How to Do a Compression Test on an Outboard, Sterndrive, Inboard or PWC

Buy or rent a compression gauge made for gasoline engines. It screws into the spark plug hole with a hose and holds the needle at the highest reading until you press the release button. Boat engines use different plug threads, so check that the kit includes an adapter that fits yours. A gauge you press into the hole by hand leaks and gives low readings; the screw-in type is far more consistent.

Every cylinder must be cranked at the same speed, or the numbers can't be compared. Put the battery on a charger until it is full. A battery that slows down as you go will make the last cylinders read lower than the first, which can point you at a problem that isn't there.

Power & Motoryacht reports that testing a cold engine gives less accurate results, so warm it to operating temperature first. Yachting Magazine notes that pros disagree on this, so the most important thing is to test every cylinder under the same conditions.

Give it cooling water: an outboard in the water, in a test tank or on external flush muffs (the built-in flush port on most outboards is for flushing with the engine off, so don't run on it unless your manual allows it). Sterndrives and inboards run in the water or with the cooling-water hookup your manual specifies. PWCs run only with water supplied as their manual describes. Then shut the engine off. The exhaust and powerhead will be hot, so wear gloves.

If the engine won't run, you can still test it cold. Just keep in mind the readings may be a bit lower across the board.

• Shift to neutral and keep people clear of the propeller.
• On sterndrives and inboards, open the hatch, run the blower for at least 4 minutes and sniff for fuel.
• On PWCs, remove the seat or engine cover and let the hull air out.
• Have a helper at the key so you can watch the gauge. Yachting Magazine recommends two people so you are not running back and forth between the helm and the engine.

Mark each plug wire or coil with its cylinder number before you unplug anything. Blow or brush away dirt and salt around each plug so nothing falls into the cylinder. Then remove all the plugs, not just one. With the other plugs in, the engine fights compression in every other cylinder and cranks slower.

Lay the plugs out in cylinder order. A plug that looks very different from the others (wet, oily, white or damaged) is a clue about that cylinder.

Pulling the plugs stops the engine from running, but it does not stop the system from making sparks or squirting fuel.

• Ignition: follow your service manual. On many engines, removing the kill-switch clip grounds the ignition. Some engines will not crank with the clip removed, so the manual will tell you to unplug the ignition coils or the stop circuit instead. Never leave high-voltage leads hanging loose near the open plug holes.
• Fuel injection: Yachting Magazine points out that many outboards still inject fuel during the test. Disable the fuel pump or injectors the way your manual describes (often by pulling a fuse or relay or unplugging the injector harness).
• Carbureted engines: close the fuel valve or disconnect the tank line if your manual suggests it, and keep rags handy.

Many service manuals call for the throttle held wide open during the test so the engine can draw in a full breath of air. Yachting Magazine notes that pros don't agree on this and that it isn't critical, but whatever you choose, do every cylinder the same way. Open the throttle only in neutral. On remote-control boats, check your manual for how to advance the throttle without shifting into gear (often a neutral warm-up feature).

Thread the gauge adapter into the first plug hole by hand, snug, so it seals without forcing it. Keep the gauge hose and any loose wires clear of the flywheel and belts. Have your helper crank the engine while you watch the needle. It should climb in steps with each compression stroke. Keep cranking until it stops rising, usually after about five or six "puffs" or a few seconds. Write down the reading.

Small and rope-start outboards: many have an automatic decompression feature that bleeds off some pressure at very low cranking speeds, so the engine is easier to pull over. A slow pull on the rope can then read far lower than the engine's true compression and make a healthy motor look worn. Crank briskly and evenly (electric start if you have it), and check the service manual for whether the decompression feature has to be disabled and what reading to expect.

If the engine refuses to turn over, or turns and then stops hard, stop cranking. Something may be locking it, such as water in a cylinder. SAIL Magazine describes how cranking against water did no damage by itself, but the engine firing on the next attempt bent a connecting rod. Do not keep trying; call a pro.

Release the gauge pressure, move it to the next cylinder and crank the same way, for the same number of puffs. Record each cylinder's number next to its reading. If one reading surprises you, repeat that cylinder once more to make sure the gauge was sealed properly.

If water or coolant sprays from a plug hole or the gauge shows moisture, stop. Water inside a cylinder means a failed head gasket, cracked part, exhaust problem or flooding, and it needs a pro quickly (see the water-in-engine-oil and submerged-engine-recovery guides).

A normal reading doesn't fully clear a cylinder if you see other clues. Look at the spark plug from each cylinder as it comes out: one that looks noticeably cleaner or "steam cleaned" next to the others, milky residue under the oil fill cap or valve cover, or a mechanic's camera (borescope) showing a shiny, carbon-free piston top or rust on the cylinder wall all point to water getting into that cylinder. Owners often find a low cylinder with exactly these signs after a head gasket or exhaust riser leak.

Look at the spread from the highest to the lowest cylinder, not just the numbers themselves.

• All cylinders close together: a good sign. Yachting Magazine suggests all should be within about 10 percent of each other. Power & Motoryacht reports experts using 10 to 15 percent, and Practical Sailor uses within 20 percent of the average of the others. Your service manual may give its own limit and a minimum value; use it if it has one.
• One cylinder clearly low: that cylinder has a sealing problem, such as a worn or broken ring, a burned or stuck valve, or a damaged piston.
• Two cylinders next to each other both low: Yachting Magazine and Practical Sailor both point to a likely head gasket leak between them.
• One cylinder near zero: a serious fault such as a broken valve or a hole in a piston. Do not run the engine until it is diagnosed.
• All cylinders unusually high: Practical Sailor notes this can mean heavy carbon buildup. A faulty gauge, or liquid (fuel, oil or water) sitting in the cylinders, can also push readings up. Check the gauge on another engine you know, and look at the plugs and plug holes for wetness before you celebrate a "strong" result.
• All cylinders low by a similar amount: could be general wear, a cold engine, slow cranking or a leaky gauge. Repeat the test with a full battery and a warm engine before drawing conclusions.

Squirt a small amount of clean engine oil (about a teaspoon, not a flood) into the low cylinder through the plug hole, then test it again.

• Reading rises noticeably: the oil temporarily sealed the rings, so worn or stuck rings are the likely cause. Power & Motoryacht describes this same check by an engine surveyor.
• Reading barely changes: the leak is more likely at a valve (on a four-stroke) or the head gasket.

Too much oil can raise the reading falsely or even hydraulically lock the cylinder, so keep it small. Crank the engine a few turns with the plugs out afterward (ignition and fuel still disabled) to clear it.

A leak-down test is the next step, usually done by a mechanic or engine surveyor. The piston of the suspect cylinder is placed at the top of its compression stroke with both valves closed, and regulated compressed air is fed in through the plug hole. A gauge shows how much air the cylinder loses, and where you hear the air escape tells you the cause:

• Out of the exhaust: an exhaust valve is leaking.
• Out of the intake or throttle body: an intake valve is leaking.
• Out of the oil fill or dipstick tube: worn or damaged rings are letting air into the crankcase.
• Bubbles in the coolant, or air hissing out of the next cylinder's plug hole: a head gasket or crack.

Air pressure can turn the engine suddenly if the piston is not exactly at the top, so hands must stay clear of the flywheel, belts and propeller. Two-stroke outboards have no valves and need their ports sealed for a leak-down test, which is a shop job.

Reinstall the plugs (or new ones if they were due) by hand first, then tighten to the spec in your owner's manual. Reconnect the coils, plug wires, fuel pump fuse, injector plugs and the kill-switch clip exactly as they were. Close the throttle and check that the shift is in neutral. Start the engine with cooling water supplied and let it idle; it should run smoothly. A rough idle usually means a wire or coil went back on the wrong cylinder.

• Even readings and the problem continues: the internals are probably fine. Go back to spark, fuel delivery, the propeller and the lower unit.
• One weak cylinder: get a leak-down test and a diagnosis before spending money on other parts.
• Pre-purchase test shows a big spread or a dead cylinder: treat it as a major repair and factor it into the price, or walk away. A full engine survey by a certified mechanic is worth it on any boat where the engines make up much of the value.