to save your engine should you want to in the future. It’s better to make the choice now rather than postpone it.
Is Fixing Piston Slap Worth It?
So, is piston slap bad? Most agree that, while piston slap is a sign of an aging engine, your car should last for a brief while longer yet. That being said, if you want any chance of preserving the motor, you should act as quickly as possible.
Let’s get this straight. Engine rebuilds are costly. Probably around $1,500 to $2,000. The parts alone might only be $200 or $300, but it’s likely to take a mechanic a full working day. You’ll find this quickly adds up with labor rates of $80 to $100 per hour.
If you have a rarer or more expensive vehicle, engine rebuild costs could approach $4,000 or more.
So, is your car worth that much to you? Or is it more economically wise to run it into the ground, scrap it, and put the proceeds toward a fresh model? That’s up to you.
