Your Engine Knocks At Start Up And Then It Stops – Here’s Why!

Have you started experiencing the problem with the engine knock on startup, then goes away? If that is the case, you are in the right place, because in this article, there will be a lot to cover on this problem.

Any type of knocking coming from the engine should be something that should worry you. Why are we saying this? Well, audible symptoms should not be ignored because if you ignore them, in the worst case, your engine can be totaled pretty quickly. And this is why we are here to help you determine how to tackle this problem.

First, we are going to cover the reasons why engine knock on startup appears and the symptoms of this problem. Then, we are going to elaborate on what you can do to sort out this problem. So, without further ado, let’s dive into the article.

Reasons for Engine Knock On Start Up

Now let’s cover the reasons for this problem with the engine knock on startup then goes away. It is worth noting that there are multiple reasons behind this problem. In the following chapters, we are going to elaborate on them in detail and see what could be causing this problem in the first place.

Timing Chain Problems

One of the most common causes for this situation, when your engine knocks at startup, is the situation with the timing chain.

Timing chains often use very complex routing and they are concealed deeply under the timing covers.

Some cars have only one, while some cars have more than one of these chains. And what can happen is that the chain can stretch.

Chain stretching is the most common cause of rattling or knocking noises appearing, especially on engine startup. So, if your engine starts rattling and there is noise from behind the timing cover, you need to address this problem.

If you don’t address the problem the chain can jump and your engine can be completely totaled.

Also, the timing chain tensioner can fail and the timing chain guides can crack on some engines and create knocking or rattling noises.

Fixing this problem can be on the expensive side but sometimes this is the price we have to pay in order to bring the engine back to top shape.

Since if you don’t do this service as we noted, the engine will get some serious damage in case the chain jumps. In this scenario, the pistons and valves will collide and a lot of damage will occur.

Using Low Octane Fuel

Another common cause for engine knocking noises, especially at a cold start is low-octane fuel. So, why is this the case?

Well, not every engine likes every gas, in other words, not every gas is the same. You have 87 octane, which is the Regular, the Mid Grade which is 89 or 90 octanes and you have Premium, which is 91 to 94 octanes.

Octane numbers don’t mean that the higher the octane count the faster the gas ignition. Quite opposite in fact. Lower octane fuel ignites faster and causes something known as an engine knock. The higher the octane rating the higher resistance to compression.

So, if you have a higher compression engine that is intended to use Premium fuel, you should always use Premium. If you use gas lower than 90 octane, the engine will start to knock because the gas will ignite prematurely.

This is why you should always fill your car with the right gas and avoid this type of situation.

Piston Slap

Another very common cause for a knocking sound at startup is something called “piston slap”. So, what is a piston slap in the first place?

Well, piston slap is a situation where the side portion of the piston is slapping against the cylinder walls. This piston slap often appears on cold start and only affects vehicles with higher mileage. More than 150,000 miles (ca. 241,402 km) to be more precise.

This situation happens because the tolerances between the piston and the cylinder have increased and there is a lot of play between the piston and the cylinder wall.

On disassembly of these engines, you will notice how the piston skirts are worn out on the sides. This uneven wear makes the piston move around at a lower RPM.

This is why the piston slap is most common on cold start and whenever the engine runs at low RPM. Once you rev up the engine, the sound will go away.

What is unfortunate is that this problem cannot be properly diagnosed unless the engine is fully disassembled. Only by disassembling the engine, you will be able to see if the pistons have this wear (high labor costs).

And if the engine is well above 150,000 miles, this is one of the probabilities. In most engines, this situation is not feasible to repair, and you will be better off installing a used engine instead of rebuilding this one. Now let’s move on to the next reason for the engine knock on startup then goes away problem.

Valve Lifter Problems

Another common cause for this problem when the engine knocks at start up are problem with lifters. But what are lifters?

Valve lifters are the components located on the valve train. The lifter is between the camshaft and the rocker arm. On one end of the lifter, there is a roller that is in touch with the camshaft and the other end of the lifter contacts the rod.

They have only one purpose, and that is to help the valves open and close correctly. Each valve has a lifter, so on a V8 engine, you will have 16 lifters.

And if one of them breaks from wear and tear, it could end up rubbing the camshaft. And in this situation, you will notice a knocking sound from the engine.

But in most cases, this sound can be described as ticking. Since it is a lot more toned down compared to knocking. In more extreme cases of wear, there could be strong knocks coming from these components.

This is why we advise that you check these components out. This can be really easy if you use an old-school pushrod V8 engine.

The downside in these cases, when the lifters are bad, is that the camshaft also has to be replaced with a new one because these lifters have probably damaged the lobes to a great extent that is unfeasible to refurbish the camshaft.

Rod Knock

Rod knock is also very common in engines, the rods simply start to knock because there is too much bearing wear.

When this problem occurs for the first time, the knock can appear only on startup and after the engine lubricates properly, the sound will disappear.

But it will be very soon that the engine starts to knock more extensively and in this case, you will have to replace the whole engine or perform a rebuild.

In most cases, this damage is unfixable and owners decide to purchase a used or brand-new engine. So, this is why it is important that you detect and diagnose this engine knock on startup then goes away problem on time and sort the issue before it totals your engine.

Carbon Deposits

Carbon deposits are another big reason why the engine starts to knock. So, what are carbon deposits, and where do they appear?

As the fuel burns, there are some carbon leftovers, these are black particles that stick like glue to the piston top and the valves specifically.

Every engine has a small amount of carbon after it first fires up. And this should stay this way. But on some engines, this carbon problem is more serious.

Especially engines that tend to leak oil from the valve stems or engines that use direct injection. They can mess up the valves pretty easily.

And whenever there is a problem like this, the engine could start knocking because the valves are not closing correctly. There is simply too much carbon on them and they will cause this problem when you have engine knock on startup then goes away.

The only way around this problem will be to refurbish the cylinder head. This can be expensive but it has to be done.

Main Bearing Wear

Main bearing wear can also trigger this problem. But what is the main bearing in the first place?

A main bearing or bearings are the ones that basically keep the crankshaft in place. So, there are bearings for the rods that tend to wear and cause rod knock.

But there are also bearings for the main caps that secure the crankshaft to the block. These main bearings can also fail and cause problems like these.

Just imagine how unhappy the crankshaft will be if it’s out of balance. It can produce more vibrations and also produce knocking noises on startup.

Oil Drain Back Valve

Oil drain back valve failure is also a very common reason for this problem. So, what is the oil drain back valve?

An oil drain back valve serves the purpose of preventing oil from going out of the filter while the engine does not work.

This valve is attached to the filter itself. And what can happen is that it can fail and cause knocking noises and also engine damage in some cases because your engine will be deprived of oil at startup.

So, if you notice these knocks, it is a good idea to flush the oil and replace the oil filter with a new one.

Symptoms Of Engine Knock On Startup

Now that we covered the most common causes for this engine knock on startup, let’s move on and discuss the symptoms associated with this problem.

As you probably know, the engine knock is a tell-tale symptom that you have something wrong with the engine in the first place. But there can be more.

More specifically, the engine can perform quite poorly. It can produce low power and feel really sluggish. The fuel economy will drop quite significantly and in some cases, when this problem prevails, there can be catastrophic engine damage.

This is the case when you see a big hole on the side of the block or the case when the rods simply destroy the bearings and the engine completely seizes. You just don’t want to end up in this situation.

How To Fix Engine Knocking At Start Up

Fixing this problem will depend on the root cause. This is why the problem needs to be properly diagnosed by professionals.

What you can do is to make sure that there is no problem with the fuel. This is the case when the fuel ignites prematurely. You can fix this problem by adding premium fuel instead of low-octane gas. You can also do an oil flush and hopefully, this will sort out the problem if it’s caused by the drain back valve.

In other cases, when there are timing chain issues, carbon buildup, rod knock, main bearing failures, and bad lifters, the engine has to be disassembled.

All of these causes are triggered by internal engine wear except for the timing chain problem which is considered normal wear and tear.

Fixing all of these problems is expensive because there are a lot of working hours to be sorted out. This is why if you determine that the engine is damaged, it is important that you consider replacing it if the damage is simply too big.


Why Does My Engine Knock When Cold Then Stops When It Warms Up?

There can be a number of reasons behind this problem.

Here are all the reasons for cold engine knocking sound:

  • low octane fuel
  • timing chain problems
  • piston slap
  • rod knock
  • main bearing failures
  • bad valve lifters
  • carbon buildup.

Many of these problems are very serious and also expensive to repair.

Can Engine Knocking Be Fixed?

Yes, it can. The easiest thing that you can try is to use premium fuel if the engine is intended to run premium gas. Since this is one of the main causes of knocking. If this doesn’t solve the problem, you probably have internal engine damage that can be expensive to fix. In some cases, it is cheaper to replace the whole engine instead of repairing the old one.

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