How To Test For A Blown Head Gasket

By on 28 October 2020 / Leave a comment Updated on: 3rd February 2021

Before any big purchase most people will put the time in for a little research. Which new phone has the best camera? Why should I pay the extra for fibre optic broadband? Is America worth the extra over Benidorm? Making big decisions, which require big monetary outlay, is something that we spent time to mull over and consider. Which makes it all the more confusing that at the first sight of an issue, most car owners will simply hand the keys to their trusted dealer/ independent with instructions to “fix it, please”. Bills of hundreds or thousands are sure to follow, while the car could be out of the owner’s hands for days of weeks.

Never is this truer than in the case of a blown head gasket. As soon as a few of the common symptoms of gasket failure appear, we run to the specialists, wallet in hand, ready to cough up and have the problems fixed. Unless the vehicle is still under manufacturer warranty, this will likely to lead to a large bill and the car being in the garage for at least a few days. Imagine then, the disappointment when your garage calls and tells you that isn’t the problem, you’ve wasted your day and theirs and have a nice little labour bill to show for it.

Now, wouldn’t it be a whole lot simpler to first, maybe check that the head gasket has actually failed. These simple tests can be performed at home, for a small outlay and without the need for specialist knowledge or complicated tools. Checking for a failed head gasket could save you stress, time and money, so is definitely worth a try.

Chemical Test

An extremely effective method for deciphering whether a vehicles head gasket is truly leaking. A chemical test can tell you whether there are any exhaust gasses present in the coolant system, a sure sign of head gasket leaks. In this example we use the KTLine CO2 Tester but there are many other alternatives. Check out our combustion leak testing kit comparison article for more information.

The kit comes with a testing chamber, adapter, pipette and testing fluid. It is completely re-usable and could be used for up to 50 applications, despite the relatively small outlay. Undoubtedly a great bit of kit to keep in the garage even if you haven’t experienced symptoms of gasket failure.

To begin, 2ml of the testing fluid is added into the chamber, after placing the adapter end into the coolant tank or radiator opening. To ensure that the coolant system does not overflow you may need to remove some coolant before beginning the test. Then, simply start the engine and allow the car to idle and warm up to its operating temperature, at which time the testing fluid will begin to bubble.

chemical test blue

The fluid will then change colour depending on whether combustion gasses are present. In this case, if the liquid stays blue then the coolant is free of combustion gasses, a yellow or green fluid however, shows the presence of combustion gasses and is a sure sign that the head gasket of the vehicle is failing. The test is easy to perform and very accurate but will only show if the gasket failure has allowed combustion gas to mix with the coolant.

chemical test yellow

Compression Test

In the case where the gasket has failed between the cylinders (where the gasses are not mixing with the coolant), a compression test is a great method to use. Compression tests are used to measure the compression within the cylinder as the engine turns over and only requires removal of the spark plugs to complete, no advanced mechanical knowledge required.

compression test

Low compression in adjacent cylinders will show that the gasket has blown between the two and is allowing air to pass out of the combustion chambers. One cylinder reading low compression is not necessarily a sign of gasket failure but irregular levels and common low readings with pistons next to one another is almost certainly an issue with the head gasket.

Positive Test Result, Next Steps

Should the worst happen and one of the tests above comes back showing a positive result, you should address the problems as soon as possible. Either by taking your vehicle into your trusted specialist for repairs or attempting a fix yourself without head gasket replacement. This is possible by using an additive solution, which can solve head gasket leaking, at least temporarily, to get your through to a time or financial state appropriate to fix the gasket permanently.

Do not, whatever the case, continue driving for long periods with a blowing head gasket. This will make symptoms much worse, while likely causing serious internal engine damage.


It may seem a simple idea but testing for head gasket failure before visiting the local garage could save you lots of money, time and stress. Having that definite answer in just a few minutes can put your mind to rest.

A positive test may not be the result that anyone would hope for, but at least this will skip the step of diagnosis by the garage, saving them time and hopefully getting the keys back in your hand sooner. These tests can be performed by almost anyone with just some simple equipment and in only a few minutes, you don’t need to be a professional mechanic to decipher if you’re vehicle is experiencing head gasket woes.

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Jack Gill

Car journalist and fully qualified mechanical engineer, Czok’s newest contributor and long-time automotive writer. Lifetime passion for all things automotive, from new supercars to rusty old hatchbacks. If I’m not behind the wheel, I’m likely under the bonnet or browsing the classifieds. Having spent over 5 years in the automotive industry and under the bonnet of my own car at home, I want to share the knowledge and learning that I have gained, with others without the time to study or the expertise, who just want their car to get them from A to B every day. We aren’t all interested in trawling through endless internet pages and forums. I aim to gather all the information up and roll it into a quick summarised format, to give only the information you really need.

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