Rod knock is something you don’t want to hear from your engine. Because it usually means an expensive repair bill in your future. But how do you identify if this problem is affecting your engine?
Rod knock is a rapping noise caused by wear or damage to the connecting rod bearings, and allows them to ‘knock’ metal on metal as the piston changes direction. You can identify the sound as rod knock if it keeps pace with your car’s RPM.
The sound of rod knock is extremely distinct and concerning. However, it can be easy to confuse this noise with issues involving the lifters, accessory belt, timing chain, or even a belt tensioner.
In this guide, we will look at what exactly is causing your engine to make that strange noise. Then we will touch on how to identify the issue, and if it’s even worth fixing.
Let’s dive into this!
What Is Rod Knock?
Rod knock is a term referring to the connection rods ‘knocking’ metal on metal. Depending on the cause of the issue, you may hear the knocking keeping pace with your car’s RPM, whether high or low.
In some case, the check engine light might also make an appearance in response to this problem.
Unfortunately, this can be one of the most expensive mechanical issues to have on a car. But, understanding the causes, symptoms and solutions surrounding rod knock doesn’t have to be difficult.
Here are some of the most common causes of rod knock:
Worn Bearings
As the pistons move, they rotate the crankshaft, which provides torque to the wheels. The bearings are responsible for facilitating the piston movement smoothly and evenly.
Over time, the bearings can wear out, allowing the piston rods to bang ever so slightly against the crankshaft.
And if your engine has already clocked up some hefty mileage, or is consuming oil like it’s nobody’s business –worn bearings might just be the root of the sound you are hearing.
Poor Engine Timing
The sparks that ignite the air-fuel mixture in the combustion chamber need to go off evenly and in a particular order.
When the timing of the firing is off, the mixture can detonate multiple times in the cylinder, creating what’s called a detonation knock.
To rectify this, the root cause of the timing malfunction must be discovered and addressed.
Faulty Knock Sensor
The knock sensor is responsible for—you guessed it—detecting rod knock. When a situation occurs that could lead to knock, corrective action can be taken straight away by the ECU.
When the sensor is malfunctioning, the system may allow the engine to continue knocking. However, a faulty sensor will trigger a check engine light alerting you to the problem.
How Do You Determine if The Sound Is Caused by Rod Knock?
If your car’s engine goes from making a knocking noise when cold, to being relatively quiet when it warms up, chances are you don’t have rod knock.
But instead, your vehicle may have a case of piston slap, an exhaust gasket leak, or issues with the accessory belt. While less severe, they could still be causing trouble under the hood.
Fact is, the sound of rod knock gets louder and more pronounced as you drive the vehicle. It’s a persistent rattling sound that is most noticeable when the engine is accelerating.
Whereas the sound from piston slap will lesson and almost disappear when the engine is warmed up or at higher RPMs.
What Can be Mistaken for Rod Knock?
There are various types of engine noises that can be mistaken for knock. The most common is lifter tick.
Lifter tick can result in a loud sound from the engine and cause damage to the lifter when the camshaft or push rod fails to connect continuously with it.
Another type of noise is piston slap, which also arises from a combustion engine.
In addition to these, there are sticking or ticking valves, worn water pump bearing, worn alternator rotor bearings, and bad exhaust leak, which can also produce different types of sounds from the engine.
It is crucial to diagnose and address these noises promptly to prevent further damage to the engine.
How Long Can You Drive With Rod Knock?
Truth is, when an engine begins to make knocking sounds, there’s a possibility that a connecting rod may break suddenly.
But, if the knock isn’t very loud, and is only apparent when the car is idling, you may get several months to a year out of the car. Just keep an eye on the oil level.
Nevertheless, the engine will inevitably fail, and leave you stranded. This may not happen for several months, or could very well happen the next time you drive the car.
It’s our recommendation you get the vehicle looked at by a mechanic as soon as possible. A mechanic will be able to determine how bad the knock is, and if it’s safe to continue to drive.
Can I Drive a Car With Rod Knock?
Generally yes, but depends on how bad the knock is. For example, if the knock sound is only noticeable while the car idles, and the vehicle runs as expected, it safe to drive.
However, if the knock is causing the car to shake and have trouble holding a steady idle, then no it’s not advisable to drive the vehicle.
What Does Rod Knock Feel Like?
Rod knock will feel like a stumble and vibration, and the vehicle will have much less acceleration power.
How to Tell The Difference Between a Rod Knock And a Lifter Tick?
Distinguishing between a knock and a lifter tick can be very tricky. Listen for a deep knocking sound that increases with speed. This is the easiest way to determine if it’s a rod knock.
However, a lighter ticking noise that remains constant or slightly increases with speed might suggest a lifter tick.
Is a Rod Knock Worth Fixing?
Yes, if the vehicle is in good condition and has been well-maintained–it may be worth fixing.
However, if the vehicle has a lot of mileage, is 10 years or older, and its value is low, it’s not worth the cost to repair the issue.
Truth is, repairing a rod knock requires pulling the engine, machining, replacing worn or broken parts, then reassembly. And it just may be more cost-effective to replace the engine or the entire vehicle instead.
Ultimately, the decision of whether to fix the engine depends on your individual circ*mstances and priorities. Just weigh the costs and benefits carefully before making a decision.
Does Rod Knock Mean New Engine?
Not always, as the engine can be repaired. And the costs to repair the engine may be less expensive then a getting a whole new engine.
But it all depends on how severe the damage is from the rod knock, and if the engine builder will provide a warranty.
But then again, a new engine will almost always come with a warranty.
Final Thoughts
Rod knock can lead to decreased engine performance, excessive fuel consumption, poor oil pressure and accelerated bearing wear. So, don’t wait long to get it checked out.
Knowing the signs of rod knock can save precious time and money. And while it can be frustrating, it’s not easily fixed problem.
In most cases, it’s best to let your trusted mechanic handle the repair of this one. Fact is the components responsible for rod knock are well out of DIY territory.
Christopher Rossen
Managing Editor
Christopher is an automotive technical writer. When he’s not at the local autocross event, he can often be found working on one of his cars. Specializes in automotive class action law, industry trends, and automotive maintenance. Email me direct, or learn more about us
