How to tell if your engine cylinder is bad

If a cylinder is “bad,” you’ll typically notice rough running, a persistent misfire, low compression in that cylinder, abnormal smoke, or unexplained coolant/oil loss—and the most reliable confirmation is a compression and/or leak-down test. In practice, start with an OBD-II scan and basic checks, then verify with compression/leak-down or a borescope to pinpoint whether the issue is rings, valves, head gasket, or a cracked head/block.

What “bad cylinder” really means

Drivers often use “bad cylinder” to describe anything from a simple misfire to serious mechanical damage. Strictly speaking, a bad cylinder fails to seal or operate properly due to worn piston rings, damaged valves or seats, a blown head gasket, or cracks in the head/block. Misfires caused by spark or fuel problems can mimic a bad cylinder, but mechanical faults show up as low compression or high leak-down.

Symptoms you’re likely to notice

The signs below are the most common early clues that one cylinder isn’t healthy. They don’t prove mechanical failure on their own, but they tell you it’s time for testing.

• Check-engine light with misfire codes (P0301–P0308) or random misfire (P0300), often worse at idle or under load.
• Rough idle, shaking, or poor acceleration; engine may feel down on power or vibrate more at stops.
• Exhaust smoke: blue (oil burning, worn rings/valve seals), white sweet-smelling (coolant, head gasket/crack), or black (overfueling from misfire/injector issue).
• Unexplained coolant loss, milky oil, or bubbles in the coolant reservoir (possible head gasket or crack).
• Increased oil consumption, oily plugs, or wet/sooty spark plugs in one cylinder.
• Fuel smell in oil (cylinder washing due to misfire), or excessive crankcase blow-by from the oil cap/dipstick.
• Ticking or tapping (valve/rocker/lifter), or hissing/whistling that changes with RPM (compression leak).
• Poor fuel economy and, over time, catalytic converter damage (misfire sending raw fuel into the exhaust).

Any one of these signs warrants further diagnosis; a consistent pattern—especially a single-cylinder misfire plus fluid loss or smoke—makes mechanical issues more likely.

Quick checks you can do now

Before deeper tests, these simple steps can help separate an ignition/fuel issue from a mechanical fault.

1. Scan for codes: Note P030X (which cylinder), fuel trim codes (P0171/P0174 lean or P0172/P0175 rich), misfire on startup (P0316), and coolant temp sensor anomalies.
2. Swap test: Move the coil, plug, or injector from the suspect cylinder to another. If the misfire follows the component, it’s likely not mechanical.
3. Plug inspection: Oily or wet plugs suggest oil/coolant or severe misfire; white crusty deposits can indicate coolant; heavy soot suggests rich running.
4. Cooling system check: Look for low coolant, pressurized hoses when cold, or oily residue in the reservoir; use a pressure tester if available.
5. PCV/air leaks: A stuck PCV can cause blow-by/oil consumption; vacuum leaks can cause lean misfires that mimic cylinder issues.

If the misfire doesn’t move with components and visual checks hint at fluids entering a cylinder, proceed to compression and leak-down testing.

Definitive diagnostics professionals use

Compression test (gasoline engines)

Compression testing compares cylinder pressure during cranking. It identifies weak cylinders and gross sealing issues.

This list outlines the basic process and what to watch for during a compression test.

1. Warm engine, disable fuel and spark, fully charge battery, hold throttle wide open.
2. Remove plugs and test each cylinder with a quality gauge; crank 4–6 pulses per cylinder.
3. Record values; repeat a “wet test” with a teaspoon of oil in the suspect cylinder.

Healthy gasoline engines typically have similar readings across cylinders within about 10% of each other; consult your vehicle’s spec for exact numbers. A wet test that raises pressure points to ring wear; no change suggests valves or head gasket.

Leak-down test (gasoline and diesel)

Leak-down testing pressurizes a cylinder at top dead center and measures how much air escapes and where it goes—pinpointing the fault.

Use this list to interpret where escaping air indicates the problem source during a leak-down test.

1. Air heard at the throttle body/intake: leaking intake valve.
2. Air at the tailpipe: leaking exhaust valve.
3. Air from oil filler/dipstick: worn rings or scored cylinder wall.
4. Bubbles in coolant/overflow: head gasket breach or cracked head/block.
5. Percentage: Under ~20% is generally acceptable; 20–30% is marginal; over ~30% is significant. Follow manufacturer limits.

Leak-down is more diagnostic than compression alone; it reveals the actual leakage path, guiding repair decisions precisely.

Borescope inspection

A borescope lets you look inside the cylinder via the spark plug hole (or glow plug/injector hole on diesels).

The list below highlights what technicians look for during a borescope check.

• Vertical scoring on cylinder walls (ring damage, debris ingestion).
• Clean, steam-washed piston tops from coolant intrusion; droplets or crusty deposits.
• Heavy oil wetness in one cylinder (oil control ring failure, valve stem seals).
• Cracks in the head near valves or between valves, and piston crown damage.

Images from a borescope corroborate compression/leak-down findings and help avoid unnecessary disassembly.

Relative compression and advanced scans

Shops may run a “relative compression” test using starter current or a scope to quickly compare cylinders without removing plugs, and use Mode $06 data to see misfire counts by cylinder. Diesel engines require diesel-rated compression gauges, with healthy compression often several hundred psi higher than gasoline engines; always use engine-specific specs.

How to interpret results

Putting the evidence together narrows the cause and the fix.

• One cylinder low compression; leak-down to crankcase: worn rings, broken ring land, or cylinder scoring.
• One cylinder low; air at intake or exhaust: bent/burned valve, poor valve seating, wiped cam lobe or lifter issue.
• Adjacent cylinders low with coolant loss or bubbles: head gasket between cylinders or crack.
• All cylinders low but even: incorrect valve timing (stretched chain/belt off a tooth), or testing done incorrectly (throttle closed, weak battery).
• Good compression but persistent misfire: ignition/fuel problem, vacuum leak at that runner, or injector fault.

Consistent findings across two or more tests (e.g., low compression plus leak-down air at the intake) make the diagnosis highly reliable.

Common root causes

A cylinder can “go bad” for several reasons; understanding them helps you choose the right repair and prevent repeat failures.

• Wear and tear: high mileage ring wear, valve seat recession, or carbon-induced hot spots.
• Overheating: warps heads, damages head gaskets, and can crack heads/blocks.
• Detonation/pre-ignition: breaks ring lands, melts pistons, damages valves.
• Contaminants and poor filtration: dirt ingestion scores cylinders; fuel dilution washes oil film.
• Timing or lubrication failures: slipped timing chains/belts, low oil pressure damaging cams and lifters.
• Coolant intrusion: corrodes rings and bores, causes hydrolock if severe.

Root cause matters—replacing parts without addressing overheating, fueling, or lubrication issues can lead to a quick repeat failure.

Repair options and typical 2025 costs

Costs vary by vehicle, engine layout, and region. These ballpark figures reflect common U.S. shop pricing.

• Ignition/fuel fixes (coil, plug, injector, wiring): $100–$600 per cylinder depending on parts and access.
• Valve stem seals (head on, some engines): $400–$1,200; full valve job with head removal: $1,000–$3,000.
• Head gasket replacement: $1,500–$3,500+ (more for V6/V8, turbo, or DOHC with timing components).
• Piston rings/cylinder work (top-end + bottom-end): Gasoline $3,000–$7,000; diesel $5,000–$10,000+.
• Reman/used engine replacement: $3,000–$8,000 for the unit plus $1,500–$3,500 labor; diesels and luxury/performance engines can exceed $12,000.
• Diagnostics (compression/leak-down/borescope): $100–$350, often credited toward repair.

Get a written estimate and compression/leak-down numbers in the report; they justify the repair scope and help you compare shops.

Is it safe to keep driving?

Sometimes you can limp a vehicle short distances; other times you risk catastrophic damage.

• Do not drive if coolant enters a cylinder (white smoke, rapid coolant loss, misfire on startup)—hydrolock can destroy the engine.
• Avoid driving with severe misfire—raw fuel can overheat and ruin the catalytic converter quickly.
• Stop if oil pressure is low, oil is milky, or there’s heavy knocking/ticking.
• If you must move the vehicle, take the shortest route, keep RPMs low, and monitor temps and warning lights.

When in doubt, tow the vehicle; the cost often beats the risk of an engine replacement.

Prevention and maintenance tips

Good habits reduce the odds of cylinder damage and extend engine life.

• Follow oil specs and intervals; fix oil and coolant leaks promptly.
• Replace air filter on schedule; ensure the intake tract is sealed.
• Use quality fuel and maintain the cooling system (thermostat, radiator, fans).
• Address check-engine lights early, especially misfires and lean codes.
• Keep timing components current (belts/chains, tensioners, guides) per manufacturer intervals.
• Allow gentle warm-up and avoid sustained detonation by using the recommended octane.

Preventive care is far cheaper than mechanical repairs; most cylinder failures start as small, fixable issues.

Summary

A “bad cylinder” shows up as a misfire plus evidence of poor sealing: low compression, high leak-down, abnormal smoke, or fluid loss. Start with an OBD-II scan and simple swaps to rule out ignition/fuel faults, then confirm with compression and leak-down tests and, if possible, a borescope. Use the test results to identify whether rings, valves, a head gasket, or cracking is to blame, and choose a repair path that also addresses the root cause. When coolant intrusion or severe misfire is present, stop driving and seek professional diagnosis to avoid total engine failure.

What happens when a cylinder goes bad in a car?

If left untreated, a cylinder misfire can lead to significant engine damage. Worse, if you experience a bad misfire while driving, it could result in an accident. This is why it’s important to treat engine misfires as soon as you detect them.

What does a cylinder misfire feel like?

A cylinder misfire often feels like the car is jerking, hesitating, or losing power, especially when accelerating or idling. You might notice heavy vibrations, a rough or uneven idle, and the engine struggling to catch its breath or even sputtering. Other symptoms can include popping or backfiring sounds from the exhaust, a strong smell of unburnt fuel, and a flashing or steady check engine light.
 
How it Feels

• Jerking or Stuttering: The car can feel like it’s hitting an invisible pothole, especially when you press the accelerator. 
• Hesitation: The engine might hesitate or stumble when you try to accelerate, rather than responding smoothly. 
• Loss of Power: You’ll notice a reduction in engine power, making it harder for the car to maintain speed or climb hills. 
• Rough Idle: The engine may not run smoothly when stopped, feeling like it’s jumping or vibrating unevenly. 
• Heavy Vibrations: You can often feel strong, heavy vibrations coming from the engine. 

Associated Sounds & Smells

• Popping or Sputtering: You might hear unusual popping, coughing, or sputtering noises from the engine or exhaust. 
• Backfiring: In severe cases, you might hear backfiring from the exhaust system. 
• Fuel Smell: There can be a distinct smell of unburnt fuel. 

What to Do 

• Get it Checked Immediately: A misfiring cylinder can cause significant engine damage, including to the catalytic converter, and can lead to stalling.
• Don’t Ignore It: While a misfire can sometimes be a temporary issue, consistent misfiring should never be ignored and needs professional attention.

Is it expensive to fix a cylinder misfire?

The costs for this repair can range from 300 to as much as a new engine, which is approximately 6000. The shop would need to diagnose the source of the issue, as many factors could be responsible, including spark plugs, coils, injectors, wiring issues, the computer, sensors, or a failed engine.

How can you tell if you have a bad cylinder?

Symptoms of a bad cylinder include a rough engine idle, loss of power, poor acceleration, increased fuel consumption, and vibrations. You may also notice a flashing or illuminated “Check Engine” light on the dashboard, hear unusual engine noises, or see smoke coming from the exhaust. A persistent overheating issue or decreased engine performance can also indicate a cylinder problem.
 
Common Symptoms

• Rough Engine Idle: The engine may shake, stutter, or run unevenly, especially at lower speeds or when idling. 
• Loss of Power: You might experience a noticeable decrease in engine power, making it difficult to accelerate. 
• Poor Fuel Efficiency: A malfunctioning cylinder can lead to increased fuel consumption, meaning you’ll spend more on gas. 
• Engine Vibration: The entire vehicle, including the steering wheel, may shake or vibrate due to the uneven operation of the engine. 
• “Check Engine” Light: The illuminated or flashing check engine light on the dashboard is a key indicator of a problem. 
• Unusual Engine Noises: A bad cylinder can cause loud knocking or tapping sounds from the engine. 
• Exhaust Smoke: The color of the smoke can reveal the issue; white smoke indicates burning coolant, black smoke points to unburned fuel, and blue smoke suggests oil is burning. 

What to Do
If you notice these symptoms, it’s important to have your vehicle inspected by a professional mechanic as soon as possible. Addressing the problem promptly can prevent further damage and more costly repairs down the line.