Piston Slap – Causes and Diagnosis (sound clip included)

Many car owners face the problem of piston slap. Most car owners might not know what pistons are or what they do, but they are important for running your vehicle. Fixing a piston slap is no easy task, but it’s possible for the determined car owner.

What does a Piston Slap Sound Like?

You can find out right here:

Piston Slapping Sound

How Does a Piston Slap Occur?

Two things cause a piston slap. Firstly, when the engine is cold, pistons have more room in their bores, and they knock against the sides. Secondly, manufacturers mistakenly put in the wrong size of pistons or rings making them knock and scrape against the sides when the engine is cold.

Essentially, this is a manufacturer’s mistake, as the pistons should be large enough that they never knock against the sides, but still slim enough so when they heat up they still fit in the bores.

Knocking indicates the manufacturers used a too-small piston for your bores. Depending on how old your car is, it could be normal for your vehicle. If you bought your car used and it’s over 30,000 miles it’s likely due to the age of the vehicle. If your car is under 30,000 miles and you bought it under six years ago, it’s a manufacturer issue you should have them fix.

Is Piston Slap a Problem?

Yes, and no. Piston slap will wear down your pistons and bores, but it can vary depending on the severity of your piston slap. It can also increase the chances of your engine getting an oil leak, so keep a close eye on your exhaust for smoke. However, the car will still run fine; it just has a consistent knocking noise when running cold.

However, a piston slap can become a bigger problem over time. It steadily wears down your pistons and bores, leading to the premature death of your car.

If you’re planning on having your vehicle for a long while yet, you will want to fix this problem sooner rather than later. Moreover, a piston slap can also affect the resale value of a car, as the knocking noise when the engine first starts up can be a turn-off to buyers. Overall, if you have the option to fix it for free or cheaply, you should take it.

Piston Slap Diagnosis

  1. There is a muffled knocking sound when you first start up your vehicle. It can sound a bit like a diesel engine
  2. The noise is coming from the front of the engine block, near the crankshaft. You can use a specialized vehicle stethoscope to determine the exact placement of the sound.
  3. When the engine is torn down you can check the pistons themselves and see if they are worn on one side.
  4. Finally, check the bores for worn spots that match the worn spots on the pistons, these are the pistons that are causing the piston slap and should be replaced.

Can You Fix a Piston Slap?

Yes, but it can be costly and difficult to do, so if you’re planning on getting a new car around 30,000 to 40,000 miles it may not be worth it to you. If you treat your car correctly, it can last upwards 100,000 miles even with a piston slap.

That being said, if your car is under six years old or is still under warranty, you should get the manufacturer to fix it. The manufacturers are liable for fixing a piston slap because it is a manufacturer’s mistake. They are the ones who installed the wrong size pistons, so they should fix it.

Another option is to buy and install the parts yourself, which is much cheaper than paying a mechanic or the dealership. However, this isn’t a fix for beginners. Replacing the pistons requires tearing down the engine, so unless you know what you’re doing you could end up with a larger problem than a piston slap.

How To Eliminate a Piston Slap

The only way to get rid of a piston slap is to replace the pistons.

This entails a teardown of your engine and replacement of parts, so it’s not cheap. That being said if your car is under six years old, you can put in a “goodwill” claim with the manufacturer.

Since the problem was originally caused by the manufacturer they will usually fix it at no cost, but this is not a sure thing, so don’t be afraid to escalate the issue. If your car is still covered by its warranty, you can also try to fix it at the dealership, but be prepared to fight an uphill battle.

Dealerships will usually give you the run-around considering it’s such a costly repair. Understand your warranty and procedures before going in.

Some people have said using a thicker oil can help with the noise, but it won’t help with the actual problem of the pistons hitting the side of the bore. That said, if all you want is to stop the noise, a thicker oil may help.

How Can a Piston Slap Be Prevented?

In short, it can’t. It’s a manufacturer’s mistake that can take years to manifest. However, there are some steps you can take to alleviate a piston slap.

Keep your car maintained, and stay up-to-date on oil changes and tire rotations. This will help you avoid a lot of problems in your car. Next, make sure to warm up your car before you drive. Piston slap usually stops after the engine reaches a certain temperature, so getting above that temperature can help you avoid a piston slap.

If you’re concerned about a piston slap, research your dealer and manufacturer to learn how they’ve dealt with cases of piston slap in the past. Good dealerships and manufacturers will fix the problem for free. Bad ones give their customers a run-around and try to get them to pay for the engine teardowns and towing fees.

See what people with piston slap say about these dealerships and manufacturers to determine if they’re trustworthy.

Piston Slap FAQ

How Long Will an Engine Last with a Piston Slap?

It depends on your car, some last only 30,000 to 40,000 miles, and others can last upwards 100,000 miles. It depends on the severity of your piston slap and how well you care for your car.

A piston slap varies in its severity, so pay attention to how loud the knocking is, and how long it lasts before the engine warms up.

If you’re in an older car with a lot of miles, it can be normal for it to knock a minute or two before quieting down and isn’t a problem you should worry about. But if you’re in a newer car with fewer than 30,000 miles, you should take it to the dealership.

Your car should still be under warranty, so do what you can to get it fixed, or it can lead to the premature death of your car.

How Much Does it Cost to Fix a Piston Slap?

If your car is under six years old it shouldn’t cost anything. Put in a goodwill repair request with your dealership, and if they’re hesitant, take it to the top, to the service manager, or even to corporate. If your car is over six years old, but still under warranty, use the same procedure. However, if you have neither option, it will cost you $1,000-$5,000 to fix the problem.

The parts themselves cost $40-$200 depending on how many pistons you have and it’s $1,000-$4,000 for labor. This isn’t a cheap or easy fix as it requires tearing down your engine to get at the pistons.

Will Thicker Oil Help Piston Slap?

Unfortunately, no. Piston slap is caused by pistons hitting the side of the bores, and no amount of oil will fix that problem. However, if you just want to fix the noise issue, many people have said a thicker oil helps. Make sure to get a high-end, thick-as-honey oil, and it should help stop the knocking noise on start-up.

Can You Drive with a Piston Slap?

Yes, you can still drive your car with a piston slap. It’s not that severe of an issue, it’s just noisy. Unless you can fix it for free, don’t bother trying to fix it, as the car will last a while as long as you take care of it properly. You can also alleviate the problem by waiting for your engine to warm up, so it’s not that big of a problem if you just want your car to get you around.

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Author: Dave Johnston

Dave Junior is a hands-on automotive technician with experience in performing service, diagnostics, and repairs on domestic and imported vehicles. He enjoys writing and sharing his knowledge far and wide.

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