Quote:
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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.
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