How Do You Check Compression on a Diesel Engine?

What Is an Engine Compression Test for a Diesel Engine? 

A diesel engine compression test refers to a test performed on diesel engines to measure the pressure inside the cylinder. This procedure uses a pressure gauge and can also determine information about components that contribute to high pressure, like how well the valves and piston rings are sealing.

Doing a compression test is sometimes critical. It can uncover and explain common engine problems you can then address to help you keep your engine in good condition and get your vehicle back on the road. We’ll cover these potential issues later on.

Follow our step-by-step guide on how to check compression on a diesel to take you one step closer to identifying and fixing your engine problems.

The first step in how to do a diesel compression test is preparing your vehicle – and yourself. Protect yourself with safety goggles and insulated gloves before diving in. If you’ve not recently used the vehicle, run the engine for up to ten minutes to get it up to its normal running temperature. Park your vehicle up, switch off the engine, and disconnect the fuel supply in order to prevent fuel injection.

Next, you need to identify the injectors in your engine and remove them. You can do this using a suitable socket wrench. Removing the injectors during a diesel engine compression test will enable your engine to spin quicker and the compression gauge to measure the cylinder’s pressure without interference from the combustion process, delivering more accurate and consistent compression readings.

Lock the compression gauge adapter into your first cylinder’s hole. This should be sealed properly against the cylinder head, as a loose grip may affect the test results. Putting this in place will help you measure the compression within this cylinder, offering an accessible point to access the combustion chamber without needing to get rid of other engine components.

To test diesel compression, crank your engine until the gauge’s pressure reading stabilises, typically taking around four times. Take note of the reading. It will be worth assigning a helper to either sit in the driver’s seat to crank the engine or take the compression gauge reading, meaning you won’t have to keep running back and forth between the two points.

Repeat this step-by-step process of how to check compression on a diesel on every cylinder, ensuring you record the compression results each time. Once you’ve tested all the cylinders, compare the results – they should be similar. If they are not similar, this may be telling of issues with specific components, such as a valve or cylinder head.

These issues may need to be addressed with further diagnostic testing or replacement parts, which our team will be more than happy to assist with. For example, we offer diesel engine air compressor reconditioning and machining services.

Low compression in your diesel engine may be a result of:

1. Worn or damaged piston rings

Worn or damaged piston rings are typically the most common cause of low compression in diesel engines. Wear can prevent your piston rings from sealing properly within the cylinder, meaning pressure can escape.

2. Problems with valves

Wear can cause valves to leak, resulting in a loss of compression. Additionally, issues with valve springs can stop the valves from closing properly, further causing compression loss.

3. Damaged head gasket

A damaged head gasket can contribute to a loss of compression by enabling combustion gases to leak out from the engine cylinder.

4. Cracked cylinder head

A cracked or damaged cylinder head can further allow combustion gases to escape. If this is the case, investing in remanufactured cylinder heads may be the solution.

You can look out for some of the common signs below, which could be telling of low compression. If you spot these signs, be sure to do a compression test on your diesel engine or allow us at Foxwood Diesel to take care of things for you.

Perhaps the most straightforward way to tell if your engine has low compression is if the check engine light on your vehicle’s dashboard flashes or lights up.

You might notice a reduction of engine power or difficulty in accelerating as a result of the components failing to generate the required pressure to achieve optimal combustion. This may lead to cost losses.

You may experience the engine running roughly or misfiring due to inconsistent combustion.

Your engine may be tricky to start, particularly in cold weather. This is due to a lower pressure, making it more difficult to ignite the air-fuel mix.

If you notice exhaust smoke coming from your engine, this could be indicative of a compression leak.

As a rule of thumb, each of your engine cylinders will have 300 to 500 PSI (pounds per square inch) if they have what is considered ‘normal’ compression. Additionally, the cylinders should measure within 10% of each other. For example, if one cylinder reads 300 PSI, the others should each be between 270 and 330.

We recommend doing the diesel compression test on a warm engine, so make sure you run it for five to ten minutes before getting started to ensure it’s at operating temperature. Doing the test this way offers a more accurate result of how effectively your vehicle’s engine seals under normal driving conditions because of the expansion of components under heat. Doing a hot compression test is also typically more accurate if you would like to compare your results to manufacturer specifications. Just try not to overheat the engine!

However, cold compression tests are also possible. They are sometimes handy when identifying more severe engine issues, such as a blown head gasket, because a cold engine can offer a more conservative compression reading across the full set of cylinders.

Please ask our team for assistance if you need help deciding whether to run a hot or cold compression test.

So, there you have it! Now that you know how to do a compression test on a diesel engine, you can transform your bus fleet vehicles, trucks, or agricultural machinery back to the peak-performing vehicles they once were.

If you’d like the experts to take care of this maintenance, don’t hesitate to contact us. At Foxwood Diesel, we know everything there is to know about how to do a diesel compression test and can take the necessary steps for you. We can also offer advice on the best replacement parts and maintenance tips should you need them.