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Old 10-04-2005, 04:48 AM   #1
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Question Leak Down Testing How To

After reading the thread concerning a blown engine, I'm looking at performing a leak down test on my engine just to be safe. The problem is I don't know where to start so I have a few questions:

What are the tools you need to perform the test?

Is there a good write up with directions on performing the test?

How often should you do this to help detect head gasket leak before it gets to the point of blowing?

TIA!

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Old 10-04-2005, 08:13 AM   #2
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Quote:
Originally Posted by deliriousga
After reading the thread concerning a blown engine, I'm looking at performing a leak down test on my engine just to be safe. The problem is I don't know where to start so I have a few questions:

What are the tools you need to perform the test?

Is there a good write up with directions on performing the test?

How often should you do this to help detect head gasket leak before it gets to the point of blowing?

TIA!
John,

There is often much confusion about what exactly a Leakdown Test is and how it's different than a Compression Test.

Essentially, a Compression Test is more a Test of how much Compression a Cylinder can make, while a Leakdown Test indicates how much pressure your Cylinder can hold.

Both Tests use a Pressure Gauge hooked up to the Cylinder through the Sparkplug Hole. But, the Compression Test relies on the pressure created by the Engine, while the Leakdown Test uses an independent source of compressed air (air compressor). Also, the Compression Tester will usually incorporate a one-way valve which allows the pressure recorded to accumulate. A release valve (usually located in the stem)zeros out gauge for the next test.

With a Compression Test, you remove all the Sparkplugs (to aid the Starter Motor in turning the Engine over easier) and with the Engine Warm (fully expanded), Ignition and Fuel Supply cut-off, Throttle Body(ies) held at WOT, you crank the Engine until the Compression Test Gauge Tops Out (usually 5-6 Revs). Then you record the value (expressed as PSI) and proceed to the other cylinders.

This is more of a comparitive test. You're looking for values which are both within Factory Spec. as well as consistency between all cylinders of say, 2 PSI. There are two (2) Types of Compression Tests, the Dry Test and the Wet Test.

In the Dry Test, you simply crank the Engine and record the Reading. In a Wet Test, you add a spoonful of Motor Oil into the Spark Plug Hole and then crank the Engine. Any difference between the Wet and Dry Tests are an indication of how well the rings are sealing as the Motor Oil will migrate around the Rings, temporarily sealing them.

So, if poor Compression is read, it could be Valves or Rings, or Headgasket. Once you add the Motor Oil, if the Readings change, the Rings are suspect. Etc. This test is really a Snapshot of your Engine's general health. It may cause you to suspect a bad HeadGasket, but cannot confirm it.

With a Leakdown Test, you use a special tool, essentially a Manifold consisting of two(2) Pressure Gauges separated by a Regulator/Shut-Off Valve. This also connects to your Spark Plug hole in the Head (again, Engine Warm so everything is expanded properly). You hand-rotate the Engine to insure that all valves for that cylinder are closed (so you have a Pressure Vessel) and then you inject a specified amount of Compressed Air into the Cylinder, say 150PSI, shut the Shut-Off Valve from the Air Supply, and wait a specified amount of time, say 20 minutes. Then you return and see how much Pressure the Cylinder has retained. You should find most (but not all) of the pressure was maintained. Some pressure drop will be due to slight leaks around rings, valves etc., or worse, a leaky HeadGasket. Once again, consistency is good between the Cylinders. This test gives you the ability to isolate the source of any leaks simply by listening for wherever a hissing sound occurs. It is much better at isolating a bad HeadGasket. I have included a pic of a Leakdown Tester below.

Leakdown Gauges are sold through any number of sources. You can also make your own quite easily from some ready-made off-the-shelf components. This is what I did, bought the required stuff (2 pressure Gauges, a Shut-Off Valve, couple pieces of hose w/ threaded fittings) at SEARS for under $50. Hope this helps...

Happy Motoring!...Jim'99

PS A Leakdown Test s/b performed whenever you have reason to believe the Engine is not in top form, or annually or whenever you change Spark Plugs so you can establish a timeLine of your Engine's performance as it ages.

Last edited by MNBoxster; 10-04-2005 at 10:39 AM.
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Old 10-05-2005, 07:00 AM   #3
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Jim
I agree with almost everything you say, except the details about using the leakdown tester. The leakdown testers I have used on previous racing engines measured a continuous leak rate by measuring the pressure drop across a calibrated orifice (much like measuring current by measuring voltage drop across a known resistor, if you are comfortable with electrical analogies). The pressure on the input side of the orifice is set (using the regulator) to a value like 80 PSI or 100 PSI, and with the input pressure still supplied the pressure after the orifice (which is the 2nd gauge, measuring the pressure in the cylinder) determines the leak rate, expressed as either a PSI difference between the two gauge readings, or a percentage. My recollection is that if you shut off the air supply, even on a cylinder with excellent leakdown (a few PSI or less) the pressure in the cylinder goes to zero within a modest number of seconds. The technical details for the leakdown tester you picture describes a continuous flow process like I described.

By the way, leakdown testing has several other advantages over compression testing. Since the process is continuous you can listen to various ports (like the intake port, exhaust port and sump) and determine what is leaking. You can perform leakdown with the piston not at TDC (by anchoring the crank in some way to withstand the air pressure on the piston) and assess ring sealing at points other than TDC.
Frank
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Old 10-05-2005, 07:17 AM   #4
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Quote:
Originally Posted by frankkerfoot
Jim
I agree with almost everything you say, except the details about using the leakdown tester. The leakdown testers I have used on previous racing engines measured a continuous leak rate by measuring the pressure drop across a calibrated orifice (much like measuring current by measuring voltage drop across a known resistor, if you are comfortable with electrical analogies). The pressure on the input side of the orifice is set (using the regulator) to a value like 80 PSI or 100 PSI, and with the input pressure still supplied the pressure after the orifice (which is the 2nd gauge, measuring the pressure in the cylinder) determines the leak rate, expressed as either a PSI difference between the two gauge readings, or a percentage. My recollection is that if you shut off the air supply, even on a cylinder with excellent leakdown (a few PSI or less) the pressure in the cylinder goes to zero within a modest number of seconds. The technical details for the leakdown tester you picture describes a continuous flow process like I described.

By the way, leakdown testing has several other advantages over compression testing. Since the process is continuous you can listen to various ports (like the intake port, exhaust port and sump) and determine what is leaking. You can perform leakdown with the piston not at TDC (by anchoring the crank in some way to withstand the air pressure on the piston) and assess ring sealing at points other than TDC.
Frank
Frank,

I agree with what you say. I did mention listening to the ports, valves etc. for any hissing indicating a leak. The leak rate test you describe can be done with this tool, but you need a specified value for the % Rate of leak (Determined by the Manufacturer and not easily available), otherwise you can only determine consistency between cylinders. One thing you didn't mention was that the Cams (or Crank/Cams in combination) must be rotated so that all the valves are closed. On some engines where there is a Valve Overlap, you need to remove the cam altogether so that the lobes don't hold the valves in the open position.

Happy Motoring!...Jim'99
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Old 10-05-2005, 07:41 AM   #5
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Jim,
You are right about needing calibration. I think this is what you meant, but for the other readers what needs calibration is the tool, not the engine being tested. The size of the orifice determines how much air flow causes how much pressure drop, and what you really want to measure is air flow (i.e. the amount of leakage). Some manufacturers of the testers give at least some guidance on what is a good engine and what is bad in terms of pressure difference. Unlike compression testing, absolute numbers matter with leakdown. That is, you can diagnose an engine where all of the cylinders have some ring leakage, given some gauge calibration (which might include pressure deltas from another, known to be sound engine using the same tester).

On rereading your original post I see you mentioned listening for the leak point, which I missed the first time through. Sorry.

Although neither of us mentioned it on the first time through (I think-), a leakdown test is normally (and most easily) done at TDC on the compression stroke. The valves are guaranteed to be closed there, and if you are exactly on TDC (or at least very close) the engine won't try to turn when the air pressure is applied. As I noted (without starting at the beginning-), if you want to do a leakdown test anywhere other than TDC you need to in some way anchor the crank. Of course you need to pick points where both valves are closed (or in some way disable the valve opening, as you noted). BTW, 100 PSI on a several inch diameter piston makes a lot of force to anchor. It is hard (or impossible) to do it by hand-holding a breaker bar on the crank nut, if I recollect.
Frank
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Old 10-05-2005, 10:50 AM   #6
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Is there a flywheel lock available for the Boxster somewhere?

There's one for the 928 that you take the slave cylinder out and pop the lock in and the teeth keep the flywheel from moving. Anything like that available out there?
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Old 10-05-2005, 04:43 PM   #7
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Quote:
Originally Posted by deliriousga
Is there a flywheel lock available for the Boxster somewhere?

There's one for the 928 that you take the slave cylinder out and pop the lock in and the teeth keep the flywheel from moving. Anything like that available out there?
Hi,

I have read in my Tech Manuals that there is a Flywheel Lock, forget the part#. But, for this test, it should be sufficient to merely put the tranny in 1st gear.

The Crankshaft Nut on my Lotus Esprit is spec'd at 174 ft.lbs. of torque (quite a lot). I can put the Tranny in first and reef on the Nut w/ a long-handled Torque Wrench until the proper torque is reached. There is some initial movement, maybe 1 or 2 degrees as all the slack in the drivetrain is taken up, but after that, it remains firmly locked just from the friction of the clutch on the Flywheel. Hope this helps...

Happy Motoring!...Jim'99

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