Ask what “engine checked” actually means.
A PPI might include a visual examination, a road test and diagnostic checks. That does not establish that the workshop has measured compression or performed a cylinder leak-down test. Before booking, ask what is included and whether either test is recommended for the particular car.
For someone considering an F355, 360, 993 or Diablo, this is a useful conversation to have early. The point is to understand how well the engine’s cylinders are sealing, and whether a result calls for closer investigation before committing to the car.
Compression: the pressure a cylinder builds.
A conventional compression test measures the pressure produced while the starter turns the engine. The workshop records each cylinder’s reading, usually in psi or bar, and compares the results with the correct specifications and with one another.
An unusually low cylinder can flag a problem, but the reading alone does not reliably identify its cause. The test setup matters. Engine temperature, cranking speed and the procedure used all belong in the interpretation. Ask for the figures and conditions, rather than only a “passed” summary. OTC compression tester manual.
Leak-down: how well it holds applied pressure.
A leak-down test uses a controlled supply of compressed air in a stationary cylinder, with the piston positioned on its compression stroke and the valves closed. The instrument indicates leakage, commonly expressed as a percentage.
Where air escapes helps direct the diagnosis: the intake can point toward an intake-valve sealing issue; the exhaust toward an exhaust valve; and the crankcase toward rings or cylinder sealing. On a liquid-cooled engine, bubbles in the cooling system can indicate another leakage path. These are clues for a specialist to investigate, not a complete diagnosis. Some leakage is normal. OTC cylinder leakage tester instructions.
Why they can be worth paying for.
The tests answer related but different questions. Compression helps identify a cylinder that builds pressure differently; leak-down can help a workshop investigate where pressure is escaping. Used appropriately, they can provide evidence that a general description of how the engine sounds cannot.
The benefit is especially worth discussing when history is incomplete, there are symptoms or unresolved concerns, or you want a documented baseline before purchase. Ask for a quote and the specialist’s reasoning. Their value depends on the car, the questions still open and how the findings would affect your decision.
Pay for a useful test, a clear record and an explanation. A reassuring percentage without context is not enough.
Do not turn one number into a guarantee.
A universal “good” compression figure or leak-down percentage is a poor substitute for model-specific interpretation. Ask which specifications, equipment and procedure the workshop used. Results obtained under different conditions are not automatically comparable.
These tests assess aspects of cylinder sealing; they do not assess every part of an engine or predict its remaining life. A good result does not establish the condition of bearings, belts, cooling components or the entire valve-guide system. An abnormal result may call for a repeat measurement or other investigation before anyone recommends a repair.
What to request in the report.
- The vehicle identity, test date and odometer reading with its unit.
- A result for every tested cylinder, clearly mapped to the engine’s cylinder numbering.
- Pressure units for compression; leakage values and test pressure for leak-down.
- Engine temperature or warm/cold condition, equipment and procedure notes.
- Any observed leakage paths, limitations and recommended next steps.
- The inspector’s explanation of the results against the applicable specification.
Agree on access and authorization with the seller and workshop beforehand. Have an appropriately equipped model specialist carry out the tests and explain what the results mean for the particular car.