5 Reasons Why A Car Engine Knocks On Startup Then Goes Away


It’s Monday morning, you had a nice relaxing weekend where you did not have to even leave the house. Now you start your car to start your morning commute to work, and you hear a knocking sound coming from your engine bay. You give it a few moments, and once your car has been running for 30-60 seconds, maybe even a minute or two, and the noise goes away and your commute goes without further issue. If you notice this each time you do a cold start on your car, there are a few possibilities as to what it may be. We have isolated the 5 most common causes, and what you can do about them, so read on for more information.

Piston Slap

Piston slap is an extremely common cause of cold start engine noise. It generally will not develop in engines that have less than 150,000-200,000 miles on them. Piston slap is what happens when the piston and the cylinder wall no longer have the tight tolerances of a new engine and the piston “rocks” side to side on a slight angle inside the cylinder instead of smooth up and down motion. 

This can happen due to physical wear on the cylinder walls, the cylinder being out of round, or it can be a design flaw. If the pistons and cylinder bores are set up and machined to the correct specifications and tolerances, the piston slap should not occur. This is one of the reasons why the sound of the slap will go away a short time after starting. If the excessive clearance is minimal, it will also frequently go away as the piston head heats up and expands. 

The proper oil pressure will often be enough to fill the excessive clearance, but if not, there are two options, you can deal with it and baby it, or you can do an engine rebuild. You can certainly just deal with it for a time, as the piston slap itself is not a potentially catastrophic failure, but the damage with the piston skirt and the cylinder walls can be. The rebuild, while much more expensive, will also allow you to inspect and replace any other potentially worn parts. If you are experienced in engine work, you may be able to do this yourself, but if you have not done it previously, you will probably want a professional to perform the work to ensure there are no issues.

Sticky Lifters

Sticky valve lifters are a common source of cold start noise that is frequently referred to as a knock, but it is more of a loud ticking sound. Your engine can develop a sticky valve lifter or two due to several factors. They can create some nuisance noise because of the type of oil that is used, the type of oil filter, the condition of the oil pump, overall mileage, cam position on stopping, and even any recent work that involves moving them.

If you are running oil that is a higher viscosity, it will take the oil longer to circulate in the engine, providing coverage to the valves and lifters, and other components. If you have recently done an oil change and used a slightly higher weight motor oil than you usually use, then that could be the reason for the change. Another oil-related cause of sticky lifters is using a filter without an anti-drain check valve on it, or a higher quality filter that has a better valve. For example, Fram filters are considered to have a cheaply made valve and often allow oil drain-back.

Another possibility is the overall mileage of the vehicle, as a worn engine with several hundred thousand miles on it will have more gaps and tolerance shortfalls that oil may not fill. This can lead to mechanical noise. Good stop-gap measures in this scenario would be running a heavier oil to cushion the components more. Occasionally the position of the cam when the engine is stopped can cause a lifter or two to drain down, but this will not happen each time, so if you notice the noise as a one-off or seldom occurring noise, you may not have to worry about it, but it is still prudent to have it looked at.

One last possibility for determining if the lifters are sticking is if you have done any recent work on the valve cover components. The lifters, rods, and springs all wear in sets, so care must be taken to keep each set together, reinstalling them in the same location with the same partner parts can eliminate a lot of subsequent issues with fit and noise.

If your lifters are sticking, and the time it takes for them to stop making noise is getting longer and longer, then it may be time to entirely replace them. This will be somewhat expensive if you take your vehicle to a shop or mechanic. If you do decide to replace your valve lifters, springs, and pushrods yourself, you will need several specialized tools and a full set of components. Do not try to simply replace one or two lifters at a time, they will all need to be replaced simultaneously.

Worn Components

Another frequent culprit for cold start engine noise, are accessories that are attached to the engine, that get mistaken for engine knock and noise. Having worn accessories like the water pump, power steering pump, alternator, and exhaust manifold can produce startup noise that is often described as a knock when relaying symptoms to a repair agent. Many of these noises can be considered normal operation and will function sufficiently until complete failure.

As they wear, things like the water pump, power steering pump, and the alternator can all display signs of imminent failure. These signs often present as harmonic noise that is heard on startup but soon goes away. This is usually due to the bearing on the pulley for one of these beginning to go bad, and so it makes noise during startup that will usually go away once oil circulates or the bearings heat up and expand slightly, making the operation smoother and quieter.

There are also reports of startup noise that sound like a ping, a knock, or a tick when there is a small leak in the exhaust manifold. Not only will it lead to a nuisance sound that will be very noticeable, but it will generally go away shortly after starting. Once the manifold heats up, thermal expansion should seal the leak, eliminating the pinging, and quieting the overall operation.

Main Bearing Wear

The main crank bearing often wears significantly with high mileage as well. The center main bearing wears down and the crankshaft no longer has the proper tolerance and repeatedly hits the thrust bearing for several seconds, until the crank heats up enough from operating that it fills the open space and stops the impact for the time being. Once the engine cools and the crank contracts slightly, this problem will reoccur. This would mean that it would be more likely to occur after long periods with the engine off, and less common when starting the engine after a brief stop.

This will not generally cause any other mechanical issues, and will simply be a noisy nuisance until it is fixed. One way to ensure that it is not worsening is to keep an eye on your oil and make sure the bearing is not leaving metal shavings, or even drop the oil pan and make sure there is no metal debris at all.

Oil Drain-Back

Oil drain-back is a very common cause of knocks or mechanical noise during engine startup. Usually, your oil filter will have a special valve, called a check valve, that prevents the oil from moving backward through the system and draining from the block and the filter. 

When this valve does not seal properly or is not present, as in some cheaper filters, then your oil pump must pump the oil through all the sensitive parts of your engine every time you start your car. This can cause significant noise for a short period until the oil is circulated and begins cushioning and protecting the engine parts.

With high-quality oil filters, the valve is also high quality, and keeps the oil in the engine block, ready for the next time the engine is started. Keeping this oil in the engine softens the effects of the next start since nothing has to run “dry” for a short period. 

With cheaper oil filters, this check valve is often cheaper or left out of the manufacturing process entirely. This means when you shut off your car, the oil drains out of your engine and collects in the oil pan, and the drain sump must completely recirculate the oil each time the vehicle is started. When this happens, you can hear significant knocks and noise while the oil is pumped to all parts of the engine.

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