A Suzuki compression test is done with the engine warmed up and then shut off, all spark plugs out, the fuel injector connectors unplugged on EFI models, the safety lanyard pulled (printed for every family up to the DF25A/30A), the throttle held wide open, and the engine cranked on the starter motor or recoil starter while you note the highest reading for each cylinder. Suzuki's compression gauge is 09915-64512; the attachment that screws into the plug hole varies by family. The figures to compare against are below by model, with the service-manual figure and the figure from Suzuki's 2025 maintenance manual shown side by side, because for one family they differ.
Two things to know before you trust a number. First, Suzuki prints this note under its compression specs: "Figures shown are guidelines only, not absolute service limits." Second, on the small motors with a decompression system (DF4/5/6, DF8A/9.9A, DF9.9B/15A/20A, DF25A/30A) the reading depends on how you crank it. The DF8A/9.9A and DF25A/30A manuals print separate recoil-starter and starter-motor figures, and they are far apart. Compare against the row for the way you cranked it.
What a compression test tells you
The DF25A/30A service manual puts it plainly: "The compression pressure reading of cylinder is good indicator of its internal condition. The decision to overhaul the power unit is often based on the results of a compression test." Keep your own record of the readings at each service, because the printed ranges are wide (the DF40A to DF100B window is 1,200 to 1,800 kPa) and a motor's own history tells you more than one reading against a guideline.
On a multi-cylinder motor the number that matters most is the spread. Every multi-cylinder service manual in our library prints the same limit: maximum difference between cylinders 100 kPa (1.0 kg/cm², 14 psi). One cylinder well below its neighbors points at that cylinder; all cylinders low and even usually points at the test conditions or at wear everywhere.
Tools: gauge and attachment by manual
Each service manual names the gauge and the plug-hole attachment it expects. The gauge number is the same in every manual that prints it; the attachment is not, so match it to your family.
| Service manual | Compression gauge | Compression gauge attachment |
|---|---|---|
| DF9.9B/DF15A/DF20A (99500-89L00-01E) and DF25A/DF30A (99500-94L10-01E) | 09915-64512 | 09915-63311 |
| DF40A/DF50A/DF60A (99500-88L01-01E) and DF70A/DF80A/DF90A/DF100B (99500-87L10-01E) | 09915-64512 | 09915-69910 |
| DF2.5, DF4/5/6, DF8A/DF9.9A, DF100A/DF115A/DF140A | Not quoted in the manual text we hold; check yours | Not quoted in the manual text we hold; check yours |
Boat Max Online stocks the Suzuki OEM compression tester attachment 09915-67010, which connects to the spark plug hole for cylinder compression testing on Suzuki outboards, and the rest of the Suzuki OEM service tools. Since the manuals print different attachment numbers by family, have the model code and serial number from the ID plate ready and we will confirm the fit before you order. A good aftermarket gauge with the correct spark-plug thread adaptor reads the same pressure; the procedure is what makes the reading comparable to Suzuki's figures.
The procedure, as the service manuals print it
The steps below are the DF25A/DF30A manual's, which covers tiller and remote models and both cranking methods. Where another family's manual adds or drops a step, it is noted in that step.
- Start the engine and allow it to warm up, then shut the engine off. The manuals all start here; a reading on a cold engine is not comparable.
- Get to the plugs. DF9.9B through DF30A: remove the STBD and PORT lower side covers. DF40A/50A/60A: disconnect all ignition coil connectors, remove the bolts securing the ignition coils, then remove all ignition coils and spark plugs.
- Remove all spark plugs. While they are out, read them; the right plug and gap for your model is on the spark plug chart.
- EFI models: disconnect all fuel injector connectors at the fuel injectors. This is printed for the DF9.9B/15A/20A, DF25A/30A, DF40A/50A/60A and DF70A to DF100B. The carbureted DF2.5, DF4/5/6 and DF8A/9.9A have no injectors; on those the lanyard step is the fuel-safety step.
- Install the compression gauge attachment into the spark plug hole, then connect the attachment to the compression gauge.
- Disconnect the safety lanyard from the emergency stop switch. The manual's warning: "If this test is performed without disconnecting the safety lanyard from the emergency stop switch, a spark from a spark plug cap may ignite fuel vapor coming out of the spark plug holes during the test, causing a fire or explosion." The DF2.5, DF4/5/6 and DF8A/9.9A manuals print the same instruction: disconnect the safety lanyard from the emergency stop switch prior to cranking the engine. The DF40A/50A/60A manual does not print a lanyard step; it has you disconnect and remove the ignition coils instead. The DF70A to DF100B manual prints neither.
- Remote control models: disconnect the remote control throttle cable from the throttle lever. Printed for the DF25A/30A, DF40A/50A/60A and DF70A to DF100B.
- Move and hold the throttle control grip (tiller) or the throttle lever (remote) in the full-open position.
- While cranking the engine with the starter motor or recoil starter, note the maximum compression pressure reading on the gauge for each cylinder.
- Reinstall the parts removed earlier (spark plugs, side lower covers, ignition coils, etc.).
The decompression note on the DF25A/DF30A
The DF25A/30A manual prints a note that explains why its spec has two rows: "Reading of the cylinder compression pressure value will be affected by the decompression system." Cranked by the starter motor, "the reading will usually be 1 500 – 1 800 kPa," even though the printed range starts at 500 kPa. That is why the warm-up is step one: a reading taken on a cold motor, or with a battery that cranks slowly, is not comparable to these figures. Take the reading with the motor hot and the battery known good before you read anything into a low number.
Suzuki compression spec by model
The first table covers the families with a service manual in our library and gives the figure as that manual prints it, in kPa with the kg/cm² and psi equivalents it prints. The last column is the same family's row in Suzuki's 2025 maintenance manual, which prints kPa only, with MZ meaning manual start and ME meaning electric start. Where the two documents disagree, both are shown with their source; we do not average them.
| Model (service manual) | Cranking method | Standard compression (service manual) | Max difference between cylinders | 2025 maintenance manual |
|---|---|---|---|---|
| DF2.5 (99500-97J01-01E) | Recoil starter | 960–1,400 kPa (9.6–14.0 kg/cm², 137–199 psi) | Not printed (one cylinder) | DF2.5 ’06 on: 960–1,400 kPa (same) |
| DF4/DF5/DF6, carbureted (99500-91J03-01E) | Recoil starter, with decompression system | 550–650 kPa (5.5–6.5 kg/cm², 78–92 psi) | Not printed | DF4/5/6 ’02–’15: 530–695 kPa. Differs from the service manual; both figures are Suzuki's. |
| DF8A/DF9.9A, manual start (99500-99J00) | Recoil starter, with decompression system | 300–900 kPa (3–9 kg/cm², 43–128 psi) | 100 kPa (1.0 kg/cm², 14 psi) | DF8A/9.9A ’06 on: 300–900 kPa (MZ) (same) |
| DF8AE/DF8AR, DF9.9AE/DF9.9AR, electric start (99500-99J00) | Starter motor, without decompression system | 1,000–1,600 kPa (10–16 kg/cm², 142–228 psi) | 100 kPa (1.0 kg/cm², 14 psi) | DF8A/9.9A ’06 on: 1,000–1,600 kPa (ME) (same) |
| DF9.9B/DF15A/DF20A (99500-89L00-01E) | Recoil starter, with decompression system | 350–900 kPa (3.5–9 kg/cm², 50–128 psi) | 100 kPa (1.0 kg/cm², 14 psi) | DF9.9B/15A/20A ’13 on: 350–900 kPa (same). Neither document prints a separate electric-start figure for this family. |
| DF25A/DF30A, recoil (99500-94L10-01E) | Recoil starter | 500–1,000 kPa (5–10 kg/cm², 71–142 psi) | 100 kPa (1.0 kg/cm², 14 psi) | DF25A/30A ’15 on: 500–1,000 kPa (MZ) (same) |
| DF25A/DF30A, electric start (99500-94L10-01E) | Starter motor | 500–1,800 kPa (5–18 kg/cm², 71–256 psi); usually reads 1,500–1,800 kPa | 100 kPa (1.0 kg/cm², 14 psi) | DF25A/30A ’15 on: 500–1,800 kPa (ME) (same) |
| DF60A, DF60AT/DF60ATH (99500-88L01-01E; the DF40A/DF50A supplements are not in our copy) | Starter motor | 1,200–1,800 kPa (12–18 kgf/cm², 171–256 psi) | 100 kPa (1.0 kgf/cm², 14 psi) | DF40A/50A/60A/50AV/60AV ’10 on: 1,200–1,800 kPa (same) |
| DF70A/DF75/DF80A/DF90A/DF100B (99500-87L10-01E) | Starter motor | 1,200–1,800 kPa (12–18 kgf/cm², 171–256 psi) | 100 kPa (1.0 kgf/cm², 14 psi) | DF70A/80A/90A/100B ’09 on: 1,200–1,800 kPa (same) |
| DF100A/DF115A/DF140A (99500-92J00-01E) | Not quoted in our copy; see your manual | 1,000–1,600 kPa (10–16 kgf/cm², 142–228 psi) | 100 kPa (1.0 kgf/cm², 14 psi) | DF100A/115A/140A ’13 on: 1,000–1,600 kPa (same) |
For the rest of the range we have no service manual on file, so the only Suzuki figure we can give is the 2025 maintenance manual's, in kPa as printed. The maintenance manual prints no maximum-difference figure and no psi column; for scale, the service manuals print 100 kPa as 1.0 kg/cm² or 14 psi. Years are model years as the manual prints them.
| Model (2025 maintenance manual) | Model years | Cylinder compression, kPa |
|---|---|---|
| DF4A/DF5A/DF6A | ’17 on | 400–700 |
| DF25 (V2) | ’06–’14 | 500–700 (MZ); 1,000–1,300 (ME) |
| DF25/DF30 | ’06–’10 | 1,000–1,400 |
| DF40/DF50 | ’00–’10 | 1,176–1,765 |
| DF60/DF70 | ’00–’09 | 980–1,520 |
| DF90/DF100/DF115 | ’01–’12 | 1,300–1,700 |
| DF140 | ’02–’12 | 1,200–1,600 |
| DF100C/DF115B/DF140B; DF115BG/DF140BG | ’22 on; ’21 on | 1,200–1,800 |
| DF150/DF175 | ’06 on | 1,100–1,700 |
| DF150TG/DF175TG | ’14–’16 | 1,100–1,700 |
| DF150A/DF175A; DF150AP/DF175AP | ’19 on; ’17 on | 1,100–1,700 |
| DF200/DF225/DF250 | ’04 on | 1,100–1,700 |
| DF300 | ’07–’11 | 1,100–1,500 |
| DF250AP/DF300AP | ’12 on | 880–1,500 |
| DF300B/DF325A | ’20 on | 570–950 |
| DF350A | ’18, ’19 on | 700–1,160 |
| DF9.9/DF15 (’00–’12) and DF200A/DF200AP (’15 on) | — | The rows for these models are printed in a way our copy cannot read reliably; see your manual |
Reading the result
All cylinders low, and close together
Check the test before the engine. Was the motor fully warmed up, were all plugs out, was the throttle held wide open, was the battery strong, and are you comparing against the row for the cranking method you used? On the decompression motors a recoil-starter number will always look low beside the starter-motor row. If the method was right and every cylinder is still under the range, the wear is everywhere rather than in one cylinder, and the overhaul decision the manual describes is a dealer conversation.
One cylinder below the others
On the multi-cylinder families the manuals set the limit at 100 kPa (1.0 kg/cm², 14 psi) between cylinders. Beyond that, the service manuals give one list of causes, and it reads the same in every manual that prints it: "Low compression pressure can indicate one or more of following: Excessively worn cylinder wall; Worn piston or piston rings; Stuck piston rings; Poor seating of valves; Ruptured or otherwise damaged cylinder head gasket."
Of those five, only the valve one has a non-invasive first check. A valve with no clearance cannot close fully, and that shows up as low compression on that cylinder; the intervals and figures are on the valve clearance chart. Telling the others apart means the cylinder head comes off, which is power-unit overhaul work in the service manuals and a dealer job. If the motor is under warranty, stop at the reading and talk to your dealer before anything comes apart.
Compression is fine but the motor runs badly
Then the problem is not in the cylinder. Work through fuel, ignition and the caution systems on the outboard troubleshooting page; the family service guides, such as the DF25A/DF30A service guide and the DF70A/DF90A/DF100B service guide, list the service data for each model.
Oil pressure: the other number worth taking
The service manuals print engine oil pressure next to compression. It is read with a gauge fitted to the engine (the DF40A/50A/60A manual has you install the oil pressure gauge in place of the service port), after warming up, in forward gear at 3,000 r/min. The DF9.9B/15A/20A manual adds that you connect the oil pressure switch lead wire to engine body ground with a jumper wire before turning on the ignition switch.
| Model | Oil pressure (service manual) | 2025 maintenance manual, kPa |
|---|---|---|
| DF4A/DF5A/DF6A | No service manual in our library | 100–500 (’17 on) |
| DF8A/DF9.9A | 240–400 kPa (2.4–4.0 kg/cm², 34–57 psi) at 3,000 r/min, oil at normal operating temperature | 240–400 |
| DF9.9B/DF15A/DF20A | 200–500 kPa (2.0–5.0 kg/cm², 29–71 psi) at 3,000 r/min | 200–500 |
| DF25A/DF30A | 200–500 kPa (2.0–5.0 kg/cm², 28–71 psi) at 3,000 r/min, oil at normal operating temperature | 200–500 |
| DF40A/DF50A/DF60A | 200–400 kPa (2.0–4.0 kg/cm², 28–57 psi) | 200–400 |
| DF70A/DF75/DF80A/DF90A/DF100B | 400–600 kPa (4.0–6.0 kg/cm², 57–85 psi) at 3,000 r/min | 400–600 |
| DF100A/DF115A/DF140A | 410–490 kPa (4.1–4.9 kg/cm², 63–70 psi) at 3,000 r/min in forward gear | 410–490 |
The manuals' causes for low oil pressure are a clogged oil filter, leakage from oil passages, a defective oil pump, a defective oil pressure regulator, a damaged O-ring, or a combination; for high oil pressure, an engine oil of too high viscosity, a clogged oil passage or a clogged oil pressure regulator. The gauge is the Suzuki OEM oil pressure gauge 09915-77311.
Parts for this job
For the test itself: the compression tester attachment 09915-67010, the oil pressure gauge 09915-77311 and the rest of the Suzuki OEM service tools. Since the plugs are out anyway, a fresh set of plugs is the cheap part of the job; the part number and gap for your model are on the spark plug chart linked above. Boat Max Online is an authorized Suzuki, Honda and Yamaha Marine dealer; orders over $149 ship free to the lower 48.
Sources: Suzuki service manuals 99500-97J01-01E (DF2.5), 99500-91J03-01E (DF4/DF5/DF6), 99500-99J00 (DF8A/DF9.9A), 99500-89L00-01E (DF9.9B/DF15A/DF20A), 99500-94L10-01E (DF25A/DF30A), 99500-88L01-01E (DF60A), 99500-87L10-01E (DF70A/DF80A/DF90A/DF100B) and 99500-92J00-01E (DF100A/DF115A/DF140A); Suzuki 4-Stroke Outboard Maintenance Manual, 2025.02 edition. Figures are Suzuki's published values; confirm against the manual for your model year and serial number.