What is Engine Knocking and How Can I Fix It?

Do you hear a strange clunking or knocking sound coming from your engine while it’s running? This can sometimes happen when your engine is at idle, while revving up, or even while rolling along smoothly. In fact, it could also be any combination of the three. This is an issue known as engine knock, and while it’s becoming increasingly rare in modern vehicles, when present, it’s still a difficult issue to fix. Running your car with engine knock leads to excessive wear and could even lead to engine failure.

In this blog, originally published in 2013 and updated in 2020 to reflect technical and industry updates, we’ll discuss what causes engine knock, how to avoid it, and what to do if you start to hear it in your vehicle.

Common Causes of Engine Knock

What causes the knocking sound coming from your engine? There are several answers to this question, but there is one that is particularly common: a bad air/fuel mixture. Your engine depends on two primary ingredients in order to fire and create the power that propels you forward: oxygen from the air around you, and fuel that can be ignited. When exposed to a spark, the ignited fuel consumes the air, creating the explosion. However, an improper mixture will cause a weaker, louder explosion that can be heard as a “knocking” sound.

A bad air/fuel mixture can also be caused by interrupted or bad timing. Timing refers to how the various parts of the engine cycle interact with one another—the pistons traveling up and down, the air/fuel mixture being brought into the cylinder, the exhaust being let out, and so much more. This is all controlled via a number of mechanical components, including your timing belt and crankshaft. Should any of these features break, bend, or otherwise adjust, the timing between all of these different sequences could be thrown off, causing your engine to run poorly or even stop working completely.

The timing on modern vehicles is computer-controlled, and the air/fuel mixture is adjusted instantly to ensure the best horsepower and performance, making fuel-related engine knock almost unheard of. However, other types of knocking can arise, including “rod knock,” which is typically a result of a bad bearing that connects the piston rod to the crankshaft. A computer can recognize this issue, but can’t automatically correct for it, as parts will need to be adjusted or replaced deep within your engine.

How to Repair Engine Knock

Engine knock isn’t always the easiest car problem to repair. Here are a few of the most common solutions to knocking issues.

Timing belt replacement: Most smaller cars, including most four-cylinder sedans, will use a rubber belt to regulate and control your engine’s timing. Over time, all rubber belts eventually stretch, corrode, or wear out to the point where they simply don’t work anymore. We strongly recommend replacing your timing belt at your manufacturer-recommended interval to prevent this from happening. Replacing a timing belt is a pretty significant service that includes readjusting your timing to line everything up properly once again. It’s not something that the average person typically has the knowledge or skill to do themselves. Additionally, most larger vehicles utilize a timing chain to keep your engine running smoothly. These are significantly more durable.

Change your fuel: One of the reasons you could develop a fuel-related knock problem is because you’re using a fuel that doesn’t have the required octane for your vehicle. Many high-compression or high-performance engines require premium fuel to run as intended. If your vehicle requires premium fuel and you discover a knocking noise coming from your engine, you should switch to premium fuel right away. You’ll likely still need to bring your car in for service, but this could prevent the issue from getting any worse.

Replace engine bearings: Replacing engine bearings is an extremely labor-intensive repair. This involves getting deep into your engine and resetting bearings that may have become misplaced or dislodged due to vibration or other faults within your engine. This requires substantial skill and considerable time to complete, however, it can restore your engine to proper operation. We strongly recommend leaving this service to a professional.

Replace engine crank: One of the mechanical reasons your timing could be off is a damaged or broken crankshaft. A broken crank prevents pistons from firing at the right time and could cause everything from bad bearings to secondary explosions and a myriad of other issues. Replacing your crank is also an extremely intensive engine repair that should always be left to a professional.

About Christian Brothers Automotive

For more than 30 years, Christian Brothers Automotive has provided professional automotive care and repair services. Whether you need a simple maintenance service or a knocking problem fixed, our technicians deliver quality results and fantastic customer service each and every time from our convenient, easy-to-access locations.

Call your local Christian Brothers Automotive to schedule your appointment.