The sound of an engine making an unexpected noise is always concerning, and a persistent metallic rapping can signal a dire internal condition. This specific sound, known as rod knock, is one of the most serious mechanical noises an internal combustion engine can produce.
It indicates that the tight operational tolerances within the engine’s rotating assembly have been compromised, requiring immediate diagnosis.
Understanding the source of this noise and the mechanics behind it is the first step toward appreciating the urgency of the problem.
What is a rod knock?
Rod knock simply means a noise or sound of one or more connecting rods hitting or rubbing against the crankshaft as it rotates in the cylinder bores. The connecting rods are connected to the pistons and the crankshaft.
And, between the con rod and the crankshaft, there is a bushing (mainly known as a bearing) made up of soft metal between the crankshaft and the con rod.
By default, car engines are manufactured with clearance tolerance for the bushing to fit in the crankcase journal.
The small clearance allows the engine oil to flow on the metal surface, preventing premature wear and tear. However, over time, the metal bushing between the con rod and the crank will start wearing off, resulting in rod knock.
In any case, rod knocks are not usually caused by normal wear because it takes hundreds of thousands of miles for the bushing to wear off.
Majorly, rod knocks are caused by premature wear. What could cause premature wear? It is usually caused by dirty motor oil, low oil level, low oil pressure, and engine overheating, amongst others.
What is a spun bearing?
Spun bearing refers to the outcome of unfortunate circumstances in the engine cylinder bores. What do I mean?
A spun bearing is the result you get when there is a lack of lubrication in the engine, and the crankshaft rotates with the bearing instead of turning inside the bearing.
This is usually caused by connecting rods or nuts stretching due to improper bolt torturing during an engine rebuild.
Several other factors, such as lack of lubrication, excessive heat, and high operating loads, can cause the bearing to wear. There are several other factors that can cause a spun bearing.
Symptoms of rod knock
There are a handful of symptoms associated with engine rod knock. Recognizing these symptoms can prove valuable when attempting to correctly diagnose the condition at hand, thereby returning your vehicle to service in an expedited fashion.
The following are several of the most common symptoms associated with rod knock.
#1. Audible “Knocking” Noise.
The most obvious symptom of rod knock is the knocking noise for which this condition is named. Rod knock can be quite loud and sounds like a deep metallic hammering noise.
This noise typically increases in parallel with engine RPM, only growing louder as this condition progresses.
#2. Noticeable Vibration.
In severe cases, rod knock can often be felt, as well as heard. This occurs when an affected connecting rod hammers back and forth on its corresponding journal with substantial force.
Once vibration of this type begins, it is only likely to grow more severe with the passage of time and miles.
#3. Oil Pressure Irregularities.
In certain instances, the presence of deteriorated bearings, like those that cause rod knock, can also lead to observable oil pressure irregularities.
This occurs whenever the oil is allowed to pass too freely through loose or worn bearings, in turn presenting flow without adequate pressure.
What Causes Rod Knock?
Without exception, the most common cause of rod knock is substantially worn connecting rod bearings. However, this condition itself can result from a number of underlying issues.
The most common causes of rod bearing wear include oil-related issues, which ultimately reduce bearing lubrication. The following are some of the most common of these issues.
- Incorrect Oil Viscosity
- Blocked Oil Passages
- Low Oil Pressure
- Excessively Hot Engine Oil
Additionally, extreme engine operation can also cause premature rod bearing wear and even crankshaft failure. This includes engine over-revving and sustained high RPM operation.
Engine operation of this type causes an excessive build-up of heat within an engine’s bearings, thereby expediting wear and eventual breakdown.
How to diagnose rod knock
Here are practical, proven steps on how to diagnose rod knock.
Check the state of the oil and oil level
- Locate your engine oil dipstick and pull it out.
- Clean it with a rag
- Check the color of the oil. If the color is muddy or dark, you have dirt or engine oil.
- Reinstall the dipstick and pull it out to re-inspect it.
- Find out the state of your engine oil and level. Dirty engine oil can cause system wear, damage, and engine knocking.
Drain the engine oil and inspect it.
Get an oil catch pan close to avoid messing up the entire garage with oil. Before draining it, look at your engine cap to know the oil viscosity your engine uses. This is important so you don’t replace the old oil with the wrong oil type.
Before draining the old oil, get a transparent container to take one cup of oil so you can take a closer look. After removing the sample for close inspection, drain the rest of the oil.
If you see metal shavings in the engine oil, it is an indication of engine components’ wear. The metal shavings could come from the con rod, crank bearing, or other inner engine components, causing the knocking sound.
If you fail to carry out regularly scheduled oil changes, the oil will get old and start to erode the brass layers on the bearing. This happens mainly on the bearings far from the oil pump because they are getting less oil flow than the bearings closer to the pump.
You should also check the color of the oil. If it is dark brown, the oil is old or dirty and needs to be replaced.
And if the oil is too light, consider replacing it. If you want to know how to extend the life of an engine with rod knock, it’s pretty simple. Always change the oil at the right interval.
How to fix a rod knock
A rod knock is a sign that a problem lies in the interior components of an engine. As we explained above, several factors can cause rod knock; some are expensive repairs, while others are not. This doesn’t mean your engine is beyond repair.
However, before consulting your dealership or mechanic, there are a few steps you can take to reduce the rod knock and possibly stabilize your engine before carrying out a thorough repair.
Step 1: Carry out an Oil Change service
Slide under your vehicle and place a catch pan directly under the draining plug. Get the sizeable socket size and wrench. Locate the draining plug on the crankcase and loosen it to drain the oil.
Drain the oil completely and reinsert the plug (the draining plug is also called a sump bolt)—Torque the sump bolt to the manufacturer’s spec.
Remove the oil filter with an oil filter wrench by gripping the oil filter and turning it clockwise. Fix a new filter and torque it down to the recommended specifications. Replace the engine oil with the same oil viscosity rating or recommended oil.
Keep checking the oil as you pour in the new oil, if you don’t know how many quarts your engine takes. A four-cylinder engine typically takes around 4-5 quarts.
Step 2: Remove the crankcase and inspect the cod rod bearing
A loosened connecting rod cap will cause a knocking noise. Inspect the bolts holding the con rod with the lid and re-torque them. Consider inspecting the cod rod bearing and replacing it as needed.
Step 3: Pour fuel injection cleaner into your gas tank
Grime and dirt might make their way into your gas tank when you refill your tank at the gas station. When the fuel pump is transporting gas to the engine, it sends that dirt and grime along, causing engine knocking.
Cleaning your fuel injection system will clear out those grimes and resolve the rod knock. If you have older vehicles that use carburetors, do not use a fuel injection cleaner. Instead, get a carburetor cleaner and spray it in the carburetor.
How Much Are Engine Rod Repair Costs?
The cost to repair an engine rod knock depends on several factors, including:
- How long has it been knocking
- The extent of the damage
- If it’s a high-performance engine
- If the engine is salvageable
The last thing you want is to find out your engine needs to be rebuilt, which can cost between $2,500-$4,000. Or worse, a complete engine replacement, which can reach as high as $10,000 or more.
How much does it cost to repair engine rods? Typically, it will cost about $2,000 to $3,000 for the parts and labor. The repair job usually requires replacing seals, gaskets, connecting rod bearings, and cylinder head bolts, as well as flushing the engine and cooling pipes.
Though, if the damage is serious, you may need to replace some extra parts, like the pistons, connecting rods, camshaft bearings, timings chains, and perhaps even the crankshaft.
FAQs.
Q: Can an oil change stop the engine knocking?
If your oil pressure is low or you have a low oil level, you’ll likely hear a splattering, tickling, or ticking noise from the engine.
If this noise is from the valves or lifters, adding or doing an oil change will stop the noise. But unfortunately, if the noise is rod knock, an oil change will not stop it.
Q: Will thicker oil stop the rod knock?
As explained earlier, a rod knock is a sound indicating wear and damage to your engine system.
So, changing to oil with a higher viscosity rating (heavier or thicker oil) will help eliminate or at least reduce the rod knock. It will potentially prolong your engine life. This doesn’t mean you should not diagnose and fix the cause of the knocking.
Q: How long will an engine last with rod knock?
If you have a rod knock in your vehicle, you might also be asking, ‘How long can you drive with a rod knock?’ Regardless of your question, once an engine starts to knock, nothing is certain to determine how long it will last.
You can have a total breakdown the next time you start your vehicle, or it can keep working for 3-6 months. However, driving with the rod for a couple of minutes is not recommended. It can blow up your engine and keep you in the middle of nowhere.
Q: Can you hear the rod knock at idle?
Yes, you can hear the rod knock upon engine startup, idling, accelerating, and decelerating. The noise will increase as you step on the gas pedal and decrease as you remove your leg.
Disregard anyone who says otherwise. Why? Rod knock is a loud banging noise upon engine startup and continues until you turn off the car.
Q: What happens when you hear a rod knocking?
Rod knocking is a problem with the internal engine moving parts. Once it occurs, you’ll hear knocking or banging sounds, have low oil pressure, and check the engine light in your instrument cluster.
If you fail to track down the cause and fix it in time, it will result in catastrophic damage to the engine.
Q: Is it worth fixing the rod knock?
A rod isn’t a problem that will go out on its own. You should diagnose and fix the problem on time before it escalates. If you notice and fix the rod knock-on time, it won’t dig a hole in your pocket. So, it is worth repairing a rod knock at an earlier stage.
Q: How Much Does It Cause to Fix A Rod Knock?
Several factors influence the cause of a rod knock. These factors include: the actual cause of the rod knock, how severe it is, hours of labor, service fees, the vehicle makes and model. However, the cost of fixing a rod knock should be around $700-2500.
Final word
Rod knock is one of those complicated engine problems that many folks overestimate, thinking it’s the end of the game with their car’s engine.
If you notice the problem earlier and take immediate action, your dealership or mechanic might resolve the problem without breaking your budget.
However, if you notice and ignore the signs highlighted above, fixing the issues might cost you a lot. At this point, replacing your engine might be the only option.