You may hear a scary engine noise called rod knock while your car is idling. It can sound like a loud banging noise that makes your heart skip a beat when you think about the expense of an engine repair. Not all knocks are rod knocks, however.
So what is rod knock, what are the causes, and how do you know the symptoms of this engine problem?
What is Rod Knock?
Rod knock is a low-frequency knocking sound from inside the engine caused by rod-bearing wear or failure. Specifically, the clearance on the vehicle’s connecting rod bearings is now too excessive so there is too much movement.
As the piston is directed to the opposite direction, the metal will now start hitting metal creating the knocks. The knocking will ramp up as load and speed increase.
Rod knock is the sound of one or more of your rods “knocking” against the crank as it change directions and goes through its rotation. The connecting rod (we’ll just refer to it as the rod from now on) is attached to the crank, and between the rod and the crank there is a bearing manufactured from a softer material than the rod or crank.
When the engine is put together there are gap tolerances (measured in 0.001-inch increments) for the bearing to the surface of the crank journal. That small gap allows oil to get in and stay on the surface of the bearing.
That oil film will make a barrier between the two surfaces. Once the bearing starts to wear, that gap will get bigger and the oil will no longer be able to make a sufficient barrier, and before you know it you have rod knock.
But it doesn’t have to be normal wear (which can take hundreds of thousands of miles), damage to the bearing or the crank journal can cause you both premature rod knock. Loss of oil pressure, dirty oil and other low oil pressure situations can ruin your day.
What does Rod Knock Sound Like?
Rod knock means there is a knocking from inside your engine that usually gets louder with speed or with a load.
Well, there are various things that can sound like rod knock (listed below), but if you really do have rod knock, it is important that you understand what that means.
Pro tip: If the knock quiets down after the engine warms up, it is probably not rod knock so check your exhaust gaskets and that sort of thing.
Causes of Rod Knock
Worn connection rod bearings are the most frequent causes of rod knock. Rod knock can be caused by only one thing, but there are several other things that can lead to symptoms resembling rod knock.
Here are several of the things that can cause rod knock, or similar symptoms as rod knock.
#1. Worn Bearings.
The only cause of rod knock per se is worn bearings. When the pistons move up and down in your car’s engine, they cause the crankshaft to rotate, which sends power to the wheels. The bearings are responsible for keeping the piston movement smooth and controlled.
Over time, these bearings wear out and sometimes come out of their position. When the bearings are worn, the piston rods begin rattling against the crankshaft, creating a particular knocking sound.
The only way to repair this is to replace the bearings, which are deep within the engine.
#2. Low Octane.
A detonation knock can be similar to a rod knock sound. If the engine is working as designed, the air and fuel mixture detonates in one place in each cylinder. Detonation knock happens when that mixture detonates in more than one place at the same time, causing the knocking sound.
One cause of this knock is the octane being too low for the engine. If you have a performance engine, you’ll require a higher octane rating than most cars. High octane burns evenly, and that detonation knock will not happen.
This issue is the easiest fix and only requires you to use higher octane fuel the next time you fill the vehicle with gas.
#3. Poor Timing.
Another cause of detonation knock is bad timing. Timing refers to when the spark plugs fire, and it is controlled by the computer.
When timing is bad, the spark won’t fire proper, and you may end up with multiple detonations within the cylinders. Hence the detonation knock. The fix is to fix the timing.
#4. Lean Air/Fuel Mixture.
Detonation knock can also happen when the air/fuel mixture is lean due to bad oxygen sensors, fuel injectors, fuel pumps, or mass airflow sensors. When there isn’t enough fuel and too much air, a lean mixture occurs.
Without enough fuel, the mixture cannot burn quickly enough, resulting in many detonations.
#5. Bad Knock Sensor.
Detonation knock can also be caused by a bad knock sensor. Newer vehicles don’t see this however, since the computer controls the air/fuel ratio, timing and fuel injectors.
The knock sensor will detect whatever circumstances lead to knock, and send a signal to the ECU for a correction to be made immediately. So, if a car is equipped with a bad knock sensor, the computer won’t know to make the correction, and will allow the engine to detonate.
#6. Bad Belt Tensioners/Pulleys.
The last reason you might hear an engine knock sounds like the source of the knock actually has nothing to do with the engine at all. If the accessory belt is not adjusted properly in terms of necessary tension on the engine or if any of the accessory belt pulleys are challenged, that can sometimes create similar knocking sounds.
When the engine spins, the belt spins. The belt is attached to a number of pulleys in the engine compartment, and the belt must be tensioned just the right way in order to operate quietly and smoothly. If the accessory engine belt is loose, your tensioner is not functioning properly or if one of the pulleys is bent it can present the same problem.
Rattling, slapping, and clicking noise can be mistaken for engine knock; to fix this condition it is simply a matter of replacing the belt, tensioner or certain pulleys.
What Are The Symptoms of Rod Knock?
The symptoms of rod knock include:
- Knocking sounds
- Low oil pressure
If a vehicle has a bad rod bearing and is not knocking for some other reason, there are two main symptoms to pay attention to. Here is some more information about these two symptoms of rod knock:
1. Knocking Sounds.
The obvious symptom of a rod knock is knocking sounds. You will probably hear this loud banging noise when you first start the car.
You will also notice it gets louder as you increase the load on your vehicle or push down on the accelerator.
2. Low Oil Pressure.
When a bearing has failed or is starting to fail, you may notice lower oil pressure than normal. This will be most noticeable when the vehicle is first started. If you have a Check Engine Oil light come on in the dash, it will usually let you know pressure is low.
If the light turns off and the pressure goes back to normal after a few minutes, this is a strong indication that a bearing has failed.
Rod Knock Repair Cost
Repairing a rod knock will cost you at least $2,500 or more. On some cars, like a Subaru Forester, the connecting rod repair can be over $5,000, with parts and labor included.
Changing a connecting rod bearing is not an easy job. The broken part is deep inside the engine. You will not only need the connecting rod bearings, but also the cylinder head bolts and engine gaskets and seals.
On top of that, the engine cooler lines will have to be fully flushed. In some cases, you may also need new pistons, connecting rods, crankshaft and camshaft bearings, and a timing chain. With all that, it may be more practical to consider an engine replacement.