Q: Car was low on oil and has knocking sound what could it be?

The origin of the noise can be pinpointed if you request an engine noise diagnostic. The mechanic will consider the knock intensity which will vary depending on source or type. Light knocking or tapping noise is caused by the valve train and camshafts not getting enough lubrication. This area is the first to experience problems from lubrication issues. Indeed, overall, most engine knocking is ultimately attributable to loss of oil pressure due to 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. Therefore, measuring engine oil pressure helps to understand the cause.

Knocking of medium intensity is often due to wear in the piston wrist pin or piston rod bearing. This noise may worsen as you accelerate due to the increased pressure on the bearings. The bearings may even spin, causing louder knocking that increases with higher engine speed. This area of the engine will be the next area to start getting damage from lack of lubrication after the valve train and camshafts. Often the loudest knocking noises will be heard in the lower part of the motor and may even sound like thumping. The noise will mostly stay the same with a little increase in volume as engine speed increases. Knocking here is caused by failed crankshaft bearings and is mostly caused from loss of oil pressure or running the engine without enough oil.

There are a few knocking noises that may not originate from worn engine internal parts and low oil pressure. For example, the front crankshaft damper can come apart and can cause a crankshaft imbalance and create a knocking noise that is sometimes mistaken for a crankshaft bearing problem. The damper can be examined externally for damage but the crankshaft cannot be so inspected. Other causes of knocking are a cracked flywheel flexplate. The plate may get cracks where it is bolted to the back of the crankshaft. This noise is sometimes mistakenly attributable to 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 a knocking 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. Your best option to get the origin of the noise pinpointed is to request the engine noise diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.