Q: What Causes Engine Knocking?

asked by on December 02, 2015

What causes engine knocking?

The engine has many moving parts that all work together to form the internal combustion engine. The engine requires a good lubrication source and a good cooling source for it to continue to work properly. The internal moving parts are lubricated by oil from the oil pump inside the oil pan on most engines. The oil pump draws in oil from the oil sump or pan through a pickup tube that has a medium sized screen to filter out any medium sized debris. The pump is gear or shaft driven by the engine. The oil pump will then pump the oil at a high volume to the oil filter. The oil filter will remove most smaller debris before the oil is pumped into the crankshaft and bearings, then to the pistons. The oil will then be pumped up to the cylinder heads to lubricate the valve train and camshafts. The oil will then drain back to the oil pan or sump and start the process all over again.

The lubrication system uses oil for lubrication and to aid in cooling the parts through reducing friction between parts. The main cooling of the engine is done by the radiator, water pump, and coolant to flow throughout the engine to radiate excess heat out through the radiator. When one of these systems is not maintained properly the engine can get damaged or have excessive wear. When the oil and filter is not changed often enough, it will start to cause wear in the bearings and piston rings of the engine. When this wear becomes excessive, the oil pressure will be pushed out the sides of the bearings and less oil will be distributed to the pistons, piston rods, camshafts, valve train, and timing chains. This is when the noise will start to come in.

Engine knocking comes in a few different varieties. There is a light knock or tapping noise that would most likely be from the valve train and camshafts not getting enough lubrication. This area is the first to experience problems from lubrication issues. There is a medium knocking sound that would be coming from the piston wrist pin or piston rod bearing. This noise may get worse as you accelerate from the increased pressure on the bearings and the bearings may even spin, causing louder knocks that increase as engine speed increases. This area of the engine will be the next area to start getting damage from lack of lubrication after the valve train and camshafts. The last knocking noise will be heard in the lower part of of the motor and may even sound like a thumping sound. The noise will mostly stay the same with a little increase in volume as the engine is increased in speed. This is caused by failed crankshaft bearings and is mostly caused from loss of oil pressure or running the engine without enough oil. The most common cause of all of these knocking problems is loss of oil pressure from a clogged filter and oil pickup screen causing oil pump failure or just running the engine low on oil from oil loss through oil burning, oil leaks, and lack of maintenance oil and filter changes.

There are a few knocking noises that may not be from the engine internal parts and oil lubrication system. The front crankshaft damper can come apart and can cause a crankshaft imbalance and put out a knocking noise sometimes mistaken for a crankshaft bearing problem. The damper can be seen externally for damage but a crankshaft cannot. The next possible noise is from a cracked flywheel flexplate. The plate may get cracks where it is bolted to the back of the crankshaft. This noise sometimes is mistaken to be from the piston wrist pins or rod knocking. The noise changes when you put the transmission in drive or park. A piston knock will be more consistent than the noise from the flexplate. Timing chain slap from a slack chain or guide and tensioner issues will cause knocking noises similar to a piston noise or slapping but will stay constant, while pistons change with rpm and load.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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