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Building and Using a Leakdown Tester

by Mike Nixon

A Better Way

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. The weakness of this test 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 tester 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 aforementioend 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 uhh, it still smokes."

But how does it work in action? 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. A good compression test combined with the smoking leads them to a diagnosis of trashed valve guides. Seems reasonable and you approve the work. And 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 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 wore 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 is still plenty of cylinder area (the pin is more than an inch below the deck) in which to build adequate pressure for a compression test.


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 on the other hand 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 find TDC (top dead center) on the compression stroke for each cylinder that is tested. Can you do it? Sure. If you can adjust your valves, you can use a leakdown tester.

Rolling Your Own

Ready-made leakdown testers are easy to find today. You don't have 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. If on the other hand you have an air compressor, that sort of implies that you're a certified tinkerer. You're probably also into making things, and for you, throwing together a leakdown tester is no big deal. Here's the rundown:


The pressure regulator

This is designed to be screwed onto an automotive paint spray gun. Grainger's is probably the cheapest, followed by Sears and Ace Hardware. The gauge that is often attached is, unfortunately, the wrong kind for our purposes.

The pressure gauge

Get a quality, back-mount, 0-100 psi gauge. As of this writing Grainger has the best deal. For the professional touch, carefully pry off the bezel and cover the faceplate with a copy of the label shown here.

The spark plug adapter

Make this by clamping an old 12mm plug in a vise (protected with wood or aluminum stock) 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 hard, the rolled seam hasn't been sufficiently removed--on some plugs you must grind partway into the hex to completely remove the seal). 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 1/2" pipe reducer, and that onto a 12" grease gun hose (about $5 at Wal Mart and just about anywhere).

The damper valve

This necessary part is merely a restriction between the regulator and the gauge. The easiest way is to plug the middle pipe with epoxy and afterward drill a 0.040" (#60 or 1mm drillbit) hole.

Using it

Adjust the cylinder to be tested to TDC compression (all modern inline fours firing orders are 1-2-4-3). Plug your tester into an air compressor line and adjust the regulator to get a "0" (or 100 psi, if you didn't remake the face) reading. Screw the hose into the spark plug hole. Connect the two. If the crankshaft turns or you hear all the compressor's air rushing out of an obviously open valve, the cylinder wasn't set exactly on TDC compression. Try again. When you get it right, the piston will stay put and the tool will indicate the amount of air that is escaping from around the rings, valves and head gasket of that cylinder. All cylinders leak a little. Large ones leak more, smaller ones less. Racing cylinders lose only 1 to 2%. Production multicylinder engines in top fiddle pass 5% and less, and no more than 10% regardless of the mileage. More than 10% leakdown means there's something wrong.


In the event of a high reading, first take the time to double check that you are in fact at TDC on the compression stroke, not on the exhaust stroke (where both valves will be open). If that checks out, and the leak is (as it is usually) a valve, remove the valve cover and, with a hammer, carefully tap on one or both of the rockers for that cylinder, watching the gauge as you do so. This will often loosen carbon from around the valve and the reading will drop to a reasonable level.

Parts List

  • Miniature air pressure regulator
  • 0-100 psi gauge, back mount, metal case, removable bezel
  • 12" grease gun hose
  • 1/4" NPT quick disconnect fittings
  • 1/4" plumbing pipe and "T" joint
  • 1/4" to 1/8" NPT reducers (2)
  • Old spark plug
  • Teflon plumber's tape


Text and illustrations © 1994, 1997

Mike Nixon

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