chmodlf

Is the leakdown test like a compression test?

I am interested in the details of how leakdown tests work and what they reveal.

Thanks and Happy Holidays

Dadofour

Until you can get a better, more technical answer, the leakdown is pumpin air into the cylinder and watching how much air is lost over a period of time.

I'm not sure if it determines whether the air is escaping past the rings or the valves....

Matt Romanowski

Leak down tests are done with the cylinder at TDC. You pressurize the cylinder and monitor how much air it takes to keep the cylinder at a constant pressure. You can then tell where the air is leaking. It can go by an intake valve, exhuast valve, or the rings (unless their is something else wrong like a bad head gasket). This give you an idea of how well the compression pressure is kept in the cylinder.

A compression test is a measure of how well everything works in a dynamic setting. It uses the motor itself to pressurize the cylinder through it's normal operation. It will not tell you where/what is leaking.

Most mechanics have different opinions on what each test is useful to find.

Matt

RoninLB

leak down in our cars is better used With a compression test. It's unreliable when used alone.

pwd72s

The mechanic reads the rate (how fast) of the air leak...and a trained ear can hear whether the air is escaping from the oil filler cap or exhaust, letting him tell if the problem is more likely rings, valves, or both...

john70t

But it won't tell you if anything outside the direct cylinder has problems:e.g. valve guides worn, intake/exhaust manifold leaks, head cracked between cylinders, rod/crank bearings sloppy.
Combustion pressures also push the rings out against the cylinder walls and a cold engine with low-drag rings(if it has been modified) will give bad readings, as will carbon stuck in the valve seat.