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newSchoolWorks.com › VoTechWorks › Transportation › Marine Engine / Outboard › Inboard and Sterndrive Systems › Lesson 9
TRN-MARI21-03 · Lesson 9 · 10.5 clock hours · lab

Compression and Cylinder Leakdown Testing

Objective. Students will perform compression and cylinder leakdown tests on an inboard engine, interpret results to locate internal faults, and document findings on a repair order.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Compression and Leakdown Testing Matter

An inboard engine produces power only when combustion gases are sealed tightly inside each cylinder. Worn piston rings, burned or bent valves, and failed head gaskets all allow pressure to escape, reducing power and increasing fuel consumption. Compression testing and cylinder leakdown testing are the two standard diagnostic procedures that locate these internal faults before a teardown begins. Together they answer two questions: how much pressure does the cylinder build? and where is the pressure going?

Safety First

Before removing any spark plugs, disconnect the battery negative cable and apply lockout/tagout (LOTO) in accordance with shop policy so the engine cannot crank unexpectedly. Wear safety glasses throughout — compressed air and combustion residue can be ejected from plug holes. Keep hands and clothing clear of the flywheel and drive belts. When connecting the regulated air supply for leakdown testing, confirm the supply pressure is set to the tester manufacturer's recommended value (commonly 100 psi) before threading the fitting into the cylinder. Never lean directly over an open spark plug hole while air is flowing.

Compression Testing: Dry and Wet

A dry compression test measures the maximum pressure a cylinder can hold when cranked at normal starter speed with all spark plugs removed and the throttle held wide open. Remove the plugs carefully, labeling each wire or coil connector to its cylinder. Thread or press the compression tester into cylinder 1, crank the engine for four to six compression strokes, and record the peak reading. Repeat for every cylinder.

Manufacturers publish a minimum acceptable pressure and a maximum allowable variation between cylinders. A common rule of thumb is that no cylinder should read more than about 15 percent below the highest cylinder, but always check the engine service manual — actual limits vary by engine family. Low readings across all cylinders suggest worn rings throughout; a single low cylinder points to a local problem.

After recording dry readings, perform a wet compression test on any low cylinder. Squirt roughly one tablespoon of clean engine oil into the cylinder through the plug hole, then retest. If the reading rises significantly (often 20 psi or more), the piston rings are worn or the cylinder wall is scored because the oil temporarily seals the ring gap. If the reading does not improve, the problem is more likely at the valves or head gasket, not the rings.

Cylinder Leakdown Testing

A leakdown tester uses regulated shop air to pressurize a cylinder held at top dead center (TDC) of the compression stroke — the point where both valves are closed. The tester has two gauges: one reads supply pressure and the other reads the pressure remaining in the cylinder. The difference, expressed as a percentage, is the leakdown rate.

To find TDC compression stroke, rotate the crankshaft by hand while watching the timing mark and confirming that the piston is coming up on compression (both valves closed). An easy check: place your thumb over the plug hole — you will feel air pressure build just before TDC. Once the piston is at TDC, thread in the leakdown fitting and apply air slowly.

While air is flowing, listen at three locations:

Generally accepted leakdown thresholds: under 10 percent is excellent; 10–20 percent is acceptable for a used engine; above 20 percent warrants investigation; above 30 percent indicates significant wear or damage requiring repair. Always confirm with the engine manufacturer's specifications.

Documenting Findings on a Repair Order

Accurate documentation is a professional and legal requirement. A repair order (RO) narrative must include: the test performed, the tool or tester used, each cylinder's recorded value, the specification from the service manual, the observed fault location (from leakdown listening), a probable cause, and the recommended corrective action. Use standard service abbreviations such as cyl for cylinder, TDC for top dead center, spec for specification, R&R for remove and replace, and WOT for wide-open throttle. Sign and date every entry. A complete RO protects the technician, informs the service writer, and guides the customer's repair decision.

Putting It Together

Compression and leakdown tests complement each other. A compression test tells you which cylinder is weak; a leakdown test tells you why. Used together and documented clearly, they allow a professional diagnosis without disassembling the engine. These skills are tested in the ABYC Marine Engine credential exam and are expected knowledge in Yamaha and Mercury factory technician programs.

Key terms8

Compression testA procedure that measures peak cylinder pressure during cranking to evaluate the sealing ability of rings, valves, and head gaskets.
Wet compression testA follow-up test where a small amount of oil is added to a low cylinder before recranking; an increase in pressure indicates worn rings rather than valve or gasket failure.
Cylinder leakdown testA test that pressurizes a cylinder held at TDC compression stroke with regulated air and measures the percentage of air that escapes, identifying the escape path.
Top dead center (TDC) compression strokeThe crankshaft position where the piston is at its highest point and both intake and exhaust valves are closed, required for accurate leakdown testing.
Leakdown percentageThe ratio of pressure lost to supply pressure, expressed as a percentage; higher values indicate greater internal leakage.
Cylinder variationThe difference in compression readings between the highest and lowest cylinders, used to identify localized wear; typically expressed as a percentage of the highest reading.
Repair order (RO)The official shop document that records the complaint, tests performed with results, diagnosis, and recommended corrective action; a legal and billing record.
Lockout/tagout (LOTO)A safety procedure requiring that energy sources be isolated and physically locked or tagged before maintenance work begins, preventing accidental engine start during plug removal.

Worked examples4

1. A six-cylinder inboard engine has the following dry compression readings (psi): Cyl 1 – 165, Cyl 2 – 162, Cyl 3 – 148, Cyl 4 – 166, Cyl 5 – 164, Cyl 6 – 160. The manufacturer's minimum is 150 psi and the maximum allowable variation is 15%. Determine which cylinders, if any, are below minimum and whether the variation is within spec.
  1. Identify the highest reading: 166 psi (Cyl 4).
  2. Calculate 15% of the highest reading: 166 × 0.15 = 24.9 psi.
  3. Subtract from the highest to find the minimum acceptable reading based on variation: 166 − 24.9 = 141.1 psi.
  4. Check each cylinder against the manufacturer's minimum of 150 psi: Cyl 3 reads 148 psi, which is below 150 psi — it fails the minimum.
  5. Check Cyl 3 against the variation threshold: 166 − 148 = 18 psi difference; 18 ÷ 166 = 0.108 = 10.8%, which is within the 15% variation limit.
  6. Conclusion: Cyl 3 fails the manufacturer's minimum pressure specification and requires further diagnosis (wet test and/or leakdown test).
Answer: Cylinder 3 (148 psi) is below the manufacturer's minimum of 150 psi. The variation across all cylinders is 10.8%, which is within the 15% limit. Cylinder 3 must be investigated further.
2. After a wet compression test on Cylinder 3, the reading rises from 148 psi to 172 psi. What does this indicate, and what should the technician do next?
  1. Calculate the pressure increase: 172 − 148 = 24 psi.
  2. A significant rise after adding oil means the oil temporarily sealed a leak path at the piston rings.
  3. This indicates that the piston rings are worn, the ring gaps are excessive, or the cylinder wall is scored — not a valve or head gasket problem.
  4. Recommended next step: Perform a leakdown test on Cylinder 3 at TDC compression stroke and listen at the oil fill cap or dipstick tube to confirm ring blow-by before authorizing an internal engine repair.
Answer: The 24 psi rise strongly indicates worn piston rings or cylinder wall wear on Cylinder 3. The technician should perform a leakdown test and listen at the oil fill opening to confirm crankcase blow-by before recommending an internal overhaul.
3. During a leakdown test on Cylinder 2, the supply gauge reads 100 psi and the cylinder gauge reads 73 psi. Calculate the leakdown percentage and evaluate the result.
  1. Leakdown percentage = ((Supply pressure − Cylinder pressure) ÷ Supply pressure) × 100.
  2. Leakdown % = ((100 − 73) ÷ 100) × 100 = (27 ÷ 100) × 100 = 27%.
  3. Compare to general thresholds: under 10% excellent, 10–20% acceptable, above 20% warrants investigation, above 30% indicates significant damage.
  4. 27% falls between 20% and 30% — this is beyond acceptable and warrants further investigation and likely internal repair.
Answer: The leakdown percentage is 27%. This exceeds the generally acceptable threshold of 20% and indicates significant internal leakage in Cylinder 2. The technician must identify the leakage path by listening at the oil fill, exhaust, and intake before recommending corrective action.
4. During the leakdown test on Cylinder 4 (27% leakdown), the technician hears hissing from the exhaust manifold but no noise at the oil fill cap or air cleaner. What is the most likely fault?
  1. Air escaping at the exhaust manifold indicates that air is flowing past the exhaust valve seat.
  2. No noise at the oil fill cap rules out ring or cylinder wall blow-by.
  3. No noise at the air cleaner rules out intake valve leakage.
  4. Conclusion: The exhaust valve in Cylinder 4 is not seating correctly — it may be burned, bent, or the seat may be worn.
Answer: The most likely fault is a leaking exhaust valve (burned, bent, or worn seat) in Cylinder 4. A valve job or cylinder head repair is the recommended corrective action.
In class: Perform a dry and wet compression test on a four- or six-cylinder inboard engine: record all cylinder readings, note the percentage of variation between cylinders, and identify any cylinder falling below the manufacturer's minimum. · Perform a cylinder leakdown test on the same engine using a regulated leakdown tester at TDC compression stroke for each cylinder; listen at the oil fill, exhaust, and air cleaner to identify whether leakage is past rings, valves, or head gasket. · Write a complete repair order narrative summarizing test results, likely fault(s), and recommended corrective action using standard service abbreviations; compare findings with a classmate and discuss.
Materials: Inboard test engine, compression tester, cylinder leakdown tester with regulated air supply, socket set for spark plug removal, service manual with compression specifications, repair order forms
Check: All cylinder compression values and leakdown percentages recorded accurately; correct diagnosis of seeded fault (instructor verified); repair order narrative earns instructor sign-off for accuracy and completeness.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Engine Diagnosis and Testing
1. When performing a dry compression test, why must the throttle be held wide open during cranking?
Engine Diagnosis and Testing
2. A wet compression test is performed on a cylinder with a low dry reading. The wet reading is nearly the same as the dry reading. This result most likely indicates:
Engine Diagnosis and Testing
3. For a cylinder leakdown test to produce accurate results, the piston must be positioned at:
Engine Diagnosis and Testing
4. During a leakdown test, a technician hears a hissing sound at the oil fill opening. This indicates:
Engine Diagnosis and Testing
5. A four-cylinder inboard engine has these dry compression readings: 170, 168, 155, 172 psi. The manufacturer's maximum allowable cylinder-to-cylinder variation is 15%. What is the approximate variation percentage for the lowest cylinder?
Shop Safety and Hazardous Materials
6. Which safety step is required before removing spark plugs from an inboard engine for compression testing?
Engine Diagnosis and Testing
7. A technician records a leakdown test result of 32% on one cylinder and hears bubbling from the coolant overflow bottle. The most likely cause is:
Service Information, Tools, and Safety
8. On a repair order, what information is essential to document after completing compression and leakdown tests?
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