A failed connecting rod is one of the most catastrophic forms of engine failure. This failure is commonly referred to as a “thrown rod” or “throwing a rod”.
When a connecting rod fails, there is a very good chance the piston or connecting rod could smash right through the engine block at very high speed, damaging anything in its path.
What Are Connecting Rods?
Connecting rods are the link between each piston and the crankshaft in an internal combustion engine. As the crankshaft spins, the pistons move back and forth very rapidly, subjecting the connecting rods to immense tensile forces.
Connecting rods have bearings that sit between the rod and the crankshaft. These bearings are lubricated and prevent metal on metal contact as the crankshaft rotates.
Likewise, there is a pin that connects the piston to the top of the connecting rod. This pin is called a wrist pin, piston pin, or gudgeon pin.
Top Symptoms of a Bent Rod
A connecting rod needs to be strong and durable for the engine to survive. If it is too weak to handle the load of the combustion, then the extreme tensile forces on the connecting rod may bend or even break it.
This would be a very bad thing for the engine because there would be no more intake strokes, exhaust strokes, compression strokes, or power strokes.
As for the driver, they will be able to notice when this occurs because there will be some very noticeable symptoms present. Here are some of the most common symptoms indicating connecting rod failure.
1) Low Compression
If the connecting rod is slightly bent, you may be able to drive on it without too much of an issue. However, the cylinder with the bent rod will probably have lower compression than the others.
If you’re worried about the possibility of a bent rod, you could start by getting a compression test. If your compression numbers are within about 10% of each other across all cylinders, your rods are probably fine.
Do note that you may experience low compression for a number of reasons. After an unsatisfactory compression test, consider performing a leak down test to help you narrow down the possibilities.
2) Loss of Power
This symptom goes hand in hand with loss of compression. As already mentioned, a connecting rod “connects” the piston to the crankshaft and transfers the energy generated by the combustion process to the wheels of the vehicle.
When the rod is bent, it can cause the piston to move unevenly, resulting in a loss of compression in the cylinder. This in turn can lead to a reduction in power and acceleration, as the engine is not able to generate the same amount of force as it normally would.
3) Engine Knocking Sounds (Rod Knock)
When your engine is turned on and running, you may hear a knocking sound coming from it. This will be a repeated knocking sound that gets faster as your engine speed increases. This is colloquially referred to as “rod knock”.
The sounds may go away after a short while, once your oil has a chance to circulate through the engine components and lubricate them.
4) Low Oil or Oil Pressure
If you have a bad or failing connecting rod, your engine will probably lose a whole lot of oil very suddenly, which would have devastating effects on your engine’s oil pressure.
There may be an oil pressure gauge on your engine which will tell you when it has low oil pressure. You likely have a warning light on your dashboard that will tell you as well.
If you have low oil pressure, then it means oil isn’t circulating well through the engine. This might lead to an overheated engine if the problem is not resolved quickly, and you will experience drastically more wear and tear on the engine because it won’t be lubricated properly.
5) Visibly Bent or Damaged Rod
If you suspect that you have a bent or damaged connecting rod, based on the other symptoms which may be present, then you need to visually inspect the rod yourself to see what its condition is.
This would require your engine to be disassembled, which is a job that only an experienced mechanic should do.
6) Seized Engine
A thrown rod is likely to stall and seize the entire engine. You will know the engine is seized because you can’t turn it over by hand (using a wrench on the crankshaft pulley bolt). Even an engine that won’t start or crank should turn over by hand in this manner.
Chances are if you threw a rod, you’ll know it. A thrown rod is often accompanied with a loud bang and metal flying out of the engine bay. You may notice a huge puddle of oil under the car, and even a big hole in the block after this happens.
What Does a Bent Rod Sound Like?
When you have a bent connecting rod, your engine may produce some unusual sounds. You might notice a light knocking or pounding noise which can usually be traced back to the connecting rods. This sound is often most noticeable when the engine is at an even RPM (such as idling), not accelerating or decelerating, and is commonly caused by a worn bearing, misaligned connecting rod, or lack of oil.
You may also hear some metallic noises. These sounds can be particularly troubling no matter what the problem ends up being, as they likely indicate that metal is rubbing against metal when it shouldn’t be. This friction can cause further damage to your engine if left unaddressed.
Causes of Connecting Rod Failure
There are various factors that can cause a bent connecting rod in your engine. Understanding the underlying problems can help you prevent issues in the future.
1) Flooded Engine
Unlike air, liquids are not really compressible. If the engine is hydrolocked (that is, filled with a fluid instead of just air) you run the risk of bending the connecting rods when you go to start your vehicle. You will have to de-flood the engine before you can start the car normally.
2) Pre-ignition or Detonation
Pre-ignition and detonation are terms for abnormal combustion of the air fuel mixture inside the combustion chamber. Fuel may combust prematurely due to a variety of reasons: carbon buildup, ignition timing that’s too advanced, or running fuel with too low of an octane rating for the vehicle.
Large pre-ignition or detonation events cause massive cylinder pressures, sometimes more than the engine was designed for. Greatly exceeding expected cylinder pressures in this way could cause catastrophic engine failure.
3) Poor Engine Management
If you’re running aftermarket engine parts, chances are you had a tuner program your ECU. If this tune is too aggressive for your running conditions or you push your engine a bit too far, you may end up throwing a rod.
Most vehicles require a tune when certain bolt on parts are installed, such as an intake, exhaust, boost controller, or a bigger turbo. Driving the car hard before the engine has been tuned to account for the new hardware could break something pretty expensive, including a connecting rod.
4) Over-Revving the Engine
As you rev the engine higher, the forces exerted on connecting rods increase because the pistons are moving faster. If you rev the engine higher than it was designed to handle, you’re probably going to be looking at buying a new engine.
Most engines prevent you from over revving the engine, and will cut fuel when you’re at or just above redline. The easiest way to over rev the engine is on a manual transmission.
If you downshift to a gear that’s too low for the speed you’re traveling, you’ll skyrocket the RPMs and your engine won’t be able to keep up. This is known as a “money shift”, because this type of miss shift ends up being very expensive.
5) Low Oil Pressure
Low oil pressure can occur for a few reasons, such as a leak in your engine, using the wrong viscosity oil, or an issue with the oil pump.
When your engine experiences low oil pressure, it can negatively impact the lubrication of the components within, putting additional stress on the connecting rods. This lack of lubrication can lead to friction, heat generation, and ultimately a bent connecting rod.
6) Engine Modifications
Many factory connecting rods are made thinner than they used to be on older vehicles. This is done in the name of efficiency and cost savings. Thinner rods don’t pose a problem at stock power levels, but stock rods aren’t always sufficient if you’re trying to make a lot more power.
If you’re trying to build your car to go faster, make sure you do a lot of research on forums for your specific vehicle to see what the typical build path is for the power level you’re hoping to make.
Assessing the Damage
Here are two common diagnostic tests that you or your mechanic can perform to determine the extent of the issue.
A compression test is a useful first step in diagnosing a bent connecting rod. This test measures the pressure built up in each cylinder of your engine while it’s cranking. To perform a compression test, you’ll need a compression tester. Here’s what to do:
- Warm up your engine.
- Turn off your engine, and remove all spark plugs.
- Install the compression tester into one of the spark plug holes.
- Crank the engine for a few seconds.
- Record the compression reading.
- Repeat steps 3-5 for each cylinder.
If you notice a significant difference in compression readings between cylinders, there may be a problem with the connecting rod in one of the affected cylinders.
Leak Down Test
A leak down test is another valuable diagnostic tool for detecting a bent connecting rod. This test measures the percentage of air pressure that escapes from a cylinder due to leakage. You’ll need a leak down tester for this procedure. Here’s how to perform a leak down test:
- Set your engine to top dead center on the compression stroke for the cylinder you’re testing.
- Install the leak down tester hose into the spark plug hole.
- Apply a regulated air pressure (typically around 100 PSI) to the tester hose.
- Note the amount of pressure drop on the leak down tester gauge.
A high percentage of air leakage may indicate a bent connecting rod leading to poor sealing in the affected cylinder.
Bent Rod Replacement Cost
In most cases, you can expect to pay between $600 and $3,000 for parts and labor, depending on the make and model of your vehicle, as well as where you get the work done.
The cost of a new connecting rod itself can range from $50 to $200 per rod. Keep in mind that some vehicles may have more than one connecting rod that needs to be replaced, which will increase the cost.
Labor charges make up the bulk of the cost as 4 hours of labor is often the minimum with some models needing 10+ hours of labor. With hourly rates varying from $80 to $150 depending on the shop, the cost adds up quickly.
Another factor to consider in the overall cost of connecting rod replacement is whether additional damage has occurred within the engine. In some cases, a bent or thrown connecting rod can lead to damaged pistons, cylinder walls, or even crankshaft issues.
These additional repairs can significantly increase the overall cost of fixing your car, so it’s always recommended to determine the full extent of the problem before proceeding.
When Is Engine Replacement Necessary?
Sometimes, a bent connecting rod can lead to a series of issues within your engine. Unfortunately, engine replacement may be necessary in some cases but will save you time and money in the long run.
If your engine experiences low oil pressure, it might be an indication that the bent connecting rod is causing some serious internal damage. Don’t ignore this! Continuing to drive with low oil pressure can lead to even more costly repairs.
If your engine has seized up and it’s due to connecting rod problems, it’s a strong sign that your engine has reached its limits. In this case, it’s usually better to simply replace the whole engine instead of dealing with both known and unknown repairs.
Extensive damage to the engine block is another clue that replacement might be necessary. A bent connecting rod can occasionally end up smashing right through the engine block. In such cases, the damage is often too severe to be repairable, and it’s best to go for an engine replacement.
In conclusion, always pay attention to signs of distress in your engine. If you suspect a bent connecting rod is the culprit, consult a mechanic to diagnose the issue and determine whether an engine replacement is needed. It’s better to be safe than sorry when it comes to the health of your vehicle!