A car engine compression test can help in detecting pressure leaks, especially those that come from valves, piston rings, and cylinders. It can also help identify a potential problem in the head gasket. The best part of a compression test is that you don’t need to turn off the engine. Here’s how to do an engine compression test. However, before we get into the procedure, let’s find out what car engine compression is.
What is a Car Engine Compression?
Engine compression refers to the pressure your engine generates within the cylinders while it is running. The amount of pressure the engine produces and how well it converts that pressure into the efficiency and power of your engine.
How to Perform an Engine Compression Test
Here we will see in detail how to perform a compression test. Remember that a compression test is a dynamic type procedure that can tell you the overall health of an engine’s mechanical components. Good compression ratios ensure good combustion and therefore better engine efficiency.
Preparation for a Car Compression Test
- Before you start the procedure, remove the spark plug when you warm up the engine for the compression test.
- Always perform an engine compression test with a fully charged battery. If a battery power drops during the test, then you may get false results.
- Make sure you use the correct compression gauge for your engine.
- Warm up the engine by driving your vehicle for a few minutes and then, leave the engine idling. This will help the valves to adjust firmly and the piston can expand and seal properly.
- Turn off the engine and open the hood. Also, make sure the engine has enough incoming air by locking the throttle open with a screwdriver.
- Remove the fuel pump or fuel injection fuse to prevent fuel from entering the combustion engine. You can check the owner’s manual to locate the fuse. The fuse is either located in the “power centre” or in the “fuse block”, under the vehicle hood. You can also find it in the side protection panel under the dash.
- Start the engine so that it can consume all the remaining fuel in the system line.
- Disconnect the spark plug.
- Clean the area around the spark plug so that dirt and debris won’t enter the cylinder.
- Deactivate the ignition system by disconnecting the coil pack electrical connector or by unplugging the ignition wire from the coil. Check the vehicle’s service manual, if necessary.
- Now, install the engine compression gauge in the first cylinder and seal the cylinder by tightening the gauge connector firmly. This won’t let the compression leak.
Car Engine Compression Test
Once your compression testing gauge is ready, begin with the test by following the below-mentioned steps.
- Step 1: Remove all four ignition coils from the engine. To do this, disconnect each of the coils one by one and move the connectors away. Then remove the electrical connection to the low-pressure fuel pump.
- Step 2: Connect a battery charger. This will help to maintain the charging for the electrical system, at a constant voltage during the initial phase of the test. This will also allow you not to strain the battery too much.
- Step 3: Before installing your compression tester, verify that the gauge reads zero and that the pipe you are going to install contains a one-way Schrader valve.
- Step 4: Install the pipe into the number one cylinder spark plug hole and connect the dipstick. Point the dipstick at the person who will be cranking the engine. Before starting, make sure the crankshaft area is free of tools, hoses, etc.
- Step 5: Run the engine until the needle on the dipstick stops moving. The goal here is to see what the greatest possible compression of the cylinder is. Don’t worry about counting the number of revolutions or how long it takes for the needle to stop moving.
- Step 6: Record and compare all of your data from each of your cylinders with the manufacturer’s specifications for your specific engine code. Note the wear limit and the acceptable variance between cylinders.
If one or more cylinders deviate from the manufacturer’s specifications, a mechanical malfunction may be present. A leak test should be performed to diagnose specific components such as valves, gaskets, head gaskets, etc.
Interpretation of the Engine Compression Test Results
The cylinder builds the pressure gradually when there is good compression. Here is how you can interpret the engine compression test results:
- The compression results generally fall between 125 and 175 PSI (pounds per square inch) when there is a healthy gasoline engine. However, it depends on the make and model of a vehicle.
- In a diesel engine, the optimal PSI generally falls between 275 and 400, with no more than 10% variation between any two cylinders.
- A compression above the manufacturer’s specifications generally points to carbon build-up in the cylinder.
- Also, the highest and the lowest cylinder rating shouldn’t be more than 15 to 20 PSI.
- The compression variation between 10 to 15% may indicate broken valves, cast head gaskets and broken rings.
When it comes to petrol vs diesel engines, the compression ratio plays an important role. The engine compression ratio in petrol engines is controlled, while diesel engines have a high compression ratio.
This is all about how to do the engine compression test. If your engine is breaking down repeatedly and you have to do frequent car garage visits then, you can consider buying a well-maintained used car for sale in the UAE.
Stay tuned to the UAE’s top car blog to find more information on car maintenance and repair.
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