What Kind of Test?
Everyone remembers how to do a compression test: Screw in the tester, flip the kill switch to "off", hold the throttle wide open, and press the starter button. The cylinder takes in air and compresses it, and the tester traps it. The maximum is reached when the gauge holds more pressure than the engine can produce. Not too difficult. The weakness of this test however is that throttle postion, engine temperature, ambient air temperature, and a host of other factors can make the results vary considerably. What's worse, a compression test checks too many engine components at the same time. A poor reading can indicate so many things, it's hard to tell which engine part is at fault without doing a lot of other tests. A leakdown test avoids this difficulty. Air is pumped into the cylinder from an outside source, and the gauge reads the percentage that escapes, which not only eliminates all of the afore-mentioned variables, but as a bonus, makes it a simple matter to pinpoint the source of the leakage by wiggling and rotating engine parts while the test is underway.

"That'll be $1,500, and Oh, Uhh...it Still Smokes..."
But how does it work? Okay. Let's say your brother-in-law rebuilt your engine. You've suspected that the guy is mechanically-challenged, and sure enough, the finished product smokes like a chimney. But he's your kin, so... Finally, you have a shop look at it. Good results from a compression test combined with the smoking leads them to a diagnosis of trashed valve guides. Seems reasonable and you approve the work. But, the engine still smokes. Now you really have a problem, not to mention the shop, and your brother-in-law. Enter Mr. Goodwrench, who produces a leakdown tester, and performs the following test. On each cylinder in turn, he finds TDCC, sets up the tester, and reads the percentage of leakage. They're all good and low. Hmm. Undaunted, our hero retests each cylinder, but this time he lowers the pressure setting on the instrument, and, rotating the crankshaft a smidge each time to slide the piston down the bore a little, picks up the problem, plain as day. On the #4 cylinder, the gauge now reads 60% leakdown when the piston is partway down the bore, indicating cylinder damage, which the teardown verifies. Seems your brother-in-law didn't get one of those pesky wristpin circlips all the way into its groove. It subsequently popped out, and the wristpin tore a handsome trench into the cylinder wall. Why didn't the shop find it when the head was pulled for the valve job? Because two of the four pistons were at TDC. Why didn't the compression test pick it up? Because despite the trench, there was still plenty of cylinder area (the pin is more than an inch below the deck) in which to build adequate pressure during a compression test. This actually happened, and it illustrates both the weakness of a compression test and the comparitive strength of a leakdown test.

Whoosh!
Leakdown testers are way cool. Not only does the amount of air escaping from the cylinder register on the gauge, it can also be heard, enabling the source of the leak to be pinpointed prior to the teardown. For example, high readings accompanied by hissing in the carburetor indicate burnt, tight, or carboned-up intake valves. The same thing in a muffler points toward--you guessed it -- exhaust valves. A breeze coming out of the dipstick hole indicates worn or heat-softened rings. And, air escaping from an adjacent spark plug hole pinpoints a blown head gasket.

Not for Everyone
There's a catch, of course. You need an air compressor to use a cylinder leakdown tester. And, you need to now how to accurately find TDCC (top dead center on the compression stroke) for each cylinder that is tested. Can you do it? Sure. If you can correctly adjust your valves, you can use a leakdown tester.

Rolling Your Own
Inexpensive, ready-made leakdown testers are easy to find today -- you don'thave to mortgage your house to a Snap-On dealer. So, if you are concerned about the condition of your engine but aren't into making things, or don't have the time, you can buy a leakdown tester for about $75 at many auto parts stores and the like. If on the other hand you have an air compressor, that sort of implies that you're a certified tinker. You're probably also into making things, and for you, throwing this thing together is no big deal. For you then, here's the rundown: