How to Stop a Car from Knocking

Co-authored by Ed Beery

Last Updated: December 30, 2022 References

A "knocking" sound from a car's engine while it's running is serious cause for alarm. This noise may be the symptom of inefficient combustion. There are several reasons that your car could be experiencing combustion problems. Some such as overheating, are simple to fix ― just turn your engine off until it cools down. Others are a bit more involved. Always try one remedy at a time so that you know what the cause was. This is a rewarding project that will keep you in touch with vehicle maintenance and help you throughout your lifetime of vehicle ownership!

Method 1

Method 1 of 5:

Checking Engine Temperature

1
An electric fan has a sending unit to turn it on when it reaches a certain temperature. Is the fan coming on when it's supposed to? If the temperature gauge or overheating indicator on your dash doesn't work you can replace the radiator cap with an aftermarket cap that has a built in temperature gauge. Also be sure to check the wiring on the fan.
2
Some vehicles have a fan shroud that directs air to the radiator to assist in efficient cooling. Be sure it is secured properly.

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3
Is the thermostat opening when it should? A typical thermostat will open around 195 degrees. One indication of a failed thermostat is when your heater doesn't put out much heat. Be sure your engine cooling system is properly filled with the proper coolant/water mix when diagnosing the thermostat this way. Whenever you add coolant mix be sure to check it with a tester that can be bought for just a few dollars at any parts store.
4
A functional water pump is critical in keeping your engine at the proper operating temp. Be sure the belt hasn't come off. Be sure to coat belts with 303 UV protectant. Your belts will possibly outlast your vehicle when protected with this solution.

Method 2

Method 2 of 5:

Increasing the Octane Level in your Fuel

1
Check to make sure you are using the proper fuel. In order for the fuel to ignite at the proper time during the combustion cycle, you have to use at least the recommended minimum octane level. In the US this, is usually 87, but there are some higher end performance cars that require higher.^{[1]
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Research source} Check your owner’s manual if you are unsure what octane rating your car uses.
2
Add an octane booster. If you discover that you have been using the wrong fuel, you can add an octane booster to your tank. It doesn’t matter much which brand you choose as they are designed to simply increase the octane level of your fuel, allowing you to use the gas in your tank.^{[2]
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Research source} Adding the booster is quite simple ― you just pour it into your gas tank.
3
Buy the right gas. Using an octane booster to get through your last tank of low octane fuel is fine, but from now, on you want to buy the right kind of fuel. You should also keep in mind that whatever low octane fuel is left in your tank when you fill up will mix with your new fuel, so if knocking continues, go ahead and use the octane booster for another fill-up or two until most of the low octane fuel is gone.
- It is also thought that using a "top-tier" gasoline such as Shell or Chevron will decrease engine deposits that can cause knocking.

Method 3

Method 3 of 5:

Cleaning Your Combustion Chamber

1
Consider cleaning your cylinders. Again, using the wrong fuel can be an issue. Not only can it cause knocking by igniting at the wrong time, it can also leave excess contaminants in your cylinders as a result of poor combustion. If you are now using the proper fuel, you may need to clean out the contaminants left by past fuels.
2
Use a fuel additive. While most fuels have a specified amount of detergent in them (at least in the US), this may not always be enough to keep your combustion chamber clean. Using top tier gasoline brands will provide more detergents that help keep your engine clean. Another way to clean your cylinders is to add a detergent to your fuel. This is as easy as buying a fuel additive at your local parts store and pouring it into your gas tank at your next fill up.^{[3]
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- This step is very simple. Just choose an additive and follow the directions on the bottle for putting it in your gas tank.
3
Flush your engine. If detergent hasn't gotten your engine running right, you can try an engine flush such as Seafoam. The flush reacts with the carbon deposits to remove them from your intake system, including the combustion chamber. Be aware that, when starting your engine for the first time after the flush, there will be a lot of smoke.
4
Test your car. Start the engine and listen closely. Your knocking engine should be running smoothly now.

Method 4

Method 4 of 5:

Inspecting/Replacing your Spark Plugs and/or Wires

1
Consult your owner's manual or a local parts store to find the correct spark plugs. A faulty spark plug OR wire can be the cause of engine knocking, and is bad for your engine overall. Check for corroded spark plug ends. If a plug wire is left disconnected it has the potential to collect moisture. Did the previous owner let this happen and now months later corrosion has taken its toll? Remember to always check part numbers on the parts you buy. ^{[4]
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2
Prepare to work on your vehicle. Find all of the tools you will need, such as a spark plug socket and gap gauge.^{[5]
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Research source} Turn off your engine and if you think the vehicle will magically try to turn on during this process you can unhook your battery terminals.
3
Check your spark plugs. You want to be sure that replacing the spark plugs will help. You can usually tell if there is a problem just by the residue left on your spark plug. A normal plug should have only brownish grey residue on the side electrode. If this is the only residue and the plug is otherwise intact, you should just clean it with a wire brush and fuel injector cleaner rather than replace it.^{[6]
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4
Remove and replace your spark plugs an/or wires. This is a relatively easy job but could take about 30 minutes to 1 1/2 hours depending on the design of the engine. Some plugs take just the right size of ratchet to fit into tight spots. Take photos to remember how parts of the engine go back on if they need removed to make the job easier.^{[7]
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Research source} If you have never changed spark plugs before you should consult How to Change Spark Plugs in a Car
- Always replace spark plugs one at a time to avoid backfiring.
5
Hook your battery terminals back up. Remember to do this in the correct order. First hook up the positive cable (usually red) and then the ground wire (usually black).

Method 5

Method 5 of 5:

Checking Your Car’s Timing

1
Find the timing mark on your engine. This is usually located in a small gap in the transmission bell housing. You are looking for a gap with short marks perpendicular to it. The marks will be numbered to eight or twelve, with zero in the middle. The word “Before” and “After” may also be stamped into the metal near the gap.
- Sometimes the gap is covered with a plastic or rubber cap. This keeps dirt out of the bellhousing.
2
Identify spark plug number one. This is the plug you should use to check engine timing. You can look in your owner’s manual or service manual if you are unsure which spark plug is number one on your vehicle. It is not necessarily the first one on either end of the block.
3
Set your park brake. Make sure that your car is parked safely and that it will not move while you are working.
4
Turn on your engine. You should let the engine warm up before checking the timing. This will yield the most accurate results.
5
Connect your timing light to your number one spark plug wire. Clip the lead over the plug wire and turn on the light. Be sure that you are using the number one plug, otherwise your readings will be incorrect.
6
Aim the timing light at the timing mark. When the spark plug fires, it will cause the strobe on the light to flash. This will show you what number the timing mark is on at the time the piston fires.^{[8]
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Research source} Record these numbers.
7
Interpret the results of your timing test. The numbers represent degrees from top dead center (TDC) of the piston in the number one cylinder. What this means, is that the mark indicates how far from the top the piston is when the spark ignites the fuel. If your numbers are in the range specified in your owner’s manual or service manual, your timing does not need adjusted.^{[9]
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Research source} If not, you will need to adjust your timing to fix the knocking.

Community Q&A

Question

How do I stop my car from knocking?

Community Answer

Follow the instructions listed in the article above.
Question

What should I do if I can't get the knocking to stop?

Doug Fontaine

Community Answer

If your car has a rod knock, chances are are this engine will need very expensive repair or replacement.
Question

My car just started stalling and making a knocking sound. It had no oil, so I topped it off, but it still rattles. Can it be saved?

Community Answer

It's done; it needs a new/rebuilt engine. Your rod bearings and main bearings are probably gone.

Warnings

Knocking is not fixed by switching to synthetic oil. IF your knocking is a result of an oil issue, it is that your car is very low on oil and you should add oil IMMEDIATELY to avoid serious damage.^{[11]
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If the procedures listed above do not cure the knock, it may be an indicator of a more serious problem. Have a professional check your car as you may be having issues with your cooling system, your drive belt components, crankshaft bearings, or flywheel. Only attempt to diagnose or repair these serious problems if you have mechanical experience.^{[12]
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Things You'll Need

Increasing Octane Level
- Octane Booster
Cleaning Your Combustion Chamber
- Fuel Additive
- Engine Flushing Agent
Replacing Your Spark Plugs
- Ratchet
- Spark Plug Socket
- Gap Tester
- Wire Brush
- Fuel Injector Cleaner
Checking Your Car’s Timing
- Hand Tools
- Timing Light