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Building and using a cylinder leakdown tester |
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. A leakdown test avoids this difficulty. 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.
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, sets 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 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.
![]() 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 easy to make your own, and it will be a lot more accurate than many for sale. |
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. Also, make sure it comes with the block-off plugs for the two passages that you won't be using. |
Pressure gauge |
Get a quality, back-mount, 0-60 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. |
Spark plug adapter |
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--on some plugs you must grind partway into the hex). 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). |
Damper 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 drillbit) hole. |
To use this tool, put the cylinder to be tested to TDC compression. Plug your tester into an air compressor line and "zero" the gauge. Screw the hose into the spark plug hole. And 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. Production multicylinder engines in good condition leak 5 to 10 percent. More than 10%
leakdown indicates wear that will affect engine performance.
You may need a helper if the engine you're testing is a big bore single or twin. First, there won't be enough friction in the engine to resist the compressed air, even at exact TDC. You have to either find a way to clamp the engine at TDC compression, or get someone to hold it there for you while you do the test. Also, for cylinders over 100mm in diameter, allow a bit more leakdown, up to 15%, before condemning it.
In the event of a high leakdown 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 and drift, carefully tap on the valve followers 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. If not, well, time to get it fixed.
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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 |
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Last updated September 2024 Email me © 1996-2024 Mike Nixon |