What Happens When a Crankshaft Goes Bad

When a crankshaft goes bad, the engine can develop severe vibrations, knocking, loss of oil pressure, misfires, and eventually seize or fail catastrophically; in many cases, the only safe fix is major repair or engine replacement. The crankshaft converts piston motion into rotational power, so any damage or excessive wear threatens the engine’s core function and can escalate quickly if driven.

What the Crankshaft Does

The crankshaft sits at the heart of an internal combustion engine, translating the up-and-down motion of the pistons into rotation that drives the transmission. It rides on main and rod bearings, relies on steady oil pressure for lubrication, and is damped by a harmonic balancer to control torsional vibration. Because it bears immense load at high speed, even small lubrication or alignment problems can snowball into failure.

What “Goes Bad” Means

“Going bad” can range from sensor-related crankshaft signal issues to mechanical wear and damage. Mechanically, journals can score, bearings can spin, thrust surfaces can wear, the shaft can crack, or it can go out of true (runout). Electrically, a failing crankshaft position sensor or damaged reluctor ring causes erratic timing signals that may mimic mechanical trouble.

Symptoms You May Notice

Drivers usually experience a combination of noises, vibrations, and performance problems as a crankshaft or its bearings deteriorate. Recognizing these signs early can prevent catastrophic failure.

• Deep knocking or rumbling that rises with RPM (often worse under load) from the lower engine
• New, harsh vibrations felt in the cabin; shudder at specific RPMs (possible harmonic balancer or crank runout issues)
• Low or fluctuating oil pressure warning, especially hot and at idle
• Metal shavings or glitter in engine oil or on the magnetic drain plug; gray sludge in the oil filter
• Hard starting, stalling, or no-start with crankshaft position sensor codes
• Misfires, rough idle, poor acceleration, and increased fuel consumption
• Burning oil smell or rapid oil darkening after a change
• Engine suddenly locks up (seizes) or throws a connecting rod in severe cases

If these symptoms appear together—particularly knocking plus low oil pressure—park the vehicle immediately to limit damage and arrange a professional inspection.

What Mechanics Find on Inspection

Inside the engine, a failing crankshaft leaves evidence that helps pinpoint the root cause and extent of damage.

• Scored or blue (overheated) crank journals; copper showing on bearings
• Spun main or rod bearings, often with bearing material smeared onto journals
• Excessive endplay (thrust bearing wear), sometimes causing clutch or torque converter issues
• Cracks at fillets, oil holes, or keyways; sometimes visible via magnetic particle inspection
• Out-of-round or tapered journals; excessive runout when measured on V-blocks
• Damaged or slipped harmonic balancer/damper or cracked crank pulley
• Reluctor ring damage or misalignment causing erratic crank sensor readings

These findings determine whether the crankshaft can be machined and saved or if the engine requires a replacement crank, short block, or complete remanufactured unit.

Why Crankshafts Fail

Crankshaft problems usually trace back to lubrication issues, vibration control failures, or excessive mechanical stress. Understanding the causes can guide both repair and prevention.

• Oil starvation from low oil level, clogged pickup, failing oil pump, or wrong oil viscosity
• Contaminated oil (coolant or fuel dilution) accelerating bearing wear
• Detonation/pre-ignition or over-advanced timing increasing bearing loads
• Over-revving, repeated high-RPM use, or missed shifts overstressing rods and journals
• Failed harmonic balancer causing uncontrolled torsional vibration
• Poor prior engine work: incorrect bearing clearances, misaligned main caps, or debris in assembly
• Manufacturing defects or material fatigue over very high mileage
• Excessive accessory belt tension or misalignment adding side load to the front journal

Often, two or more factors combine—such as marginal oil pressure and a worn damper—leading to rapid deterioration.

How It’s Diagnosed

Technicians combine electronic scanning with mechanical checks to separate sensor faults from real crankshaft damage and to quantify wear.

1. Scan for codes and data: look for P0335–P0339 (crank sensor) and P0016–P0019 (crank/cam correlation); review live CKP signal stability
2. Verify oil pressure with a mechanical gauge, hot and cold; compare to spec
3. Listen with a stethoscope for lower-end knocks; perform cylinder cutout to see if noise changes
4. Inspect oil and filter: cut the filter open for metal; send oil for analysis if needed
5. Check the harmonic balancer for separation or wobble; inspect crank pulley and belts
6. Scope the CKP waveform; verify timing correlation with the cam sensor
7. If indicated, drop the oil pan: inspect bearings, measure clearances (Plastigage), and check endplay with a dial indicator
8. For rebuilds, measure journal sizes, taper, out-of-round, and runout; perform crack checks (magnetic particle or dye penetrant)

This structured approach prevents misdiagnosis—especially mixing up a bad sensor with actual bottom-end damage—and helps decide between in-car repairs and full teardown.

Can You Keep Driving?

It’s risky and generally not advised. A failing crankshaft can seize without warning, causing sudden loss of power assist for steering and brakes, and can eject connecting rods through the block. Continuing to drive also multiplies repair costs by damaging the block, pistons, and heads. If knocking or low oil pressure is present, park the vehicle and tow it.

Repair and Cost Expectations

Costs vary widely by engine, parts availability, and whether the block is salvageable. Here are typical paths shops consider in 2025.

• Crankshaft machining: polish ($60–$150), grind undersize ($200–$400), then install matching undersize bearings ($50–$200)
• Replace harmonic balancer/damper if degraded ($80–$400) and often the oil pump ($120–$400)
• Bearings and gaskets for a bottom-end refresh ($150–$500 in parts)
• Labor for in-vehicle bottom-end work: roughly 10–20 hours at $100–$200/hr, depending on access
• Severe damage: replacement crank or reman short block/long block; total installed cost commonly $2,500–$7,500+ for many vehicles, higher for trucks/luxury/performance engines
• Crankshaft position sensor replacement if the issue is purely electronic ($30–$150 part; 0.5–2.0 hours labor)

Shops will also assess block main bore alignment; if out of spec, line-boring/honing may be required, or a replacement block may be more economical.

Preventing a Bad Crankshaft

Routine maintenance and attention to warning signs go a long way toward protecting the bottom end.

• Keep oil at the correct level and change it on schedule using the specified viscosity
• Address oil leaks and low oil pressure warnings promptly; fix pickup screen and pump issues
• Replace a suspect harmonic balancer; don’t ignore wobble or rubber deterioration
• Avoid sustained over-revving; ensure rev limiters are working
• Fix cooling problems to prevent oil thinning and detonation
• Use quality parts and proper torque specs during engine work; verify bearing clearances
• For manual transmissions, avoid riding the clutch; for automatics, address line-pressure faults that load the thrust bearing

These steps reduce stress on journals and bearings and help maintain stable oil film under high load.

Related Fault Codes and Terms

Crankshaft issues often appear alongside specific diagnostic codes and terminology that can guide troubleshooting.

• P0335–P0339: Crankshaft position sensor circuit faults
• P0016–P0019: Crank/cam correlation errors (timing or reluctor issues)
• “Rod knock”: deep metallic knock from worn rod bearing
• “Spun bearing”: bearing shell rotates in its bore, cutting oil flow and damaging the journal
• “Endplay” or “thrust”: axial movement of the crank; excessive indicates thrust bearing wear
• “Runout”: bending or misalignment of the crankshaft measured as wobble

Seeing these codes or terms in a work order typically calls for deeper mechanical checks rather than sensor replacement alone.

Summary

A bad crankshaft threatens the engine’s core integrity, showing up as knocks, vibration, low oil pressure, and poor performance before potentially culminating in seizure or catastrophic failure. Prompt diagnosis—separating sensor issues from mechanical damage—can save an engine, but serious cases often require machining or replacement. Preventive maintenance, correct oil management, and a healthy harmonic balancer are your best defenses against crankshaft trouble.

How do I know if my crankshaft is bad?

Knocking or Rattling Sounds: A broken crankshaft may produce knocking, rattling, or grinding noises, especially when the engine is running. Excessive Vibration: If you notice unusual vibrations coming from the engine, it could indicate that the crankshaft is damaged or misaligned.

Is it worth fixing a crankshaft?

Crankshafts are precision components, and mistakes can lead to engine failure. If you’re skilled in engine repair and follow exact specifications, it’s possible — but for most people, professional repair or replacement is the safer option.

Can a car run with a bad crankshaft?

Yes, there is nothing to keep you from driving – another car. You will not be able to continue to drive the car that needs a crankshaft replacement. It’s really rare to need a new crankshaft. Usually, when a crankshaft gets damaged, something else is to blame and that kind of damage is usually catastrophic.

What happens when your crankshaft goes?

Loss of Power: – Reduced Engine Performance: A failing crankshaft can affect the engine’s ability to generate power, leading to sluggish acceleration or overall poor performance. Engine Misfires: – Misfiring Cylinders: If the crankshaft is damaged, it can cause timing issues that lead to engine misfires.