How to Tell if Your Crankshaft Is Failing

Your crankshaft is an extremely robust moving part of your engine. It is placed low in the engine, close to the oil supply. The crankshaft’s function is to convert the up-and-down motion of the pistons into rotating motion that can be used to smoothly power your vehicle.

As the pistons are pushed up and down in their cylinders by the process of combustion, they are linked to the crankshaft by connecting rods. The design of the crankshaft allows the lower end of the connecting rods to move in a circular direction, turning the crankshaft. This rotary motion of the crankshaft, along a linear axis, is directed into your car’s transmission and then to the driving wheels, moving the car forward or backward, depending on the driver’s needs.

Crankshafts need to be very strong, so that they can stand up to the huge forces transmitted to them by the pistons and connecting rods, and to transmit this power into the transmission. Crankshafts are usually made of a single piece of steel bar, using a forging or casting process. Some engines use assembled crankshafts, which are made of several pieces of steel attached together. Others may use crankshafts that are machined out of a single piece of remelted steel, using milling machines and lathes.

Inside the engine, your crankshaft is held in place by several bearings, which are lubricated by the engine oil. The bearings allow the crankshaft to spin smoothly in its place, while also stabilizing the crankshaft as it is subjected to the powerful forces of the pistons and connecting rods that are rotating it.

Signs of a bad crankshaft

Because the crankshaft is such an integral part of your engine, a bad crankshaft will usually be noticed immediately. Some of the underlying causes of a bad crankshaft are:

• A forging or casting flaw
• Insufficient lubrication
• Bearing wear
• Loose bearing main caps
• Distorted bearing bores
• Improper bearing clearance
• Bearing failure

The results of these conditions can be a cracked or even broken crankshaft. You may hear a knocking noise from the engine, or notice that the engine’s oil pressure is low (from a gauge or warning light). In severe cases, the engine will simply stop running.

If you notice any of these signs, pull your car over immediately, and shut off the engine. Do not try to drive it any farther. Arrange to have your car towed to your mechanic for diagnosis and repairs.

How do you fix a bad crankshaft?

Unless you have extensive experience in repairing and rebuilding engines, this is definitely a job for your mechanic. The first step for the mechanic, once your car arrives in the repair shop, is to disassemble your engine and see exactly what the problem is. Depending on the severity of the damage, repairs may be possible and cost-effective, or may not be worth the expense. Here are a few possible scenarios:

• The crankshaft has been damaged, but nothing else inside the engine is affected: The crankshaft may be repairable, or it may need a replacement. Remanufactured crankshafts are available, which are a cost-saving solution, compared to buying a new crankshaft. If your car is very old with high mileage but otherwise runs fine, you might be able to find a cheap used crankshaft in a junkyard, to save money and keep your car running a bit longer. The crankshaft bearings should also be thoroughly checked, to confirm that they are still in good condition. Bad bearings will quickly destroy your new crankshaft.

• The crankshaft and its bearings have been damaged: This type of damage is more extensive, because the bearings must also be repaired. The forces that caused the crankshaft damage have also damaged the bearings, causing excess wear, or making their formerly round surfaces into an irregular shape. A new or remanufactured crankshaft and bearings may solve the problem, but if the damage is too extensive, you might consider getting a remanufactured replacement engine (if the car is fairly new). If your car is old and worn out, it may be time to say goodbye and send it to the junkyard.

• The crankshaft, the bearings, and the top end of the engine have been damaged: This is the most catastrophic type of crankshaft-related damage. In addition to damage to the crankshaft and its bearings, the connecting rods and the pistons attached to the crankshaft have also been damaged. And if the pistons hit the tops of the cylinders, they can cause serious damage to the intake and exhaust valves. Now you have a huge and expensive mess that involves repairs to your entire engine. A replacement engine may be your best option if the car has some value left in it. Otherwise, unless you are covered by some type of mechanical warranty, it may be best to cut your losses and junk it — or as we say today, “recycle” it.