What does an engine compression test tell me about the health of my
There are different ways to check engine compressions, and they
can reveal specific internal defects.
- Remove all spark plugs.
- Ground center spark plug wire AWAY from the cylinders
- Make sure you have a healthy battery & the car is at operating temp.
- Attach compression gauge on cylinder 1
- Put car in neutral & have friend push in clutch & accelerator
- Have friend crank engine, note FIRST reading and reading after
pressure stabilizes (3-5 cranks).
- Note down readings and repeat for all cylinders
- A healthy engine should have all readings near mfg-ers spec,
and should be about +/- 10% of each other. If not...you are in trouble.
- If one or more is low, but not adjacent cylinders (e.g. 1 & 3) suspect either a burned valve or worn rings. If adjacent
cylinders are low, also suspect a bad head gasket or a warped head.
- To determine whether it's rings or valve, do a leak down test =>Add a couple spoon full of regular engine oil to the engine through
the spark plug hole of the offending cylinder
- Repeat measurement. If it remains low => Valve/Head, else if the
readings jumped up => Rings.
- To distinguish between head vs valves => see a professional. They'll lock the drive shaft, put air pressure on the cylinders
and watch for bubbles in the coolant fluid.
- Also the first reading and the final readings should not be
too far apart.
Compression should come up to around 180 - 200 psi.