The purpose of a compression test is to monitor the condition of the valves, the engine, and other internal components. When a car’s not running well, compression tests can show if one of the internal components has worn out. Fortunately, it doesn’t take going to a mechanic to perform one of these tests. You can perform one at home with a compression gauge. Here are some useful tips on how to do a compression test.
1. Get your engine to its running temperature.
When a car hasn’t been driven in some time, the engine is going to be cold. Start your car and keep your engine running for about 5 to 10 minutes. You’ll be able to feel the heat coming from the engine when you’re near it. Be sure not to overheat your engine by letting it run too long before you conduct the test. When it’s warm, turn the engine completely off with no fuel or electricity flowing to it. Remove your key from the ignition, so you can safely remove the components of the engine bay. If you just took your car for a long drive, then let it cool for 30 minutes first. The engine should be feeling warm and not burning hot. While you can still do a compression test on a cold engine, your test will be more accurate if the engine is warm.
2. Use insulated gloves and safety glasses.
You should have on heat-resistant gloves to protect your hands from any hot parts when you’re reaching for the engine compartment. The safety glasses will protect your eyes from oil spray or possible gas when you’re detaching engine components. If you’re doing the compression test when the engine is cold, you won’t need any gloves.
3. Remove the injection fuse or fuel pump.
The fuse box will usually be located in the engine compartment. You’ll need to open the black case to expose the plastic tops of the fuses that are plugged into different slots. The fuses for fuel pumps are usually blue, but it can vary depending on your specific vehicle. Once you find it, pull it out with some tweezers to prevent any gas from flowing when performing the compression test. If you don’t see the fuse box in the engine compartment, it may be located inside the passenger glove box or underneath the steering wheel. You can look at your owner’s manual to be sure. It will direct you to the fuse or fuses that need to be removed.
4. Disconnect the ignition coil fuse.
When you disconnect the ignition coil fuse, it will disable your ignition system, so it won’t send any spark of electricity into your engine’s spark plugs. You can look in your owner’s manual to locate where it is, and then remove it. Be sure to keep it separate from your fuel fuses, so you’ll know which one goes where after you’ve done the test. If your car has no ignition fuse, search for the big ignition coil located in your engine compartment. It will look like a cylinder that’s perched on the top of your engine. Then, pull off the big wire that’s plugged into the top part of the coil.
5. Detach wires from spark plugs.
Check your engine where you should see a series of black cables that come out at the top end. Grab each wire at the end and twist it, while pulling it up to detach it from the engine block. The opposite end of the wires will still be plugged in, so you’ll be unable to take them out of the car. You can push them aside, so they’re off the engine.
6. Remove spark plugs.
Fit your wrench with an extension handle and a spark plug to make the removal process easier. Then, fit the socket into the engine holes uncovered by the wires that were removed. Once you have the wrench on the spark plug inside, turn it counterclockwise until you can lift it out of your engine. Be sure to label the spark plugs with masking tape or chalk to remember which cylinder they belong to. You can also check them for any damage while they’re out. They should be replaced if they look worn out.
7. Perform the test.
Look down on your engine to find the cylinder that’s closest to the front of the engine. Once you’ve found it, put your test compressor’s hose in the spark plug slot and turn it clockwise by hand until it’s locked in place. Make sure to check the sizing label on your hose, and match it to the size of your spark plugs. When testing a diesel engine, use a gauge designed for diesel since it has a high compression tolerance. You’ll then check the end of the gauge for a metal connector that can fit on the end of the hose adapter.
You’ll then crank the engine four times to complete your test. You should turn your key as far as it will go in the ignition, then release it. Do this without shutting off the engine, as it should be running throughout the entire process. When you’ve done that, check the compression gauge to see the result. Expect the needle on the gauge to stop moving and point to a number. If it doesn’t, crank your engine for up to 10 seconds. You can have another person sit in the driver’s seat and do it for you, so you can watch the compression gauge.
Then, you’ll unscrew your hose adapter and do the same test on all the other cylinders. You’ll keep a record of each number. You should test the cylinders in order from the first one and working straight down the line to your engine’s opposite end.
8. Interpret results.
Most engine cylinders are usually around 125 and 175 PSI. Remember that ratings can vary based on your vehicle and its overall condition. For diesel engines, the optimal PSI is around 275 and 400. Low cylinder readings indicate a problem. The pressure difference between the highest and lowest cylinder rating shouldn’t be more than 15 to 20 PSI. If it is, that may indicate engine problems that need fixing.Performing a compression test is an essential part of keeping up with the condition of the internal components in your car. Keep these tips in mind when you perform your next test. You may discover as you’re looking in your car that you need a new fuel injection pump. You can find information about one at this link: https://goldfarbinc.com/collections/fuel-injection-pump-new-used-rebuilt.