How to do an Engine Cylinder Leak Down Test & Interpreting Results
What Is a Leak Down Test & What Does a Leak Down Test Tell You
Spend much time working on cars and one of the following scenarios will sound very familiar to you. You have encountered an engine that is losing power or has developed a miss and a tune up doesn’t solve the problem, or you’ve just hauled an engine or even a complete car home from the scrapyard and want to know if it’s any good. In both of these scenarios (and many others), an engine cylinder leak down test will provide you with some answers. What is a leak down test? A leak down test is a method of using compressed air and a specialized gauge set that reads differential pressure to evaluate the condition of an engine’s cylinders, pistons and rings, and valves and seats without disassembling the engine.
How To Do a Leak Down Test | Cylinder Leak Down Test Instructions
A leak down test is relatively simple. Likely the most time-consuming part will be the process of getting each cylinder to top dead center. Below are step by step instructions for how to do a cylinder leak down test.
- Remove the spark plugs.
- Rotate the engine until the cylinder to be tested is at top dead center (TDC). Remember that on a four-stroke engine, the piston is at the top of its stroke twice for every complete cycle. It’s critical that the piston is at the top of the compression stroke when both (or all) intake and exhaust valves are closed. For more on finding TDC, check out our guide on how to find top dead center.
- Install spark plug adapter to the cylinder to be tested.
- Connect shop air to cylinder leak down gauge and set regulator to the desired pressure.
- Read the second gauge, which records differential pressure, which translates to leakage, or the amount of pressure going through the cylinder past the rings, valves, etc.
- Repeat on every cylinder that needs to be tested.
What Tools Do I Need for a Leak Down Test
The beauty of this test is that you don’t need much in the way of specialty tools.
- A leakdown tester tool is the most critical component here. Note that a leak down tester will differ from an engine compression tester. A cylinder leak down test kit will typically consist of the primary guage set which includes a pair of gauges, a regulator to set the pressure of incoming shop air, and a hose to connect to the spark plug adapter.
- A spark plug adapter fitting. Many tools come with a common size (such as 14mm) fitting already on the hose. For other applications, the proper adapter will need to be sourced.
- A source of compressed air. This doesn’t need to be big or fancy, but preferably the air compressor will be capable of 100 psi.
How Much Pressure For Cylinder Leak Down Test
Some testers have a “set” mark on the shop air gauge and the other gauge will be marked accordingly. For tools like the Speedway Motors leakdown tester used in the video, both gauges simply read psi. For this type of tester, we prefer to set the incoming air pressure on the regulator to 100 psi. Why 100? While this process will work with just about any amount of compressed air, putting 100 psi into the cylinder makes your math easy when reading the other gauge and calculating differential pressure.
Interpreting Leak Down Test Results
So now that you have your reading, how do you interpret the cylinder leak down test results? The pair of gauges make the reading as simple as calculating the difference in pressure between the gauge measuring the pressure at the regulator and the gauge that measures the pressure loss in the cylinder. As discussed above, this is made even easier if you can achieve 100 psi into the cylinder.
Let’s refer to the example in the video above, our incoming air was 100 psi, while the second gauge reading differential pressure indicated 70 psi. That means that the cylinder has 30% leakage.
Acceptable Leak Down Test Results of Each Cylinder
What’s acceptable will vary from engine to engine. Different ring styles and other variables can impact a cylinder leak down test acceptable result. In general, a cylinder with less than 10 percent loss will be in great condition. 10 to 20 percent is marginal, and 20-30 percent means time to do some further investigating. Additional resources exist, such as pressure leakage diagnosis charts, that can help you interpret your results. Also, most of us are working on engines with more than one cylinder, so the difference between the cylinders can also tell us a lot. If seven cylinders are 10 percent or less and one is 30 percent, that can indicate trouble.
Let’s go back to that LS engine in our video. The lone cylinder we checked leaked 30 percent. Totally shot and time to rip it apart, right? Not necessarily. As Lake Speed mentions in the video, oil acts as a gasket, and the best time to perform a leakdown test on an engine is when it’s up to temperature and all the sealing surfaces are coated with oil. The engine in our video had been sitting in the corner of the shop for quite a while, so it’s likely dry as a bone and that 30 percent loss may be totally acceptable. Likewise as mentioned above, if that cylinder leaked 30 percent and all the others leaked 10, then that’s a bad sign and time to investigate more closely.
Also note that the compressed air that leaks out of a cylinder is going somewhere, and if you can ascertain where it’s going, that can be valuable information to help you in your diagnosis. If you can hear air escaping through the intake, then there is likely a leak at the intake valve. Air coming out of the header can indicate exhaust valve trouble. Air escaping from the oil dipstick can indicate piston ring or cylinder wall issues, and bubbles in the coolant can mean a head gasket failure or a cracked block.
