How to Perform a Leak Down Test | Autance

There is a frankly huge range of tests that you can perform to evaluate your cars performance. It’s kind of…

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How to Perform a Leak Down Test | Autance © How to Perform a Leak Down Test | Autance

There is a frankly huge range of tests that you can perform to evaluate your cars performance. It’s kind of cool that one of the most useful is also one of the easiest to perform. We’re talking about Leak Down Tests.

All you need to perform one of these are some pretty rudimentary tools, a little knowledge and some time to spare on a sunny afternoon. In this article we’re taking a deep dive look at Leak Down Tests. What they are, what they do – and how to perform one.

Is a Leak Down Test a Compression Test?

First things first and we have to tackle a common misconception here – a Leak Down Test is not a Compression Test. They are quite similar, so we understand why they can be confused with each other. But they are definitely different kinds of tests.

With a compression test, a pressure gauge is connected to the engine in place of the spark plugs. When the engine is cranked, the gauge can measure this pressure. This is a compression test. It is a great indicator of the engines ability to generate pressure, which in turn is needed to crank and turn the engine cylinder.

Poor pressure equals a weak cylinder that in use will translate as an underperforming engine that wheezes instead of roaring.

What is a Leak Down Test?

If that is a compression test, then what is a Leak Down Test? Well, hang on in there and we’ll tell you exactly what it is. Ready?

A Leak Down Test is a Compression Test in reverse.

As we outlined above a compression test is all about measuring the pressure that the engine can generate itself. A leak down test also includes putting a pressure gauge into each cylinder in the engine – though in this instance it is a Leak Down Test device that is inserted, with a display that incorporates two separate dials or displays.

It is this dual dial display that really makes the Leak Down Test both different from the Compression test, but also helps make it so useful. One gauge measures the air pressure that is being introduced into the cylinder by the compressor. The second dial then tells you the percentage of the air that is leaking from the cylinder (if any!)

You can see then that with a compression test, you are measuring the engine’s ability to generate pressure. With a Leak Down Test you are measuring the ability for the engine to hold that pressure in. Both tests are therefore important ways to measure engine health and performance – and they are also completely different.

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.

For the Leak Down test to work properly, then you first need find the Top Dead Center for each cylinder before hooking up the air compressor. To do this then you will need to remove the spark plugs as above. When they are removed, insert something long and thin into the spark plug hole – a screwdriver will do the job. Turn the engine by hand by using a wrench on the crankshaft. When the screwdriver is at the point where it is no longer rising or falling with each turn of the crankshaft, the cylinder is at Top Dead Center.

Remove the screwdriver and hook the Leak Down Test tool into the spark plug hole. Place the car into gear if you can, but also engage the parking brake so it won’t roll. Chocks behind the wheels can also be useful.

When the Leak Down Test tool is attached to the cylinder, the next task is to attach the tool itself to the air compressor. Once the compressor hose is connected, turn the regulator clockwise to allow the air to flow into the cylinder.

The right hand dial will give a reading as a percentage. This indicates the pressure loss that is occurring (i.e. the amount of air escaping the cylinder). But more on that in a minute!

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

A leak down test will give you two – yes TWO – sets of data. One is pretty easy to interpret, whilst the other will require you to become something of an automotive detective.

The first set of results is right there in your hand, displayed on the right-hand dial on the Leak Down Test tool. The dial will be laid out with percentages, usually ranging from 0% to 100%.

First of all, do no expect a 0% reading! There are precious few engines with the build quality and strength to withhold 100% of air pressure. We hate to break it to you but you are not going to find that kind of engine in your minivan.

Instead, check out this rating system:

  • 1 to 10% Loss – All is good. This the typical range of pressure loss that you will find in a normal, happy engine.
  • 10% to 20% – Pressure loss in this range is a red flag. Your engine will still be working fine at this range, but you are going to start to notice a loss of performance. As the pressure loss moves up toward 20% that performance loss will get more pronounced. If any of your cylinders are testing in this range, it is not cause for immediate concern, but do add engine repairs to your upcoming to do list.
  • 30% or More – If you are seeing engine pressure loss measured at 30% or more than your engine is in big trouble! If you are seeing readings in this range then get ready for some exploratory work to find – and repair – the problem.

Also remember that each cylinder works independently. So whilst it may be a pain in the butt, you do need to check out each one and perform a full Leak Down Test on each cylinder.

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

As we said, a Leak Down Test will give you two sets of data. The readings on the tool are a very useful indicator of engine health. But a leak Down Test can also help you to pinpoint where the problems actually are inside your engine.

Assuming of course you actually do have any problems that is. If you the dial reading is under 6 or 7%, we’d suggest you don’t even bother checking out this section, unless you are interested in fine-tuning your engine. If however your pressure leak readings are worryingly high – 10% or over – then you need to bust out another key set of tools in your mechanics armory.

Your ears.

Yep, the other set of data that you can get from a Leak Down Test comes from listening to where the pressure is leaking. So for example, check out:

  • Intake Valve

If you hear air coming out of the air intake, out of your carburetor or out the throttle body then that suggests your problem is with the air intake valve itself.

  • Exhaust Valve

The issue could of course be down the other end of the car. If you can hear air escaping from the exhaust manifold, from the turbocharger or from the tailpipe itself then your problem is likely a leaky exhaust valve.

  • Piston Rings

If the air is escaping through the dipstick tube or the oil filler cap, the issue is likely a worn piston ring.

  • Cracked Cylinder Head

No one wants to hear they have a cracked cylinder head. But if you are running your Leak Down Test and you notice that your coolant is bubbling up out of the radiator neck, then a Cracked Cylinder Head could well be your problem.

That, by the way, is why we like the Leak Down Test so much. Not only does it help give you an idea of the health of your engine, but it also helps to diagnose what the problem is with your engine too.

That’s pretty cool if you think about it.


There you have it folks, our comprehensive guide to the leak down test. We’ve told you what it is, we’ve told you how to do it and we’ve told you how to interpret the results that you get. Heck, we’ve even told you exactly why it is such a useful test to perform.

We hope that you can see why it is so useful too, and we really hope that you will perform a Leak Down Test on your own car.

Good luck guys, and happy testing!


  1. Leak-down tester – Wikipedia
  2. How to Do a Compression Test – YourMechanic
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