Got a Honda with a steady misfire on #3, coils and plugs swapped, injector swapped, still on #3.

Compression Test First — it's quick and tells you a lot.

When to do it: Anytime a cylinder acts up after ruling out ignition and fuel. Since you've swapped coils, plugs, and injectors with no change, mechanical is next.

Compression Test Procedure:

• Warm engine, all plugs out, throttle wide open
• Crank 4-5 compression strokes per cylinder
• Compare readings

What's "mechanical":

• Low compression on #3 only = mechanical problem in that cylinder
• Normal: 140-180 psi, within 10% of other cylinders
• If #3 reads significantly lower (say, 90 psi while others are 160+), you've got a mechanical issue

Then go to Leak-Down:

• Only after compression test confirms low pressure
• Requires compressed air (90-100 psi)
• Listen for where air escapes:
• Hissing from intake = intake valve
• Hissing from exhaust = exhaust valve
• Bubbles in radiator = head gasket or cracked head
• Hissing from oil cap/PCV = rings/piston

Red flags requiring a pro:

• Any coolant loss or overheating history
• Bubbles in cooling system during leak-down
• Head gasket failure on aluminum Honda blocks often needs machining

Bottom line: Compression test first. If #3 is low, leak-down to pinpoint. If it's valves or gasket, you may need a shop unless you're experienced with head work.

Do a compression test first. It’s faster, requires only a gauge, and tells you if the cylinder can build pressure. If compression is within 10–15% of the other cylinders (typically >150 psi on a healthy Honda), the rings, valves, and head gasket are likely sealing well enough to run. Move to ignition/sensor diagnostics (CKP/CMP correlation, wiring, ECU driver).

Go straight to leak-down if:

1. Compression is low on #3.
2. Compression is "good" but the misfire is load-dependent (suggests a sealing issue only under dynamic conditions).
3. You suspect a burnt valve, bent valve, or head gasket breach between cylinders.

*Leak-down tells you where the air goes:*

• Intake manifold (throttle body): Intake valve sealing failure (burnt/bent/carbon).
• Exhaust pipe/tailpipe: Exhaust valve sealing failure.
• Radiator/coolant overflow (bubbles): Head gasket breach or cracked head/block.
• Dipstick tube/PCV/oil cap: Ring/cylinder wall wear or piston damage.
• Adjacent spark plug hole (pull #2 & #4 plugs): Head gasket breach between cylinders.

Mechanical "Smoking Gun" Thresholds (Honda typical):

• Compression: < 100 psi = major mechanical fault. > 20% variance from highest cylinder = mechanical fault.
• Leak-down: > 20% total leakage = engine needs work. > 10% difference between cylinders = fault in the high-leak cylinder. Listen at the ports to pinpoint the component.

Pro Tip: On Hondas, verify valve adjustment before leak-down. A tight (zero lash) valve holds the valve open, mimicking a burnt valve on leak-down and killing compression. Check lash cold per FSM specs.