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 the most pressure the engine can produce. Not too difficult. The weakness of this test however is that a compression test checks too many engine components at the same time--you don't know whether the problem is the cylinder or the valves. A leakdown test tells us which it is, and for that reason is often the followup test after a compression test. Air is pumped into the cylinder from an outside source, and the gauge reads the percentage that escapes. This pinpoints the source of the compression loss.

A real life example
You have a smoking engine. Good results from a compression test combined with the smoking leads you to a diagnosis of worn valve guides. Seems reasonable . But, just to make sure, you perform the following test. On each cylinder in turn, you find TDCC, set up the tester, and read the percentage of leakage. They're all good and low. Hmm. Mysterious. So you then lower the pressure setting on the instrument, and, rotating the crankshaft a smidge to slide the piston down the bore a little, you pick 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. Looks like a piston pin circlip wasn't put in carefully and the wristpin tore a handsome trench into the cylinder wall. Why didn't the compression test indicate this? Because an engine builds pressure mainly from the upper half of the cylinder. This scenario actually happened.

Leakdown testers are 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 engine 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 cylinder wear or worn or heat-softened rings. And, air escaping from an adjacent spark plug hole pinpoints a blown head gasket.

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. But if you can correctly adjust your valves, you can use a leakdown tester.

Drawing by Mike Nixon

Inexpensive, ready-made leakdown testers are easy to find today. If you're not into making things, or don't have the time, you can buy a leakdown tester for less than $100 at many auto parts stores and the like. But it's also easy to make your own, and it will be a lot more accurate than many of those for sale.

This is designed to be screwed onto an automotive paint spray gun. As of this writing in the 1990s, Grainger's was the cheapest. The gauge that is often attached is, unfortunately, the wrong kind for our purposes. We need a lower pressure one. Also, make sure the regulator comes with the block-off plugs for the two passages that you won't be using.

Get a quality, back-mount, 0-60 psi gauge (I know, the picture shows a 0-100, but a 0-60 is more accurate). For the professional touch, carefully pry off the bezel and cover the faceplate with a copy of the label shown below.

Make this by clamping an old spark plug in a vise (protected with wood or aluminum stock, of course, and on the hex--not the threads) and whacking off the porcelain with a well-aimed lateral hammer blow. Then grind off the rolled-over seal above the hex, grind off the ground electrode, put the plug back in the vise and drive out the remaining porcelain with a drift (if difficult, the rolled seam hasn't been sufficiently removed). Tap the hex end with a 1/4" pipe tap (NGKs are hardest to tap but seal the best afterward). Screw the bugger onto a 1/4" to 1/8" pipe reducer, and that onto a 12" grease gun hose (about $5 at Wal Mart and just about anywhere). Or simply buy a compression tester hose at any of several sources. It's the same hose, only without the internal Schrader valve.

This essential part is merely a restriction between the regulator and the gauge. The easiest way to make it is to plug the middle pipe with epoxy and afterward drill a 0.040" (#60 or 1mm drill) hole. I have also simply made an aluminum plug to a tight fit and drilled that.