Engine Knock On Startup And Then Goes Away? 5 Reasons Why

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It’s Monday morning, and as you start your car for your morning commute, you hear an engine knocking sound coming from under your hood. Oh, boy. This isn’t the first time you’ve heard your engine knock on startup, and then it goes away.

Sure enough, you give it a few moments, and once your car runs for 30-60 seconds, maybe even a minute or two, the noise goes away and your commute goes without further issue. If you notice this when you cold-start your car, there are a few possibilities as to what it may be.

We’ve isolated the 5 most common causes of this. It could be piston slap, sticky lifters, worn accessory components, worn main bearing, or oil drain-back. Now we’ll tell you what you can do about them, so read on for more information.

Reasons for engine knocking:

Piston Slap

Piston Slap Diagram

Piston slap is an extremely common cause of cold-start engine noise. If you’ve clocked over 150,000 miles on your vehicle, you might be well-acquainted with the distinct noise it produces during those cold starts. Yep, that clacking sound is likely the product of piston slap.

Here’s the lowdown: as engines age, the once snug fit between the piston and the cylinder wall gets a bit loose. Instead of the smooth up-and-down motion, the piston does a bit of a side-to-side shuffle, creating a knocking sound.

Now, why does this happen? It could be a few different reasons. Wear and tear on the cylinder walls, an out-of-round cylinder, or just plain engine design can all play a part. In an ideal world, where engines remain in pristine condition forever, you wouldn’t have to worry about such a problem. But moving parts WILL wear out over time.

Interestingly, the noise often mellows out shortly after you fire up the engine. Why? As the piston warms up, the metal expands a tiny bit, tightening up the fit between piston and cylinder wall. If the extra clearance is minimal, the noise might just fade away. Oil pressure also lends a hand, filling in the tiny gaps.

What to do about Piston Slap:

Now, when you experience this engine knock on startup, you’ve got a couple of choices:

One: You can learn to live with it, and limp it along until something major breaks. Sometimes this approach makes sense if the vehicle is not worth dumping a bunch of money into, or you are on a tight budget.

There is a variety of fairly inexpensive additives (especially compared to the price of a rebuild) out there that may help quiet it down a bit and may even extend the life of the engine to some degree.

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Two: There’s the nuclear option – an engine rebuild.

Yes, it’s certainly an investment, but it’s also a chance to give your engine some much-needed love, swapping out worn parts and ensuring everything is shipshape.

If you’re feeling competent, and don’t mind getting your hands oily, a DIY engine rebuild might be in the cards. But, if you’re not familiar with the innards of an engine, it’s probably wiser to let the professionals handle it.

With piston slap, there could be some serious wear and tear, likely requiring machining work. So, keep that in mind as you contemplate tackling the project yourself.

Sticky Lifters Causes Engine Knocking On Startup

Sticky lifters may be reasons for engine knocking

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.

High Viscosity Oil

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’ve recently used a higher weight motor oil than usual, then that could be the reason for the change.

Check Valve Not Present

Another oil-related cause of sticky lifters is using a filter without an anti-drain check valve. Or perhaps the filter is low quality and the valve is stuck open. For example, Fram filters are considered to have a cheaply made valve and often allow oil drain-back.

High Mileage

Yet another possibility is the overall mileage of the vehicle. 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.

If this does not happen each time, and 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, just to be safe.

Improper Repair

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 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. You will need to replace all of them simultaneously.

Worn Accessory Components Cause Engine Knocking On Startup

Another frequent culprit for cold start engine noise, are accessories that are attached to the engine, that get mistaken for engine knock and noise. Worn-out accessories like the water pump, power steering pump, alternator, and exhaust manifold can produce startup noise that often sounds like a knock.

Alternator, water pump, power steering pump
Courtesy of Reddit poster r/fordranger

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 themselves as harmonic noise heard at startup, but then it soon goes away.

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Typically, this occurs because the bearing on one of these pulleys is starting to wear out. Consequently, it generates noise during startup, which tends to diminish as oil circulates and the bearings warm up. The bearings then expand slightly and lead to a smoother, quieter operation.

Additionally, startup noise resembling a ping, knock, or tick may be due to a small leak in the exhaust manifold. While this creates an annoying and easily noticeable sound, it usually subsides shortly after starting.

As the manifold heats up, thermal expansion tends to seal the leak. This eliminates the pinging and contributes to a quieter overall operation. But over time, the leak will tend to get louder as the heat and pressure further warp the manifold surface.

Eventually, it will get too far out of shape and no longer seal back up, even after the engine warms up.

Read: How To Tell If Your Exhaust Manifold Is Leaking

Leaking exhaust manifold causes engine knocking

Worn Main Bearing Causes Engine Knocking On Startup

The main crank bearing often experiences significant wear, especially in high mileage vehicles. The center main bearing wears down, disrupting the proper tolerances around the crankshaft surface.

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As a result, the crankshaft may repeatedly hit the thrust bearing for several seconds until the crank heats up enough during operation. This expansion of the metal fills the gap, and thereby temporarily halts the impact.

This problem tends to reoccur each time the engine cools and the crank contracts slightly. Therefore, you will notice this sound more after long periods with the engine off. Rather, you won’t hear it so much after just a brief stop.

While this issue typically doesn’t cause additional mechanical problems, it can be a noisy nuisance until addressed. To monitor its progression, it’s advisable to check your oil regularly, ensuring the bearing isn’t leaving metal shavings.

Additionally, inspecting the oil pan for any metal shavings can help confirm that there’s no worsening of the situation.

main crank bearing wear causes engine knocking

Oil Drain-Back Causes Engine Knocking

Oil drain-back is a frequent culprit behind knocks or mechanical noise during engine startup. Typically, your oil filter comes equipped with a check valve. This is a special one-way valve that prevents oil from moving backward through the system and draining from the block and filter.

In cases where this valve fails to seal properly or is absent, as seen in some budget filters, the oil is allowed to drain back into the oil pan. Your oil pump then has to circulate oil through the critical parts of your engine every time you start your car. This process can lead to considerable noise for a brief period until the oil circulates and starts lubing and safeguarding the engine components.

Premium oil filters usually feature a high-quality valve that effectively keeps the oil in the engine block, ready for the next startup. Retaining this oil in the engine lessens the impact of the next start, as the moving parts don’t have to run “dry” for a short period.

Oil filter check valve causes engine knocking
Courtesy of Golden Hugeline

Conclusion

So if you’re hearing a knocking sound coming from your engine compartment, but only for a short time while it is cold, now you have a few things you can check. Remember, the culprit could even be more than one of these items.

It’s not uncommon for multiple components to wear out simultaneously and have similar sounding symptoms. That can really throw you for a loop when trying to troubleshoot!

But, just methodically check each part independently and you should be able to hone in on the bad actor. I often use a length of fuel or air hose with a bolt in the end to help me pinpoint the exact location of various noises that pop up.

It’s a cheap diagnostic stethoscope and works great!

homemade diagnostic stethoscope

Related: Low-Speed Accelerating Knocking Sound

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