Engine Knocking at Startup [Causes & Fix]

Engine knocks on startup affect a lot of cars. It’s not an uncommon issue, but it can be a very serious problem. In this article, I’ll explain the cause of your engine knocking noise. Is it something as simple as carbon buildup or something more complex like bearing damage?

1. Too Much Octane Rating

The primary reason for engine knock is the use of too much octane rating. In other words, your car is taking in more fuel than it can burn, so the resulting combustion process causes a “knock” or “pinging.”

This usually occurs during the initial startup when there’s a sudden increase in engine speed. Most modern cars have self-adjusting timing and compression to correct this problem by reducing the air/fuel mixture and increasing spark advance until the knocking stops.

2. Carbon Buildup on Engine

Carbon buildup can cause engine knock. If your engine is knocking, you should check the oil level and make sure it’s full. Even if you have a low oil level, as long as your car is running, the oil pump will be able to move enough of the fluid through the system to keep everything lubricated and cool.

But if there’s too little fluid in there, it could lead to serious damage over time including carbon buildup on your pistons that results in constant knocking sounds.

If you’re experiencing problems with your vehicle due to an issue with its engine or transmission (such as knocking), it’s important that you bring this problem up with someone who knows what they’re doing right away so they can look into other potential causes before deciding whether or not replacing these components will help solve this problem for good.

3. Oil Drain-Back

This is a common problem among many vehicles, and it’s easy to diagnose. A worn-out engine will allow the oil to drain back into the pan rather than stay in the engine where it belongs. This is called oil “drain-back.” When this happens, you lose oil pressure which can even cause your engine to fail.

The fix is simple: replace your oil pan gasket with a new one from an automotive parts store. This will prevent any future drain-back issues and ensure that all of your vehicle’s parts stay lubricated properly.

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4. Worn Engine Parts

If your car has a worn engine part, it can cause an engine knock. Engine knock is when the parts in one of the engine’s cylinders start to break down and clatter together while they’re supposed to be sliding easily past each other. On startup, this can create an audible “knock” sound – hence the name.

If you notice your engine knocks on startup and then goes away, this obviously means that there’s some sort of problem with your car’s engine.

A worn cylinder head gasket or valve stem seals are commonly known culprits for this condition, but there may be other causes as well. A technician will need to diagnose why these noises occur and help you fix them before they become something more serious like overheating or oil leaks which could cause even more damage down the road.

5. Sticky Engine Lifter

The engine lifter is a part of your engine that lifts the camshaft. It’s made of metal, and it can get sticky over time and cause knocking sounds when you start the car up.

If you notice that this is happening, take your vehicle to a mechanic so they can inspect your car’s lifters for wear or damage. The good news is that if it has become damaged or worn down, it can usually be replaced without having to buy an entirely new engine.

6. Piston Slap

The rattling noise you hear is a piston slap, which occurs when the pistons are moving in and out of their cylinders. A piston slap shows up as a rattle or tapping sound that comes from the engine. It’s usually caused by loose piston rings, but it can also result from the excess clearance between the cylinder walls and pistons.

Pistons are supposed to seal off combustion pressure inside the cylinder so it can push fuel into your engine’s chambers; however, if there is too much clearance between these components (i.e. if one component isn’t tightly fitted), then combustion gases escape past them instead of getting compressed by them. The escaping gases cause a vibration called “piston slap.”

7. Bearing Wear-Out

Bearings are the most common cause of engine knock on startup and they can be replaced. The problem with bearings is that they’re made of metal, and over time, the metal wears out.

When this happens, it’s better to have the engine rebuilt than to just replace one or two parts. The reason for this is that there’s a chance that other parts will fail soon after because they aren’t working as they should be due to wear on the bearings.

Also worth noting: if your car has been sitting for a while (such as during winter or while you were away), it may take some time before your engine warms up enough so that there won’t be any knocking when you start it up again.

Read: Drivetrain Malfunction BMW

How to Fix Engine Knock on Startup

1. Check the Spark Plugs

If they’re worn or clogged, they won’t fire properly and you’ll get misfires. If your car has a distributor cap and rotor, check those too, they can cause ignition problems as well.

2. Check the Fuel Filter

Which filters out dirt from your gas before it enters your engine’s cylinders to keep them clean. You should replace it every 30k miles or so (or sooner if you live in an area with lots of dust or debris). It’s cheap, easy to do yourself, and will prevent big trouble later on down the road if you take care of it now.

3. Check the Air Filters

Air filters catch dirt that gets sucked into your engine while driving along dusty roads or through large cities with poor air quality so they should be cleaned regularly (every 50k miles) using high-quality filter oil lubricant.

This is also something that can be done easily by yourself – make sure never forget this step because otherwise over time these tiny particles could damage critical components inside like valves/cylinders, etc which could lead to expensive repairs later down the road when needed most.

Read: Clunking Noise When Braking

How to Prevent Engine Knock on Startup

1. Change Your Oil and Filter

The average person should change the oil and filter in their car every 3,000 miles or three months. This will help keep the engine lubricated properly, which is especially important when starting it up in the morning.

2. Change Your Spark Plugs

Spark plugs need to be changed at least once a year (you may need to change them more frequently if you drive a lot of miles). Spark plugs burn out over time, causing poor performance and difficulty starting the car when it’s cold outside.

3. Clean or Replace Your Air Filter and Fuel Injectors if Necessary

Dirt buildup can occur inside air filters or fuel injectors over time as contaminants get sucked into these components during normal use of the vehicle even though these aren’t necessarily externally visible components themselves.

4. Check Engine Light Frequently

It’s important to regularly check the status of your engine light. If you ignore warning signs, you could end up in a more serious situation than necessary. Instead of waiting until it becomes a problem, keep an eye on things and make sure the engine light is always clear.

5. Drive Gently

Avoid sudden acceleration, hard braking, hard acceleration, or any other kind of fun with the throttle.

If you’re going to make a turn while driving, don’t do anything too drastic and if it’s unavoidable (such as when you want to turn off onto an exit), avoid making any sudden swerves or off-roading that could jostle your engine unnecessarily.

Read: How to Clean Purge Solenoid Valve


When you hear a knocking noise in your car, it’s often an indication that something isn’t right. This type of problem is difficult to diagnose because the cause could be anything from your engine being too hot to the pistons being worn out, to carbon build-up, bearing worn out or too much octane rating, oil drain back, or worn out engine parts.

Luckily there are several things you can do if your engine knocks on startup including replacing the oil filter, driving slowly, regular maintenance, and changing worn-out parts. These will help you narrow down the possible causes so that they can be fixed quickly before any damage occurs.