How to: Perform a Leak-Down Test

[The Mic]

When your street or race engine starts showing signs of “lack-of-power”, a common and yet simple test that can be performed is a “Leak Down” test. A “Leak Down” test pinpoints where you are losing compression and exactly how much before you open up your engine.

The test involves a special gauge and adding compressed air to measure the percentage of leak in the cylinder. One face of the gauge measures the pressure of the compressed air that you are forcing into the cylinder, and the other face measures the percentage of this pressure that is being lost. There is a knob on the gauge that lets you control the pressure going in so you get even results.

There are many manufacturers of these gauges and they are available at just about any auto parts house. Depending on the particular “Leak Down” test gauges you get, the actual test will go something like this:

1. Take out the spark plugs.

2. Bring each cylinder to TDC (top dead center) so that all it’s valves are closed.

3. Screw the gauge adapter into the spark plug well and connect the gauge. The gauge has a fitting to connect the extender hose to the spark plug well, and another to connect the air hose.

4. Apply the compressed air and modulate the knob to get a steady readingfrom both faces on the gauge. Use the same pressure on all cylinders.

5. LISTEN to where you can hear the compressed air.

At the crankcase: Remove the oil filler cap. If you can clearly heara whooshing/howling by listening at the oil filler cap, you're losingcompression through the rings.

At the tailpipe: It is your exhaust valves if you can hear it (or evenfeel puffs on your hand) at the tail pipe.

At the intake manifold: It is your intake valves if you can clearlyhear a whooshing/howling by listening at the throttle body/intake manifold.

6. Repeat steps for each cylinder.

If the results are inconclusive, it could be a blown head gasket. Symptoms of a blown head gasket are: reduced power, white or blue smoke out the tailpipe, oil in the coolant or coolant in the oil. You can check for a blown head gasket by removing the coolant cap and watching the coolant level when you add the compressed air. If it rises or you see bubbles, you have a leaking head gasket.
Modified by S13GUY at 12:12 PM 2/1/2005

[eddiec]

thanks for the effort. now i have a good reason to go buy another tool...

[onosqv]

awesome man, I gotta do this test. Question: How do I bring each cylinder to TDC?

Thanks!

[8ggalant]

rotate the crank w/ a ratchet *ducks*/// i think thats how...lol but i have limited mechanical knowledge so sumone might wanna jump in

[sdtouge]

^thats how ive done it in the past...

[eddiec]

yep big breaker bar and appropriately sized socket for the crank pulley bolt. stick a straw or some other non marring object like a straw or something in the spark plug hole and watch it rise and fall. that will tell you tdc, but it will not tell you if its tdc exhaust stroke or tdc compression.

[boke_as_hell]

How does one know when the cylinder is at TDC without a feelier gauge and a screw driver....I've done that and it takes two people. I also know the notches on the crank pulley, i think it's the second one, but can't really remember.

[benemorius]

It's the second notch from the left on the pulley. But again you won't know if that's tdc compression or tdc exhaust.

Pull the spark plug for cylinder 1 and cover the hole with a finger, leaving a small opening. Rotate the pulley while feeling/hearing the air whoosh around your finger. When you get to the second notch from the left and the air has just finished whoosing out of the cylinder, you are at tdc on the compression stroke. Make sure you get it centered on the notch well (or if you're not doing cylinder 1, use whatever means possible) so that the piston is at the very top of it's travel. If it's not, it may be pushed down by the compressed air of the leakdown test. For the remaining cylinders, you must rely on either the whoosing air or a skinny object stuck down the plug hole riding up and down on the piston to determine when each is at tdc. Keep track of what stroke each cylinder is on or once again you will not know whether a tdc is on the compression or exhaust stroke. The best way is to do them in order: after cylinder 1, cylinder 3 will be next in line for tdc compression, followed by 4, then lastly 2.

[Fenvy]

I think all 4 of mine ranged from 89% to 92%

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