This is, I am sure, Leakage Testing 101 -- for Dummies.
Bob Hoover gave a good writeup a couple days ago on how to determine where a cylinder might be leaking using a Differential Leak-Down tester such as Harbor Freight sells at
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Bob's description of where to listen and how to interpret the results is clear. That tester gadget has a couple of gauges, and a valve. My experience with Harbor Freight is that they don't provide instructions.
My guess is that the device has a pressure-regulator valve on the input side. The air pressure you select indicates on the gauge next to it. Since this is referred to as a "differential" test, the gauge on the output side should show the difference between the pressure coming out of the regulator, and the back-pressure from the cylinder. It is calibrated in red-yellow-green ranges, bad-ok-good would be my guess.
So . . . you increase pressure and, unless there is a real leak you read good, then you turn it up more until the red-yellow-green difference gauge starts to swing into yellow-red indicating that the input pressure is matching output pressure, which then tells you how much pressure the cylinder is holding.
You can also use this gadget to simply pump air into the cylinder and listen for the leaks as described by Bob.
Now that I've rattled on, could someone give me the quickie summary on how to use the tester?
At less than combustion temps & pressures, all cylinders leak. But not a lot.
Using a pressure of 90 psi as your standard, a properly broken in VW cylinder should give you a reading between 80 and 87 psi with the higher number indicating the lower rate of leakage. You're expected to know something about the engine, such what type of rings are fitted and the precise location of the geometric TDC (which is where the engine is positioned to do the test) but once you have established a base-line reading for a given engine, a periodic pressure test provides a quick and reasonably accurate indication of its condition.
As the engine wears, leakage will increase. If the jug will only hold 67 pounds or thereabouts, the leak-down rate is above 25% and the engine is telling you it's time for an overhaul.
So why's this any better than a compression check? Because to do a proper compression check you're supposed to crank the engine over at starting speed for ten revolutions... for each cylinder. And after doing them all dry, you give each jug a measured amount of oil and repeat the test to get the 'wet' figure, using it as a measure of merit for ring sealing.
Lotsa airplane engines got 14 cylinders. Or more. (up to 48) And lotsa airplanes got more than one engine (HK-1 has eight of them suckers).
The common compression, while useful on smaller engines, can literally wear out a big one, assuming it can be performed at all. (Takes a lot of horsepower to spin-up a big engine, even with all the plugs out.) Leak-down test, you pull one plug, bring that jug to TDC, do the test, move on to the next.
Plus, as I've already explained, when you're pumping outside air INTO a cylinder it's fairly easy to determine where it is leaking OUT... if the leak is big enough.
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Most engine manufacturers publish leak-down ratios for their engines, as well as wet & dry pressure-test figures. But even if they don't, you can. Indeed, as I've mentioned in other posts, your engine is perfectly capable of telling you how it feels. The manuals describe a set of generic feelings, applicable to all engines, which is just dandy if you're a line mechanic. But every engine differs in some small way from every other.
Normal leak-down is typically less than 10% for a new engine (after it is broken in). As wear accumulates the rate of leak-down grows. By the time it hits 25% it's a good idea to withdraw the engine from service; replace it or give it an overhaul or whatever. Those figures aren't carved in stone; they can vary a few points for any particular engine. But they can serve as a rough guide for an engine of unknown provenance.
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Modern-day vehicles are transportation appliances and treated as such. But when your life -- or your livelihood -- depends upon the reliable performance of a powerplant, you need to pay more attention to its condition.
A periodic leak-down test allows you to plot the progession of wear on an engine. Knowing the amount of wear, you can determine if the engine has enough life left in it to safely complete a particular task, such as getting you through the next harvest without breaking down -- or carrying you safely on a long flight over water.
Not quite the same thing as puttering around in a bug or bus but the principle applies to all.
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