5 Reasons Your Engine Knocks on Startup Then Goes Away

Well, imagine one of those busy Mondays after a weekend filled with fun and bonding with family.

You did not have to use your car for the two days you were home.

When you start your car in the morning to work, you hear some knocking from your engine bay.

You give it a minute or two when your vehicle is running, and the noise vanishes, and you then drive to work without any issues.

If your engine knock on startup then goes away, there has to be a problem with your car’s engine.

Here are all the possible causes of engine knock and tips on fixing them.

What Is Engine Knocking?

Engine knocking happens when the ignition of part of the fuel or mixture of air in the cylinder is not from the dispersion of a flame erupted by the spark plug. (Source)

Still, the ignition comes from air pockets or a mixture of fuel blown out of the envelope of the standard front combustion.

The fuel-air should be ignited by a spark plug only at a particular position in the piston’s stroke.

Engine knock occurs if the combustion process peak does not occur at the optimum point of the four-stroke cycles.

Thus, the characteristic metallic “pinging” sound is created by the shock wave, and cylinders pressure increases dramatically.

It is important to note that knocking is not pre-ignition though pre-ignition can be followed by knocking.

What Does An Engine Knock Sound Like?

The engine knocking sound is the “pinging” metallic noise similar to shaking metal balls in a tin can.

However, when accelerating, some cars can have little engine knocking noise.

The sound is metallic tapping noise that results from the engine compartment and is likely to change with speech.

Some causes of engine knock sound include; building up of carbon in the engine, fuel usage with low octane, and a poorly tuned engine.

Is Engine Knocking Bad?

Yes, to some extent, Engine knocking is terrible; it can cause unnecessary expenses.

The damage caused by engine knocking on piston surface, crankshaft bearing, and cylinder walls is costly.

Fortunately, engine knock is not a rampant issue with modern vehicles since the computer controls the air to fuel ratio, timing, and fuel injectors.

Additionally, modern cars have a knock sensor that detects any engine knock and reports to the engine control unit, and hence gets controlled automatically.

Therefore bad knock sensor can permit the knocking of the engine. So, if your car is modern and has a knock engine, checking the sensor will help diagnose the problem.

Engine Knock on Startup Then Goes Away Quick Fixes

Here are the causes of engine knocks and how to fix them promptly

1. Piston Slap

Piston slap is a prevalent cause of engine knocking on startup. It does not occur to engines of less than 150,000- 20,000 miles.

A piston slap occurs when the piston and the cylinder wall lack a new engine’s tight tolerance, so pistons move side to side at an angle in the cylinder instead of a smooth up and down motion. It can occur because of wear of the cylinder walls or due to design flaws.

The pistons and cylinder bore are helpful to correct specifications and tolerance, and then the piston slap cannot occur. The reason is why the knocking sound evades soon after starting.

Fixing: You can do an engine rebuild.  It allows you to do inspection and replacement of any worn-out parts. If you are experienced and comfortable doing engine work, you can do it yourself. However, if you have never done it before, look for a professional mechanic. Though it is expensive, it will ensure no arising of other issues.

2. Main Bearing Wear

The main bearing wears out with time due to high mileage.

When the center main bearing has worn down, the crankshaft lacks adequate tolerance and hits the thrust bearing repeatedly for a few seconds. It happens till the crank heats up adequately from running that it fills the open space stopping the knocking sound for that particular time.

Again, when the car’s engine cools down, the crank contract a little the problem repeats. Thus, it often happens when the engine is off longer and less common when you start the engine after only a slight stopping. However, it does not have a mechanical problem, but the noise is disturbing.

Fixing: Regularly check on your oil and ensure the bearing does not leave metal shavings. You can also drop the oil pan and avoid metal debris altogether.

3. Oil Drain Back

Oil filters have an exceptional valve, known as a check valve. The check valve helps prevent oil from going backward via the system and draining from the filter and the block.

If you don’t seal the check valve correctly or use cheaper filters, the oil pump will move oil via every sensitive part of your engine when you start your vehicle. Thus the knocking sound occurs until the oil circulates fully to protect engine parts.

Fixing: Get a high-quality filter since it will also have a high-quality valve. Thus the check valve will keep the oil in the engine block waiting for the next starting of the engine. When there is oil in the machine, it softens the next starting of your vehicle and prevents running “dry” for a short time hence no noise from knocking the engine.

4. Sticky Lifters

Sticky valve lifters on the engine result from the oil used, condition of oil pump, type of oil filter, and vehicle mileage.

For instance, if your engine is running oil of higher viscosity, the oil takes longer to circulate the engine to cover the valves and lifters. Also, using slightly higher-weight oil when changing your car’s oil can cause the knocking engine.

In addition, the mileage of a vehicle has a significant impact. A car with a worn-out engine will likely have more gaps that only oil can’t fill.

Fixing: If your lifters continuously stick and take longer to stop making noise, it would be best you replace them entirely. If replacing them on your own, you will need more specialized tools and a set of all components. However, taking your car to the mechanic is a bit expensive but worth it. Do not replace one or two lifters; you should replace all simultaneously.

5. Worn Components

When your car has worn accessories like a power steering pump, water pump, alternator, and exhaust manifold can cause a knocking engine when starting up the vehicle.

As these components wear, they show signs of continued failure, and they present some noise that comes when starting up a car and goes away.

The noise goes away after the oil has circulated or the bearings have heated up and expanded a little to make the drive quieter and smoother.

Fixing: Regularly check on your oil level and replace the worn-out components.

Final Thoughts on Engine Knock on Startup Then Goes Away

Have you ever experienced an engine knock on startup then goes away?

The “pinging” sound that gets louder as you accelerate or go uphill is a sign of engine knock.

However, engine knock has a variety of ‘noises’ that can result from individual issues.

Thus, if your car’s engine knocks on startup then goes away, you need to figure out the source of the noise.

While it is cheaper to fix some of these problems on your own, it would be best to visit a professional mechanic if you lack any experience handling engines.

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