Knowing how to perform a compression test is a very easy and straightforward process that gives you some information about the motor in question. However when you are looking to test the integrity of the combustion chamber and the cylinder rings, a leakdown test is required.
The majority of us know how to perform a compression test on an engine. This process involves installing the test device to the cylinder and cranking the motor to see how much the cylinder is compressing or capable of compressing. This PSI reading is then a determination to see whether or not the engine is in good condition. To learn how to compression test a vehicle check our guide here.
But the compression test isn’t the end all be all to testing an engine, after all several different factors can affect this PSI reading which could lead to misinformation. For example, certain engine components will definitely lower the compression test reading, camshaft profiles, piston configuration and rod stroke all affect the compression readings you will get once you test the engine.
Because of these factors, many times a healthy performance engine will provide lower compression readings than a healthy stock unit. Not only that, but a compression test isn’t exactly the best way to diagnose the problem with your engine. After all, when you have a low reading in a cylinder, how do you know if it’s the rings, valves, seats or head gasket? This is why a leakdown test is the preferred method to testing an engine.
Knowing how to leakdown test is a crucial step to checking your engine, or selecting an engine donor for an engine swap. The leakdown test should be performed with the compression test to give you the most information possible about the engine in question. The act of the leakdown test in theory is very simple, you are testing the combustion cylinder to see how much the cylinder is leaking air with all the valves closed.
In simple terms if you are compressing the combustion chamber with 100 PSI of pressure and your leakdown tester reads 95 PSI you have 5% of leakdown. To leakdown test your engine, you will need a leakdown tester and there’s several different kinds of testers available. From single to dual gauge, most leakdown testers with all the features you are going to want will cost you around 150 dollars.
What are the differences between a single and dual gauge leakdown tester?
There’s no difference in accuracy between the single and dual gauge type of leakdown tester, however the dual gauge makes life a little bit easier for you. Where as the single gauge leakdown tester only reads the input pressure and then switches manually to read the cylinder pressure feedback, the dual gauge does both at the same time. Using a dual gauge can make life easier for you, especially if you are working solo because you’ve got to rotate the engine and crankshaft to Top Dead Center (TDC).
As with compression tests, the leakdown test should be performed when the engine is warmed up and properly expanded. To begin our How To Leakdown Test guide, remove your spark plugs and disconnect your EFI connections to insure you do not burn out any ignition coils.
What you need for this How To Leakdown Test Article
- outside air source or air compressor
- dual gauge or single gauge leakdown tester
- Spark plug socket
- Crank pulley wrench or crank sprocket drive
How To Leakdown Test Your Vehicle
With the spark plug removed, you must rotate the engine until the cylinder in question is at TDC. This means that there should be no opening in the combustion chamber with your leakdown tester connected.
- Turn cylinder to be tested to top dead center (TDC)
- Connect air hose to the leakdown tester
- With airline connected, turn down pressure all the way.
- Remove radiator caps
If your dual gauge tester should have a valve that leads from the air inlet to the outlet gauge and hose. Open this valve now to have the outbound gauge rise up in pressure and fall to 0. If the second gauge does not return and settle at zero, you have the pressure turned up still, release all the pressure from your device first and foremost.
Once your leakdown tester reads zero pressure it’s ready to be tested. Your leakdown tester may have come with extra fittings or adapters to fit into your spark plug hole. Connect the outbound hose to the cylinder that you’ll be testing.
Remember that this cylinder should be at operating temperature and warmed up properly. Failure to do so or testing on a cold motor will lead to inaccurate readings, which is not the right way on how to leakdown test your car. Also, you want your vehicle in a quiet area where you can listen to where the air could be leaking from.
Once you have the spark plug removed, you are ready to learn how to leakdown test your engine. Connect the outlet hose to the cylinder you are testing, and turn up the inlet air pressure. The purpose of the dual gauge is to tell you what percentage of air is leaking out of your completely closed cylinder. If your gauge does not hold pressure, make sure your cylinder is at top dead center and pressurize your cylinder.
How to Diagnose Leakdown Test
Diagnosing the leakdown test is simple and straightforward so long as your vehicle is in a quiet area. Start listening for the escaping air in your air intake, your exhaust and / or throttle body. This will give you a good idea of what is leaking or what is the problem with your engine internals.
Air Leaking from Exhaust – When you can hear the air hissing and escaping from your exhaust and / or headers, it’s a dead giveaway that your exhaust valves are burned or not seating properly.
Air leaking from Intake or throttle body – Bad intake valve or bad intake valve seat is the culprit here, you must service your cylinder head(s).
Air hissing from other cylinders – This is a sure sign that your head gasket has blown or if you own an engine with an open deck, there may be damage to your cylinder walls.
Air leaking into coolant or radiator – Bubbling coolant is never a good sign when you are performing a leakdown test on your engine. A blown headgasket or cracked head may be the issue.
Most commonly asked questions when it comes to leakdown test or compression tests :
Q: What if I don’t know what my compression reading should be?
A: Not to worry, if you are unsure of what your compression reading is a better idea is to measure all the cylinders in your engine. You should have variation between the cylinders, but they should all be within 5-10 % of each other. Anything more than that is a sign of a problem.
Q: I have bad compression but not much leaking
A: This may be a more subtle problem with your engine, your cams may be worn or there may be an issue with your valve springs or collapsed lifters.
Q: All my cylinders have poor compression but not much leaking, but my car runs fine
A: Sounds like your timing belt may be off a tooth, check for vacuum when the car runs to see what condition your engine is in .
Q: My compression is good and not much leakage but my cylinder is misfiring.
A: This is a sign of another related issue, like a clogged fuel injector or bad spark plug wire or plug.
That does it for our How To Leakdown Test your Engine, if you have any questions about testing your engine, or questions about this method leave them for us below!