How To Perform A leak Down Test?

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Frankly, there is a wide range of tests available that can be performed to analyze how your car is performing. The good news is that one of the most useful ones of these is also among the easiest to perform. Yes, we are referring to the Leak Down Test.

To perform the test, you need some simple tools and basic knowledge about it along with some free time. This article will take you through Leak Down Tests elaboratively, and tell you what they are, what’s their purpose and how can you perform it yourself.

Is a Leak Down Test a Compression Test?

Before moving on, let’s first clear a very popular misconception. No, a Leak Down Test is not the same as a Compression Test. Nevertheless, they are very similar and are hence confused together, but in reality, they are distinct tests.

For a compression test, a pressure gage is connected with your vehicle’s engine near the spark plugs. The gauge measures the pressure as the engine cranks. This is what a compression test is, and it is a superb indicator of your engine’s ability for generating pressure needed to turn and crank the engine cylinder.

Poor pressure means the cylinder is weak and will in turn lead to an engine tat underperforms..

What is a Leak Down Test?

To put it simply, a compression test when reversed is a leak down test.

As mentioned above, a compression test measures the ability to generate pressure by an engine. For a leak down test, a leak down test device is entered into each cylinder in the engine having two distinct displays or dials.

This is what is the major difference between the two kinds of tests, and what makes the leak down test so useful. Air pressure introduced by the compressor in the cylinder is measured by one gauge, while the other one indicates the exact air percentage leaking from the cylinder.

This can be used with a compression test as well, as you are measuring the ability of your engine to generate pressure, while the leak down test measures the ability of the engine to hold this pressure. Both of these are nevertheless very important methods to measure the performance and health of your engine.

How to Perform a Leak Down Test?

So we’ve taken a look at why you should perform a Leak Down Test. In the section after this, we’ll look at how to use and interpret the data that the test will generate. But before you can get to that stage, you need to know how to actually perform a Leak Down Test.

The first thing you are going to need is to gather your equipment! You are going to need:

  • A Leak Down Test Tool
  • An Air Compressor
  • A Socket Wrench with a Spark Plug Socket

The first thing you are going to have to do is to remove the spark plugs from each cylinder with your socket wrench and spark plug socket. In order for the test to work appropriately, you much locate the Top Dead Center for every cylinder before you hook up the air compressor to it. For this, the spark plugs should be removed as described above. Insert a long, thin item like a screwdriver in it when they are removed.

Now remove the screwdriver and simply hook up the Leak Down Test Tool in the spark plug hole. You can put the gear as well if you want, but make sure the parking gear is engaged to avoid the car from rolling. Placing chocks behind the wheels will also be useful.

When you attach the leak down test tool to the cylinder, the next thing you should do is attaching the tool with the air compressor. Once its hose is connected, simply turn the regulator clockwise so the air flows into the cylinder. You will get a reading on the right-hand dial as a percentage, which indicated how much pressure is being lost.

How to Read the Results of a Leak Down Test – Part 1

You will get two sets of data from a leak down test. one of them can be interpreted very easily, while the other one needs some kind of automotive detection. The right-hand dial on your leak down test tool gives the first set of results in your hand right there, with the percentages laid out ranging between 0-100%.

Firstly, you shouldn’t expect a 0% reading at all as there are only a few precious engines that have the strength and quality to hold all the air pressure. It isn’t pleasing to hear, but your minivan does not have such an engine.

So, we have prepared a rating system for your ease:

1-10% Reading:

Everything good. This is the most common range of pressure loss which is found in a normally operating engine.

10-20% Reading:

If your pressure loss is in this range, it’s a red flag for you as the engine will begin to work fine in this range but the loss of performance will be noticed thereafter. As the pressure loss goes over 20%, the loss will be more noticeable. If your cylinders are being tested in the range, this isn’t a cause of immediate fear but you should add engine repairs to your next to-do list.

30% or Above Reading:

If your pressure loss is 30% or over, it is a great trouble for you now. If you get readings in this range, be prepared for tentative work to find and repair the issue.

You should also remember that the cylinder should work independently, so while it may be a pain for you, you must check all and perform a leak down test on every cylinder.

How to Read the Results of a Leak Down Test – Part 2

As discussed above, a leak down test will provide you two data sets. Your engine’s health is indicated very well by these readings, but the test will also point to the exact area of the problem in your engine.

Assuming that there are problems with the engine, but the dial reading is below 6-7%, it is suggested to not bother checking this section unless you need to fine-tune your engine. However, if your pressure leak readings are over 10%, you must smash another essential set of tools in your mechanics armory.

Yes, the next data set you can get from a leak down test comes from where your pressure is being leaked. So for instance, check the following:

Intake Valve:

If air is coming out from the carburetor, air intake, or the throttle body, it is suggested that the problem is with the air intake valve itself.

Exhaust Valve:

The issue is definitely from the car’s other end. If you hear air escaping from the exhaust manifold, tailpipe itself, or from the turbochargers, then the issue is in a leaky exhaust valve.

Piston Rings:

If the air escaped from oil filler cap or dipstick, the issue is most probably a worn piston ring.

Cracked Cylinder Head:

Having a cracked cylinder head isn’t great news for anyone, but if the leak down test is being run and you notice the coolant bubbling up from the radiator neck, the problem would be in your cracked cylinder head.

This is why the leak down test is so much, which not just helps to give the idea of improving your engine but also helps diagnose the issue in the engine.

In Conclusion

With it, this guide comes to an end. We have described how it is, it is described how to do it and how to interpret the results, and also told you why is it a great test to perform. You can see it as being useful as well and we hope that you perform the same test on your vehicle as well. Happy testing! 


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