If you have taken your car for a long drive recently, give it at least 30 minutes to cool down. Make sure the engine feels warm instead of burning hot. You could also try doing a compression test on a cold engine. The test is more accurate when the engine is warm, but it may still alert you to problems that need to be fixed in order to keep your vehicle in good shape.
In most cases, you won’t get any gas or oil on you during the test, but you’re still better off safe than sorry. Put on the safety glasses just in case. If you’re performing the test on a cold engine, you won’t need the gloves.
The fuse box can be located somewhere else in your car, such as underneath the steering wheel or inside the passenger glove box. It differs depending on the vehicle. Refer to the owner’s manual or look for a diagram on the fuse box case. It will show you the location of the fuse or fuses you need to remove. If you don’t have the manual or a diagram, search for your car’s make and model online to see if you can find one.
If your vehicle doesn’t have an ignition fuse, look for the big ignition coil in the engine compartment. It looks like a cylinder perched on top of the engine. Pull off the big wire plugged into the top part of the coil.
Label the wires so you know which spark plug each one connects to. The spark plug wires are generally well-routed to avoid confusion, but keep them separate anyway to reduce the possibility of switching them. You don’t need to take the wires out to perform the test, but consider taking the opportunity to inspect them and replace worn-out ones. Some vehicles have ignition coils instead of spark plugs, but they can be removed the same way.
Socket wrench kits, along with compression gauges and replacement parts, are available online or at most auto part stores. Label each spark plug with chalk or a piece of masking tape so you know which cylinder they belong to. Lay them out in a secure spot near your vehicle. Consider checking the spark plugs for damage while you have them out. If they look worn out, replace them. Burned oil or other debris could be a sign of an engine problem.
Keep in mind that compression testing kits often come with multiple adapter hoses. Use one that fits well in your vehicle’s engine. Check the sizing label on the hose and match it to the size of the spark plugs.
Make sure the gauge is well-connected to the hose. If it feels loose, it will affect the test. Keep in mind that some compression gauges plug directly into the engine and don’t require a hose. However, most gauges you will come across make use of a hose adapter.
The needle on the gauge should stop moving and point to a number. If it doesn’t stay in place, crank the engine for up to 10 seconds. Ask a friend to sit in the driver’s seat and crank the engine for you. That way, you can keep an eye on the compression gauge. If your vehicle has a remote starter, you could also use it to avoid having to get behind the wheel.
Test all of the cylinders in order, starting with the first one and working straight down the line to the engine’s opposite end. On your paper, label it “1, 2, 3” and so on. Keep the test results orderly so you know which cylinders they correspond to. Once you’re done testing, you can remove the compression gauge and hose adapter.
For diesel engines, the optimal PSI is between 275 and 400. Low individual readings indicate cylinder-specific problems like worn-out piston rings.
For example, a series of low readings could indicate that the valves between cylinders have worn out. It could also be a sign of overall engine failure.
The PSI typically rises when you do a wet test instead of a dry one. The change can help you determine what is wrong with the cylinder. If the test worked, a large change could mean the piston rings have worn out.
If the low reading is from a single cylinder, check it for worn-out piston rings. If the test results were both the same, the cylinder could have a bad valve. If you notice 2 nearby pistons with low PSI readings, you most likely have a blown head gasket to replace. The gasket is between the 2 cylinders. Low compression in all cylinders could mean your engine needs a new timing belt. If that doesn’t work, the engine may need a tune up. If the engine no longer runs very well, you may be better off replacing it with a new one. Fixing an engine can be expensive, so sometimes getting a new engine from a scrapyard is more cost-effective.
