If your car or truck has been running poorly and doesn’t seem quite as peppy as it once did, it may be a good idea to compression test your engine. Compression is a huge part of how your engine runs, as the piston compresses the fuel and air mixture before your spark plug ignites it. This ignition and combustion is what drives your engine and your car.
When a cylinder or cylinders lose compression, your car will not run smoothly. Lack of power and a rough idle can be a couple of key indicators. You might interpret this rough idle as misfire, but if there’s no code for misfire, you can save yourself some money by learning how to compression test your engine.
If the loss of compression is great enough, the cylinder just won’t ignite causing the sensation of misfire in your engine. If there’s less pressure than what the cylinder is supposed to hold, the mixture won’t ignite as efficiently. This causes the lack of power and response in your engine, and you’ll need to know how to compression test your engine to find out what the real culprit is.
Q : Where is my cylinder pressure going?
A : Because of the nature of your engine and combustion chamber, there’s only two places for this pressure to go. If you have poor compression in a cylinder, the pressure could be escaping upwards through the valve seats or valve seals. Typically a bad or worn valve seal will also smoke at startup or when your engine is cold.
The other more common area your pressure is escaping to is past your rings. Because of the mileage and wear on your rings, they are not capable of scraping the cylinder walls and compressing air as they used to be. When this happens, pressure and compression ratio go down while oil consumption and engine wear go up.
Tools you will need to perform this compression test
- Compression tester
- Spark plug removal tool
- wrench or socket to disconnect distributor (if applicable)
- bypass plug or tool to disconnect igniter (if applicable)
In order to learn how to compression test your engine, you will need someone to crank the engine while you measure the compression rating. It’s a good idea to ask a friend to give you a hand for this job. Before you begin our DIY guide on how to compression test your engine, make sure your vehicle is in PARK or NEUTRAL and has the parking brake on at all times.
Step 1 – Disable spark and / or fuel
If you own a distributor driven vehicle, disconnect the big wire from your distributor to your coil. This should be normally located on the center of your distributor cap. Disconnect this cable and rest the connector against a grounded metal surface as far away from your spark plugs as possible.
Testing on a vehicle that doesn’t use a distributor? Easy to take care of, as the only thing you’ll be unplugging is your igniter. You may or may not have to pull an EFI fuse or unplug your injectors to prevent from flooding your engine.
Step 2 – Remove spark plugs
If your vehicle in question utilizes coil on plug like our test subject, a 2005 Acura RSX then you will need to unplug these coils first. Begin by undoing the 10mm nuts and bolts that cover the ignition coils
With this spark plug cover removed to your K20, you can now access the coil on plug and disconnect the ignition harness.
Remove the 10mm bolts that hold these coil on plugs in place, and then remove the ignition coils. You are now ready to remove the spark plugs, make sure to do this carefully to ensure nothing falls into your motor, or this will become a pretty complicated compression test.
Begin testing by locating your #1 cylinder on your engine. In our test subject it’s the closest cylinder to the actual timing belt. Screw in your compression tester firmly, and now you’re ready to learn how to compression test your engine.
Step 3 – Crank Engine
With the compression tester firmly installed into the test cylinder, have your friend crank over the motor with the throttle opened. If you have a carburetor, use a tool or flathead screwdriver to hold open the choke if you happen to own an automatic choke.
Instruct your buddy to crank over the motor until the needle slows down or stops increasing. Tell him to stop and watch the compression to make sure it doesn’t rapidly fall all of a sudden.
Record the compression rating for cylinder 1 and then move onto the next cylinder.
Once you are done compression testing all the cylinders in your engine, go back and inspect your findings. Your cylinders should all be within 10-15 psi of each other, meaning your engine is a good relative health.
If one of your combustion cylinders reads way lower than the rest, try to retest this cylinder. If the findings do not change, move onward to Step 4.
Step 4 – Pour Oil into Cylinder
Using an oil gun or even Mighty Vac, gently squirt the troubled cylinder through the spark plug opening. Retest your problem cylinder and if the compression stays the same, chances are your cylinder head will need to be serviced.
If the compression rating increases however, this means that your rings or ring lands have worn and your piston cannot hold the compression required. This most likely means you will need new rings in that cylinder, as well as machine work to have the cylinder walls rehatched.
Now that you know how to compression test your engine, you know what the problem with your engine is. You now know whether or not rebuilding your engine is worth it, or at the very least know what kind of work it will entail.
Have a question about our how to compression test your engine DIY guide? Leave them for us below!