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What is a "leak down" test?

Engines take lots of abuse. Mostly because they tend to be used in harsh conditions. They may power a chain saw, a go-kart, a leaf blower, or a dirt bike. So they may be exposed to dirt that clogs their air filters and moisture that corrodes their parts. They may be subjected to sudden accelerations or may be kept running at peak power for longer periods than they were designed. Here is all about the automobile engines that may help you to troubleshoot your problem or know more about your automotive engine. A leak down or a "cylinder leakage" test is about the same as a compression test because it tells how well the engine's cylinders are sealing. But instead of measuring cylinder pressure, it will measures cylinder pressure loss. A leak down test requires all the spark plugs be removed. Turn the crankshaft so that each piston is at top dead center (both valves closed) when each individual cylinder is tested. It is a good practice to start with the number one cylinder and follow the engine's firing order. It saves time in the long run and less turning of the crankshaft to get things lined up. Attach a threaded coupling to a leakage gauge and screw it into a spark plug hole. Attach Compressed air (80 to 90 psi) and feed it into the cylinder you are testing. An engine that is in great condition should show only 5 to 10% leakage/loss. An engine that is still in pretty good condition may show up to 20% leakage/loss. But more than 30% leakage/loss indicates trouble. A cool thing about a leakage test (as opposed to a compression test) is that it can be a faster and easier way to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor points to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn. A leakage test can also be used along with a compression test to help diagnose other kinds of problems. A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc. If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two. If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).
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