Compression and leak-down test
Two workshop tests of how well each cylinder seals — one measures pressure built, the other measures air lost.
Also called: compression test · leak-down test · cylinder leakage test
What it means
Definition
A compression test cranks the engine with a gauge in the spark plug hole and records the pressure each cylinder builds. A leak-down test instead holds the cylinder at top dead centre, feeds in regulated air, and measures how much escapes. The first shows which cylinder is weak; the second shows where the air is going — past the rings, a valve, or the head gasket.
Why a buyer cares
What it changes
Both are things a buyer can ask an inspecting mechanic to perform. Neither depends on a warning light being on.
In practice
Where you meet it
On a pre-purchase inspection report, listed per cylinder as a pressure reading or a leakage percentage.
Two things that look alike
Not the same as
A compression test says a cylinder is weak; only a leak-down test indicates why.
Read next
Related terms
Pre-purchase inspection (PPI)
A mechanical inspection of a used car by a mechanic the buyer chooses, carried out before the sale closes.
Head gasket
The seal between the engine block and cylinder head that keeps combustion, coolant and oil apart.
Engine oil consumption
Oil an engine uses internally between changes, as distinct from oil lost through a leak.
Where this comes from
Sources
The definition here is written from what the manufacturers’ own manuals say, but those documents could not be opened and quoted, so nothing is being presented as a citation. How we source data
NO CITABLE SOURCE. These procedures are documented in paid service manuals and technician training; everything freely readable was a shop, parts or tool-seller page, all outside the source policy. The description above carries no pressure or percentage figures for that reason. Publish through the empty-source path or hold.