There are quite a few head gasket related questions on the site. Coolant or water in the oil, oil in the coolant or water in a radiator, etc. I'm thinking that some instruction on howto use a leak down tester is in order. With that, I am interested in hearing responses related to the following.


1. How do I operate a leak down tester?

2. Why is there an air pressure regulator on a leak down tester

3. What are some of the issues I can troubleshoot with a leak down tester?

4. If I have a blown head gasket where would on the car would I listen for an air leak in order to determine if my head gasket is blown?

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    This is a good question
    – cloudnyn3
    Feb 22, 2016 at 20:56
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    Thanks. I'm hoping putting links to this and some others in those head gasket questions will be good. Love your answer btw. Feb 22, 2016 at 20:57
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    @cloudnyn3 I didn't put your name in my previous comment. Feb 23, 2016 at 0:27

2 Answers 2


There are many leak down testers on the market. You can actually make your own with some hardware store parts and a fish tank.

Operation of "most" leak down testers

The procedure of operation depends on whether or not you have the cylinder head attached to the engine block or not. If the head is attached, your leak down tester should have an attachment that screws into the spark plug hole. Once fastened you can use your compressed air (usually around 100psi) to fill the the cylinder with air. It's very important to ensure that you have the cylinder that you are testing at TDCC (Top Dead Center Compression). This means that the cylinder is completely sealed and all valves are closed on that particular cylinder, thus giving you a nice platform for testing.

Testing with head off

When testing with the head off you often use a method similar to this. https://www.youtube.com/watch?v=A26KfDj6wAM

I do not swear by this method, and the guy seems to take a while to get to the point. So, you can form your own opinion on this. It's more fit for finding cracks on the head, however though I have found that sometimes my leak down tester finds a fault and it turns out to be a cracked head; That can be difficult to diagnose with the engine assembled; Just food for thought =)

Air Pressure Regulator

We use the air pressure regulator for a couple of reasons. Some cars have a higher compression rating than others, so you need to be able to regulate the pressure accordingly. When you're talking about pressure in the cylinder, we have a constant and combustion pressure. You want to be able to test up to your max combustion pressure, but also test a constant pressure that is a bit lower for fine tuned diagnostics. The reason for this is that the cylinder is not under CONSTANT 210psi and the cylinder actually jumps up a little more due to fuel and oil lubrication when running. To perform a good leak down test that yields good results, you need to be able to regulate pressure. The valves may be holding at 100psi, but your compression rings on the piston may only hold to 90psi.

Issues you can trouble shoot with a leak down tester

Mainly when using a leak down tester, you'll be troubleshooting an issue with a head gasket, leaking valve seals, and bad piston rings. Essentially anywhere air could rapidly escape from the combustion chamber that wasn't intended when the engine was engineered and that creates an issue while it's running. Cracked heads are a different story, as I have seen some leak oil into antifreeze and you wouldn't have known unless you actually pulled the heads off to see the cracks. Thats a different topic of itself.

Where to find the leak

When using the leak down tester, you want to listen where your oil goes in and your coolant reservoir or cap comes off. This will allow you to audibly hear where exactly the leak is. You should hear a small bubbling noise, or something like that. If the leak is significant, it will most likely bubble up in the radiator or coolant reservoir. If it's bad enough it will hiss loud enough to hear.

When doing this you want to start out at a lower PSI and gradually increase. You also want to leave it to sit for about 30 minutes and see how much it's decreased in pressure. A few PSI isn't anything to worry about if it's an engine with a lot of miles on it. If all of the pressure is gone within 5 minutes, you may have some additional issues that need to be addressed. Mainly you want to pay attention to any noise coming from your coolant reservoir or radiator. If you set your pressure on the leak down tester too high, you can actually move air past the piston rings and give a false diagnosis. Keep that in mind. Removing the oil pan or drain bolt and listening for hissing is also an option. Have a rag or towel on hand.

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    Extreme bonus points for incorporating a fish tank into any answer :) Feb 22, 2016 at 20:57
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    Yeah, fish tank would fall into the 'next level' category for sure. Feb 23, 2016 at 0:29

1. How do I operate a leak down tester?

Directions differ depending on whether it's a one gage or two gage system. It basically gets down pulling a spark plug, getting that piston to Top Dead Center (TDC), threading the tester into the spark plug hole, hooking up compressed air (at a minimum pressure), bringing the cylinder pressure up to a set amount, and then watching how quickly it comes down again, and if it settles anywhere.

2. Why is there an air pressure regulator on a leak down tester?

To put an upper limit on the cylinder pressure, and to allow calibrated readings. The percentage leakdown is only meaningful for a given pressure- meaning you can't compare one cylinder's reading to another if the peak pressure is different, which (without a regulator) would occur from cylinder to cylinder as the air tank runs down (and then builds up again when the compressor runs)

3. What are some of the issues I can troubleshoot with a leak down tester?

Worn rings and valves, bad head gaskets and cracked or warped heads.

4. If I have a blown head gasket where would on the car would I listen for an air leak in order to determine if my head gasket is blown?

The coolant system should be monitored carefully for air bubbles as an indication of a bad head and head gasket seal. Unless it's an unusual system, the excess air should come out of the PCV, intake and exhaust systems, never through the coolant.

In extreme cases, air might vent to the outside at the head/block juncture. An ungloved hand, or a visual indicator such as talcum powder can be used to verify.

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