<VV> Re: Leakdown Test

[chapmanmd@charter.net]

Not to be disrespectful Matt, but there is a different theory regarding leakdown tests... ESPECIALLY for low compression engines. You guys are all smarter than me, but I want to throw my 2 cents in as I have become a believer, the hard way, in leakdown tests for low compression and turbo engines.

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 wi!
 ggling and rotating engine parts while the test is underway.

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 gau!
 ge 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.

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. 

Just a thought... IMHO of course... Please don't flame me for the length of this.

Cheers!

Mark Chapman
Group Red 66 Monza and too many other cars, motorcycles, and junk to keep up with.