A compression test tells you whether each cylinder can build and hold pressure. A leak-down test tells you where the pressure is going. Together they reveal more about engine mechanical condition than any other test short of disassembly. Used correctly, they prevent misdiagnoses that send engines to the machine shop unnecessarily — or conversely, that let mechanical problems get misread as ignition or fueling faults.
When to test compression
Any of the following situations justifies a compression test before spending further diagnostic time on ignition, fuel, or sensor causes:
- Misfire codes (P0300–P0312) that persist after spark plugs, coils, and injectors have been tested and confirmed good
- Cylinder-specific misfire on one cylinder that does not move when you swap the coil and injector to a different cylinder
- Rough idle or loss of power with no ignition or fueling faults found
- Blue-gray smoke from the exhaust under acceleration or on deceleration — suggesting oil burning
- Excessive oil consumption without external leaks
- White smoke from the exhaust at operating temperature — suggesting coolant entering a cylinder
- Coolant consumption without external leaks, low coolant level, or coolant contamination in oil
- Engine won’t start after confirming spark and fuel are present
- Pre-purchase inspection on any used vehicle
Cranking compression test
The standard compression test measures the maximum pressure each cylinder builds when cranked with the throttle open and the spark plugs removed.
Tools needed
- Compression gauge set with the correct spark plug thread adapter for the engine (14mm most common; 12mm on some import platforms)
- Screw-in adapter for accurate readings — push-in adapters give inconsistent results
- Remote starter switch or an assistant to crank the engine
- Battery charger to maintain voltage during the test — consistent cranking speed is required for accurate results
Procedure
- Warm the engine to operating temperature before testing. Cold oil is thicker and causes artificially low readings from seal leakage that disappears when oil is warm.
- Disable fuel and ignition to prevent fueling the cylinders during cranking. Pull the fuel pump fuse and the ignition fuse, or disconnect both the ignition control module and the fuel pump relay.
- Remove all spark plugs from all cylinders. Testing one cylinder at a time with the others sealed gives false-high readings because the engine must compress only one cylinder. With all plugs removed, each cylinder is tested against the same cranking resistance.
- Thread in the compression gauge adapter hand-tight. Do not overtighten — the adapter must seal the cylinder but you need to remove it easily.
- Hold the throttle wide open during cranking. A partially open throttle restricts airflow and causes artificially low readings.
- Crank the engine for 4–6 compression strokes (typically 4–6 seconds at normal cranking speed). Record the maximum reading and note how quickly it builds — a cylinder that builds slowly is a different finding than one that builds quickly and peaks low.
- Record each cylinder and compare.
Interpreting compression results
| Finding | Likely cause |
|---|---|
| All cylinders within 15% of each other at or above manufacturer minimum | Compression normal — mechanical causes for the symptom are unlikely |
| One cylinder low — all others normal | Single cylinder mechanical fault: burned valve, broken ring, scored bore, head gasket on that cylinder |
| Two adjacent cylinders low | Strong indicator of head gasket failure between those cylinders — combustion gases cross-leak through the gasket |
| All cylinders uniformly low | Engine wear across all cylinders, timing chain/belt advanced far behind specification, or throttle not held fully open during test |
| One cylinder builds slowly to a low peak | Ring seal problem — pressure builds through several strokes rather than quickly (compared to a valve that stays near zero from the first stroke) |
| Compression increases significantly after adding oil (wet test — see below) | Ring seal issue — oil seals the rings temporarily and raises the reading |
| Compression unchanged after adding oil | Valve seating problem — oil does not reach the valves |
The wet compression test
If a cylinder reads low, add approximately 15 ml (1 tablespoon) of clean engine oil into the cylinder through the spark plug hole. Re-test immediately. If the reading increases significantly — typically more than 15–20 psi — the rings are worn or the bore is scored. The oil is temporarily sealing the ring gap. If the reading does not change, the rings are sealing adequately and the problem is at the valves — either a burned valve, a seat issue, or a valve that is not fully closing.
Running compression test
A running compression test measures cylinder pressure at idle with the engine running rather than cranking. It is less commonly used but reveals different information — specifically, valve timing issues and intake manifold vacuum leaks that do not show up on a cranking test.
With the engine running at idle and a compression gauge or transducer installed in a cylinder, a normally functioning cylinder shows a consistent reading in the 60–90 psi range (specification varies widely — use service data). More importantly, each stroke should be identical. An intake valve that is not fully closing causes low and varying readings on the intake strokes. An exhaust valve problem causes high and varying readings on specific strokes. A cylinder with good valve timing and sealing shows very consistent readings stroke-to-stroke.
A running compression test is most useful when cranking compression is borderline or normal but drivability symptoms persist — it catches valve timing issues that appear mechanical in operation but do not reduce peak static compression significantly.
Leak-down test
Where the compression test shows that pressure is low, the leak-down test shows where it is going. This is the test that separates a ring problem from a valve problem from a head gasket problem — and it identifies it without disassembly.
Tools needed
- Cylinder leak-down tester — a two-gauge tool that connects regulated compressed air to the cylinder and measures the percentage of pressure lost
- Spark plug thread adapter for the engine
- Compressed air source (minimum 100 psi shop air)
- Breaker bar and socket to rotate the engine by hand
Procedure
- Bring the cylinder being tested to top dead centre (TDC) on the compression stroke. Both intake and exhaust valves must be closed — TDC on the compression stroke, not the exhaust stroke. Use the compression stroke TDC mark on the timing cover or feel for when the cylinder pressurizes as you rotate toward TDC.
- Lock the engine at TDC. With compressed air flowing into the cylinder, the piston will try to move if the engine is not locked. Use a ratchet on the crankshaft pulley bolt to hold the engine, or have an assistant hold the breaker bar.
- Connect the leak-down tester and apply compressed air (typically 80–100 psi). Read the leak percentage on the gauge. Most testers have an inlet gauge (showing applied pressure) and an outlet gauge (showing actual cylinder pressure). The difference is the leak percentage.
- Record the leak percentage and listen for where air is escaping:
- Air from the oil filler cap or PCV valve — rings leaking into the crankcase
- Air from the intake manifold (felt at the throttle body or air filter) — intake valve not seating
- Air from the exhaust pipe — exhaust valve not seating
- Bubbles in the coolant reservoir — head gasket or cracked head (combustion gas entering coolant passages)
- Air from an adjacent spark plug hole — head gasket failure between cylinders
- Repeat for all cylinders.
Interpreting leak-down results
| Leak percentage | Assessment |
|---|---|
| 0–5% | Excellent — new or like-new condition |
| 5–10% | Good — normal for a used engine with some mileage |
| 10–20% | Fair — acceptable for higher mileage, worth monitoring but not urgent |
| 20–30% | Poor — internal wear is contributing to symptoms; repair likely needed |
| Above 30% | Severe — cylinder is not sealing adequately; engine repair is required |
The specific leak direction matters as much as the percentage. 25% leakage into the crankcase means ring wear. 25% leakage into the intake manifold means a burned or poorly seating intake valve — a different repair entirely. An engine with 8% leakage into the coolant (head gasket) is a more urgent and expensive repair than one with 25% into the crankcase that has good oil pressure and just uses oil.
Frequently asked
My compression is at the manufacturer minimum in all cylinders. Is the engine okay?
At-minimum compression is not good compression — it means the engine is at the edge of the acceptable range. Consistency between cylinders matters more than absolute numbers in many cases, but a cylinder at minimum with others significantly higher indicates that cylinder is worn relative to the others. A leak-down test on the low cylinder tells you whether the wear is in the rings or valves and guides the repair decision.
Can I do a compression test on a diesel?
Yes, but the procedures and tools differ. Diesel compression is significantly higher — typically 400–600 psi — and requires a diesel-specific compression gauge rated for that range. Diesel glow plugs are removed rather than spark plugs. Minimum compression thresholds and the significance of variation between cylinders follow the same logic, but the absolute values are completely different. Consult manufacturer specifications for the platform.
The compression test looks fine but I still have a misfire on one cylinder. What now?
Normal compression rules out most mechanical causes but does not rule out all of them. A running compression test may find a valve timing issue that static cranking compression misses. Beyond that, move to ignition (scope the coil primary, check secondary resistance), fuel (monitor injector pulse width and duty cycle with a scope or current clamp), and cylinder contribution testing with a scan tool — disable cylinders one at a time and measure RPM drop to confirm which cylinder is actually misfiring versus which one is reporting it.