How to do a compression test

A compression test diagnoses many engine problems. If the compression test is below the manufacturer's specifications, this indicates an internal engine problem.

Over time, you may have noticed that your car doesn't perform as well as it did when you first bought it. There may have been a stall, stumbling, or misfire. It may run rough at idle or all the time. When your car starts to work this way, a lot of people think about tuning it. Replacing the spark plugs and possibly the ignition wires or boots can fix the problem - if that's the problem. If not, then you may be wasting money on parts you don't need. Knowing how to perform additional diagnostics, such as a compression test, can help you diagnose your engine correctly, which can save you money because you won't be buying parts you might not need.

Part 1 of 2: What does a compression test measure?

When diagnosing most engine problems, it is important to do a compression test as this will give you an idea of ​​the overall condition of the engine. As your motor spins, there are four strokes, or up and down motions:

Intake stroke: This is the first stroke that occurs in the engine. During this stroke, the piston moves down in the cylinder, allowing it to draw in a mixture of air and fuel. This mixture of air and fuel is what the engine needs to be able to produce power.

Compression cycle: This is the second stroke that occurs in the engine. After drawing in air and fuel during the intake stroke, the piston is now pushed back into the cylinder, compressing this mixture of air and fuel. This mixture must be pressurized for the engine to produce any power. This is the turn in which you will perform the compression test.

power stroke: This is the third stroke that occurs in the engine. As soon as the engine reaches the top of the compression stroke, the ignition system creates a spark that ignites the pressurized fuel/air mixture. When this mixture ignites, an explosion occurs in the engine, which pushes the piston back down. If there was no pressure or very little pressure during compression, then this ignition process would not occur correctly.

Release cycle: During the fourth and final stroke, the piston now returns to the cylinder and forces all the used fuel and air out of the engine through the exhaust so that it can then start the process over again.

While all of these cycles must be efficient, the most important is the compression cycle. In order for this cylinder to have a good, powerful and controlled explosion, the air-fuel mixture must be at the pressure that the engine is designed for. If the compression test shows that the internal pressure in the cylinder is significantly lower than the manufacturer's specifications, then this indicates an internal engine problem.

Part 2 of 2: Performing a compression test

Necessary materials:

• Compression tester
• Computer scan tool (code reader)
• Ratchet with various heads and extensions
• Repair manual (paper or electronic for vehicle specifications)
• spark plug socket

Step 1: Position your vehicle safely for inspection. Park the vehicle on a level, level surface and apply the parking brake.

Step 2: Open the hood and let the engine cool down a bit.. You want to test with a slightly warm engine.

Step 3: Locate the main fuse box under the hood.. It's usually a big black plastic box.

In some cases, it will also have an inscription showing the diagram of the box.

Step 4: Remove the fuse box cover. To do this, disconnect the latches and remove the cover.

Step 5: Locate the fuel pump relay and remove it.. This is done by grabbing and pulling straight up from the fuse box.

• Functions: Refer to the repair manual or the diagram on the fuse box cover to find the correct fuel pump relay.

Step 6: Start the engine and let it run until it shuts off. This will mean that the engine has run out of fuel.

• A warning: If you do not shut off the fuel system, fuel will still flow into the cylinder during the compression test. This can wash out the lubricant from the cylinder walls, which can lead to incorrect readings and even damage to the engine.

Step 7: Remove the electrical connectors from the ignition coils.. Press the latch with your finger and disconnect the connector.

Step 8: Loosen the ignition coils. Using a ratchet and an appropriately sized socket, remove the small bolts that secure the ignition coils to the valve covers.

Step 9: Remove the ignition coils by pulling them straight out of the valve cover..

Step 10: Remove spark plugs. Using a ratchet with extension and spark plug socket, remove all spark plugs from the engine.

• Functions: If the spark plugs haven't been changed for quite some time, it's time to replace them.

Step 11: Install a compression gauge in one of the spark plug ports.. Pass it through the hole and tighten it by hand until it stops.

Step 12: Crank the engine. You should let it spin about five times.

Step 13: Check the compression gauge reading and write it down..

Step 14: Depressurize the compression gauge. Press the safety valve on the side of the gauge.

Step 15: Remove the compression gauge from this cylinder by unscrewing it by hand..

Step 16: Repeat steps 11-15 until all cylinders have been checked.. Make sure readings are recorded.

Step 17: Install the spark plugs with a ratchet and spark plug socket.. Tighten them until they are tight.

Step 18: Install the ignition coils back into the engine.. Make sure their mounting holes line up with the holes in the valve cover.

Step 19: Install the heat exchanger mounting bolts by hand.. Then tighten them with a ratchet and socket until they are snug.

Step 20: Install the electrical connectors to the ignition coils.. Do this by pushing them into place until they make a click, indicating that they are locked in place.

Step 21: Install the fuel pump relay into the fuse box by pressing it back into the mounting holes..

• Functions: When installing the relay, make sure that the metal pins on the relay are aligned with the fuse box and that you gently press it all the way into the fuse box.

Step 22: Turn the key to the working position and leave it there for 30 seconds.. Turn the key off and on again for another 30 seconds.

Repeat this four times. This will prime the fuel system before starting the engine.

Step 23: start the engine. Make sure it works the same way it did before the compression test.

Once you've completed the compression test, you can compare your results to what the manufacturer recommends. If your compression is below specifications, you may be experiencing one of the following problems:

Punched cylinder head gasket: A blown head gasket can cause low compression and a number of other engine problems. To repair a blown cylinder head gasket, the top of the engine must be disassembled.

Worn valve seat: When the valve seat wears out, the valve can no longer seat and seal properly. This will release the compression pressure. This will require a rebuild or replacement of the cylinder head.

Worn piston rings: If the piston rings do not seal the cylinder, the compression will be low. If this happens, then the engine will have to be sorted out.

Cracked ComponentsA: If you have a crack in the block or in the cylinder head, then this will result in low compression. Any part that is cracked must be replaced.

Although there are other causes of low compression, these are the most common and require further diagnosis. If low compression is detected, a cylinder leak test should be performed. This will help in diagnosing what is going on inside the engine. If you don't think you can do this test yourself, you should seek help from a certified mechanic, such as from AvtoTachki, who can perform a compression test for you.