Rod Knock – information, causes and fixes

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 Engine rod knock is one of those things that you cannot ignore. The noise from lose rods is unbearable and if not handled immediately it is a signal for further engine trouble. The internal combustion engine works firstly, by the ignition of a fuel/air mixture. The explosion causes the pistons to move up and down along the rods hence moving the crankshaft. This is what gives a car motion.

The major problem in rod knock is that they have bearings attached to them which when worn out will cause the rods to be out of place. Some people describe the rod knock as someone picking a hammer and hitting a metal pan with it. If you do not solve the problem immediately, the rods will start hitting on the crankshaft.

The sound of rod knock will depend on the car speed. If you are accelerating fast then the noise is higher than when the car is idling. There moments the rod knock will subsidize only to come back again.

What causes rod knock?

To understand what causes rod knock you first have to understand how the internal combustion engine operates. When you start the car and press on the fuel pedal, the fuel valve will release fuel into the combustion chamber.

This is mixed with air and a resulting ignition via the sparkplugs sets the pistons in an up and downward motion. This is what moves the crankshaft and subsequently causes the car to move forward.

To connect the crankshaft to the pistons is a connecting rod. To bolt this rod to the crankshaft are smooth bearings. Oil passes through these metallic parts, preventing them from colliding with each other and causing friction. The oil fills in the gap between the crankshaft and the bearings.

The problem arises when your bearing become worn out or your car runs low on oil. This leads to the rods being lose hence the noise. Rod knock should be differentiated from piston slap which is a situation whereby the wrist pin wears out. The wrist pin is what holds the piston to the connecting rod at the top. When lose it wobbles in the cylinders hence making noise.

If you do not solve the problem immediately it can render an engine completely useless. The bearing wear results in a small deviation in rod space but this is enough to cause metals to bang into each other. The longer you wait to solve the problem the wider the space becomes. Rod knocks can lead to holes developing in your cylinder and the connecting rod breaking.

Here are some other contributors to premature wear of the bearings:

  • Low oil pressure
  • Poor oil viscosity – as a result of buying the wrong oil for your car
  • Excessive high engine oil temperatures
  • Excessive engine revving
  • High level of combustion pressure

How to fix Rod knock

If you do not fix rod knock immediately then the problem is going to become more severe with the worst case scenario of permanent engine failure.

As you continue driving with a car that experiences rod knock the bearings continue wearing out until the connecting rod collapses on the crankshaft. There instances the connecting rod will jam leading to a situation called thrown rod.

The first thing your mechanic will want to know is how long you have being having the problem. They will them examine the engine to see the extend of the damage. If you take the car to your mechanic the moment the problem starts then they maybe able to salvage your engine.

Most of the bearings in most modern cars are actually made from lead-free aluminum alloy. These have a smooth texture and offer long lasting protection against rod knock.

Every engine has something we call a gap tolerance. This are small spaces left in between the crankshaft and the connecting rods. This gap tolerance key function is to allow a certain amount of oil to pass through.

This allows the oil to act as a thin layer of protection against friction. This oil will prevent the crankshaft journal and the rod bearing from coming into contact with each other. Due to wear and tear this gap widens hence cannot hold engine oil effectively.

If you starve your engine of oil then it likely to overheat. Heat causes premature wear and tear of the bearing material.

Changing engine oil

Ramps: to access the rod knock damage you need the car in an elevated position. This means you may need some ramps to raise the car to a vintage point. An elevated car also helps you perform an oil change.

New oil/filter:

Since most of the rod knocks are caused by lack of oil in the engine, a simple oil change can solve the problem if the damage was not extensive. While doing an oil change you will need to get an oil pan to collect the used oil from the engine.

You will also need to use an oil wrench to remove the old oil filter and replace it with a new one. Once you have drained the oil to an oil pan the next step is to refill your car with fresh oil.

Draining your oil is simple

First drive your car along the mechanic ramps until it is on an elevated position that you can easily see the oil pan. Then identify the oil pan and open the cap to drain the oil.

Once you have drained the old oil, your need to replace the oil filter and pour in new oil. Fixing the oil filter will require an oil wrench as it can be tight at times. If you have completely drained your car off oil, then you need to do a refill of about 5 quarts of oil.

This is enough to keep all your movable parts well lubricated. Your fuel injector could be having signs of carbon deposits, which may necessitate changing it. You can use a fuel injector cleaner to clean it or a carburetor cleaner for those cars with carburetors.

What we have described above is a solution for rod knock repairs that are as a result of low engine oil. If you have a more serious problems like faulty connecting rods then repairs can go up and be more complicated.

In some cases the average repair costs for the connecting rods can be as high as $2,500 and other cars racking up repair costs more than $5,000. You may also need new pistons, camshaft bearings, connecting rods, new crankshaft or timing chains.

The repairs can be extensive leading to a complete engine overhaul. The problem with rod knock repairs is it affects other engine parts. You will find yourself paying extra for replacement of new gaskets and seals.

Conclusion

Rod knock is a serious car mechanical problem. You identify the problem by the noise produced by the rods, which is like hitting a metallic surface with a hammer. The major cause for rod knock problems is low engine oil.

Oil is the lifeblood of the engine. Without it metallic parts knock on each other producing friction and excessive heat. The bearings connecting the crankshaft and the connecting rod to the piston are also subject to wear.

If you detect the problem immediately a simple oil change should solve the problem otherwise it can lead to complete overhaul of the engine.

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Steven Reilly
Steven Reilly is a qualified mechanic and his passion for cars goes beyond just the technical aspects. He is also an amateur racer and all round car enthusiast. When he is not driving them, he can often be found in his garage under the hood of a rare model. Steven Reilly has lost track of the number of hours he has spent setting up his fine collection of rebuilt models. He believes that cars can provide a constructive and fun opportunity to teach the youth important life skills. In line with this, he is developing a community outreach program, potentially dubbed ‘Cars for change'.

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