Compression vs. Leak-down Test

[GregM.]

Thanks for helping out a rookie here:

I've seen a compression test done before... But I don't know what a leak-down test is.

Can you all explain the differences to me?
How do you do one?

Thanks for your time

[Dink]

Leakdown test is done the same as a compression test, except pressure is applied to the cylinder and a different gauge is used to measure the loss of pressure,BTW it is done one cylinder at a time.

Dink

[Steve]

One of the differences between the two is that a compression test will tell you what the engine is capable of doing. If the numbers are a bit low, run a leak-down test to see where it's going and how fast.

With a compression test, you are refreshing the compression every time the piston comes up on the compression stroke, which is a couple of times every second. There is no time to tell if the compression itself is low or there is plenty of compression, but a lot of matching leakage.

A leak-down test will use an air source ( I have used a modified spark plug ), to apply compressed air to a cylinder that is at top dead center (tdc). By monitoring the pressure, you can see how fast it is getting past the valves or rings, then compare with the other cylinders.

In a liquid-cooled engine, this is also useful for determining blown head gaskets, as you will see bubbles in the coolant when pressure is applied to the cylinder. If the rings are leaking, you might be able to hear the rushing noise through the oil filler. If the valves are leaking you could hear it through the intake or exhaust ports.


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[GregM.]

Thanks, I'm getting a clearer picture now. So a leakdown test is probably more expensive to have a mechanic do? You're using more equipment right?

I think I may be burning too much oil (I'll figure it out exactly over the next few weeks)... would either, or both of these tests show what's going on? Then I'd know what repairs would be neccessary.

Thanks for the insight