Reason Why an Engine Knock on Startup then Goes Away [Solution]


There is a tendency to believe all is well when the noise you hear from your engine after igniting dies again. This can result from minor issues such as stocky filters, carbon buildup, piston slap, worn-out vehicle constituents, or engine parts.

You should be concerned if your engine knock on startup then goes away. This article will guide you on how to identify the problem and what you can do to resolve it. Moreover, you will also learn the cost of fixing an engine knock issue and essential points to note.

How do you diagnose engine knocks?

It would help if you recognized an engine knock when you heard it. An engine knock is described as weird “knocking” sounds from your car’s engine when driven or after a startup. It could also occur when accelerating your engine.

How do you diagnose engine knocks

Instead of the usual low reverberating sound you hear when starting the car, the engine knocking causes a repetitive beating or clanging sound. This occurs when the air-fuel combination is incorrect, triggering fuel to burn in irregular pockets rather than gusts.

Subsequently, it will damage the cylinder wall and piston if prolonged. In addition, knocking can also begin due to a lack of lubrication in the upper cylinder region. Finally, unfamiliar sounds from your engine indicate a problem. However, your engine could also rattle to call your attention; here is why.

Why does the engine rattle on cold start?

A cold start refers to turning your vehicle’s ignition after being idle for long, under 50 degrees Fahrenheit, or as low as zero degrees, depending on the environment. The denser the air is, the more challenging your car engine works to yield power so your car can accelerate.

A rattling noise on a cold start could be due to an oil problem or a tear of the variable valve actuator O-ring seal. The oil may lack the viscosity property needed to lubricate and guard your engine components. It could also be due to no oil at the top of the engine when you crank it.

The variable valve actuator needs oil to move the camshaft. Without lubricant, the actuator would insert a pin and lock its position. Eventually, the actuator develops a fault and makes a rattling noise.

The actuator remains in one position when you start the car due to the lock. Ordinarily, the rattling noise goes away after two seconds. However, if the noise persists, the actuator is possibly damaged and needs a replacement.

In addition, your engine’s efficiency can be affected by the ignition-system knocking and other related issues. A deeper look into the common causes of knocking will enlighten you on what goes on in your engine.

Causes of Engine Knock-In Your Car

Engine knock arises when fuel is kindling in the engine cylinder head. This causes a piston in the engine to misfire since the timing is inaccurate. In addition, the piston returns a backward thrust to your crankshaft, therefore pushing against the natural drive of your engine. The following are common causes of engine knock in your car.

  • Low Octane Rating of Fuel
  • Fuel-burning restriction
  • Wrong spark plugs
  • Imbalanced Air-Fuel Combination
  • Non-lubricated Cylinders

Low Octane Rating of Fuel

Using fuel with a low octane rating can produce a knock. This measures the fuel’s ability to counterattack the engine’s early explosion of the air-fuel mixture. In addition, it explains how prone the fuel is to engine knocking while the internal combustion chamber compresses it.

To prevent the “knocking” or “pinging” sound you hear, use gasoline with an octane fuel rating of your manufacturer’s recommendation. An octane booster can help reinstate the correct octane rating and stop the knocking.

If you drive a vehicle that requires a highly compressed fuel and air mixture, then use fuel with a high octane rating of 91 or above. Otherwise, the air and fuel mix will explode before the spark plug creates the spark.

Fuel-burning restriction

Vehicle gasoline is required to have carbon cleaning detergent, but this does not avert carbon deposits from forming. Gas and diesel contain multiple carbon molecules, which allow their remnant buildup on valves, spark plugs, and other components participating in combustion.

This decreases the volume in the cylinder and raises the amount of compression. In most cases, using a special fuel injector cleaner or additive to clean carbon deposits on engine components can resolve this issue. Regular maintenance done by a mechanic could work as well.

Wrong spark plugs

Spark plugs that the manufacturer does not recommend could lead to engine knock noises. They have specific heat ranges and different resistance levels to hotter temperatures withdrawn from the combustion chamber.

Spark plug in bad condition

If you use a spark plug that cannot bear your engine’s heat, it will malfunction. In addition, engine knocking can also occur when the spark plug gap is not appropriately set. The spark plug gap is where the spark plug explodes the air-fuel mixture that powers the car.

When the narrow gap created is too feeble to perform this task, the engine could go bad. On the other hand, a wide gap may halt a spark from firing or fail too quickly.

Imbalanced Air-Fuel Combination

When the internal combustion chamber does not receive proper air and fuel combination equilibrium, fuel will burn haphazardly. If this lingers, your cylinder wall and piston might have gone bad.

This will develop strange knocking noises from the cylinder wall and piston. Consider replacing these components immediately to prevent further damage.

Non-lubricated Cylinders

Lastly, the upper area of the cylinder head needs to be lubricated appropriately to prevent metal-on-metal contact. This occurs due to wobbly lifters or valves not receiving the correct amount of oil required.

Check to confirm that your oil is flowing accurately to the cylinder head area; if not, you may have an oil leak. In addition, verify your engine’s status and attempt to resolve an engine knock promptly to prevent further damage to your vehicle.

How to fix Engine Knock Problem In Your Car?

Before incurring a massive bill at the mechanic’s, there are a few steps you can follow to get rid of the knocking yourself.

  1. Purchase better quality fuel that matches your manufacturer’s recommendation. Confirm the octane rating before use. For example, 87 octane fuel is fine for regular vehicles, while high compression or high-performance vehicles require 91 or even 93 octane fuel.
  2. Flush out dirty cylinders stained with carbon residue from carbon components of gasoline. Small amounts of carbon deposits remain on your cylinders and build up over time as fuel is used. Do a periodic flush before the buildup becomes too severe.
  3. Add a cleaning additive or soap to your gasoline. The popular one is a fuel injector cleaner which cleanses the residue from the combustion chambers and injectors.
  4. Verify that you are using the correct spark plugs. Please take out the spark plugs and note down the model numbers printed. Compare these to the numbers listed in your owner’s manual. If they align, you are good to go, but replace them with the recommended ones if the spark plug model number does not match. Also, clean them if they are filthy.
  5. A simple habit of changing your oil can fix an engine knock problem. However, if your engine is not well greased, it can discard the scheduling of many of your engine’s parts and result in complications.

Important Note: An engine knock can scare you because it requires expensive repairs which you think you cannot afford. However, the best decision is to address it earlier to prevent accumulating expenses.

How much does it cost to fix an engine knock?

Replacing the spark plugs in a car can repair engine knocking. This can cost a couple of hundred dollars and will be done quickly. But allowing your engine knock to be prolonged would develop into a more extensive repair job.

You could find yourself with worn-out engine bearings costing a couple of thousand dollars, if not more. However, you should expect to spend $2,000 – $5,000 for a rebuild or as low as $250 -$500 for carbon buildup cleaning and engine oil/oil filter changes.


Can You Drive a Car With a Knocking Engine?

You should not drive your car with a knocking engine to avert danger.

Can engine knocks disappear

Specific engine knock causes have simple fixing solutions. However, it does not promise the knock will go away as there might be more than one cause; pay a visit to your mechanic for further diagnosis.

Wrapping Up

You have understood what the strange sounds you have been hearing are and why it needs your attention. An engine knock on startup then goes away is no more a riddle, and you can resolve it at once by following the steps on how to fix the engine knock problem in your car. You will preserve the life of your vehicle by paying more attention to your engine issues.

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