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Leak Down Test Informational Thread

Old 01-23-2008, 01:20 PM
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Leak Down Test Informational Thread

So I have been noticing alot of engines having the ever so happy erection of there dip stick. Its very vital that u be very thorough on ur investigation before u call it good to go. Doing a compression test is a good thing to do when u have this happen to u but u shouldnt stop there just because it read good.Things like cam profile and engine cranking speed can affect the compression test so its good to double check everything and run a leak down test.
So heres the run down on what a leak down test is and what it checks for and why its important:

A leak down or "cylinder leakage" test is similar to a compression test in that it tells you how well your engineís cylinders are sealing. But instead of measuring pressure, it measures pressure loss. A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engineís firing order. A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder. An engine in great condition should generally show only 5 to 10% leakage. An engine thatís still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble. The cool thing about a leakage test (as opposed to a compression test) is that itís faster and easier 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 would point 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 in conjunction with a compression test to 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).

All in all dont short change ur engine because it can be more expensive down the road if u dont know whats really going on.
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Old 01-23-2008, 03:17 PM
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a compression test will warn you if you have something wrong. then you jump to the leak down. my car has always pulled retarded high compression test numbers anyways. {compared to others} leak downs are not that easy so to speak. finding tdc is easiest if you pull the valve cover off and check them vs the cam. if need be, stick a long screw driver in the spark plug hole. when it is at it's highest point, and both cams are up, or the valves are closed. you are at tdc.
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Old 01-23-2008, 03:26 PM
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but ive seen engines pull good compression test then ran a leak down and failed 60%, and good input on taking off the valve cover...forgot to mention that
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Old 01-23-2008, 03:38 PM
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it's a two step deal. comp test, then leak down. if you have a bent valve, or blown ring land it will show up on the comp test
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Old 01-26-2008, 11:16 PM
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My leak down test results, taken March 20th, 2007...

Cylinder 1: 8%
Cylinder 2: 11%
Cylinder 3: 10%
Cylinder 4: 80%

Hmmm, I wonder if there was something wrong?
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Old 02-08-2008, 04:14 PM
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Can you guys advise the procedure for doing a compression test? I want to ensure I do it properly, and I am uncertain if it can be done the same as on old cars.
One howto I read said to hold the throttle down all the way and then crank. I am not certain if this will be safe. Also, should the other spark plugs be left in the engine while test each cylinder?
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Old 02-08-2008, 04:18 PM
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Remove spark plug, turn motor until desired cylinder is at top dead center (all valves closed), screw air fitting into spark plug hole. Initiate test.
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Old 02-08-2008, 11:09 PM
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Originally Posted by Blown 4-banger View Post
Remove spark plug, turn motor until desired cylinder is at top dead center (all valves closed), screw air fitting into spark plug hole. Initiate test.
Actually I was talking about compression test not leak down.
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Old 02-10-2008, 11:16 PM
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Disconnect ignition, screw in air fitting, crank motor over 3-5 times.
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Old 02-13-2008, 05:13 PM
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Originally Posted by Blown 4-banger View Post
Disconnect ignition, screw in air fitting, crank motor over 3-5 times.
Thanks Guys! I'll be doing both tests this evening, wish me luck!
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Old 11-19-2008, 08:56 PM
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just a quick question, can these testers be rented anywhere?

i went to auto zone and the guy said they only have them for sale...not sure that these are something I really need to buy.
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Old 12-26-2008, 08:32 PM
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yeah...how do I get a compression test. where do I go. idk how to do it
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Old 02-25-2009, 12:36 AM
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Take out all the spark plugs. not one at a time, and make sure your throttle is open if you want a correct reading. That is why you may have good compression but fail the leak down test....Head gasket!
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Old 11-25-2011, 01:07 PM
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keep in mind, when you pressurize the cylinder it will force the piston down and open the valves. I ended up pulling my cams to close the valves and let the piston drop with the pressure. However this will cause your engine to be out of time. gotta put the cams/crank back in time and that was a bitch.

prevent the harmonic balancer (crank pulley) from turning with a screwdriver or other locking tool.
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Old 04-19-2012, 08:21 AM
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Canada Here is your answer right here!

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