Cooley On Cars
How to do a compression test on your engineCooley shows you how to do the king of all engine tests.
It's the god of all engine tests, the compression test. If you're even remotely mechanically inclined, this is a good car tech 101. Now remember, your engine is really all about the cylinders. And the cylinder is nothing more than an elaborate pressure chamber. As long as it's sealed Everything's going well. If it's not sealing well it all goes to hell. You're going to be losing power. You're going to be burning a lot of gas for no reason. You're going to be burning oil and running that low and gumming up your engine. It's just a disaster. So, the compression test will tell us how well is this apparatus around the cylinder compressing? Three problem areas. Here where the round piston seals against the cylinder around it. That's done by these rings. When they wear, you lose compression because it isn't sealing well. Up here by the valves, when these open and close, they have to do so tightly against these seats. They're carefully machined, so that they don't have any leakage between them. And a third place this can all go wrong is in this gap between the block. And the head. See that seam right there? Now, this is a demo engine. On a real engine, you'd see a gasket in there. And if that's blown, that's a blown head gasket. The compression test we're about to do probably isn't a great way to read that, but these other areas that are much more common? We're about to find out if they're healthy or not. Or not. First thing you want to do is disable the flow of fuel to your engine. Now on a modern car, not like this one, easiest way is probably just to pull the fuse that goes to the fuel pump, the fuel injection system, or it might be two different fuses. That way it's not pumping gas in that you don't want to burn anyway right now. Now. Step two, you also wanna disable ignition, or the spark. Again, a fuse on a modern car, easiest way to do that. You can look that up in your owner's manual. Here on the Cougar, I'm gonna do it in a simpler way. We're gonna go to the central wire that comes off the coil and feeds the high tension electricity out to the distributor, and just pull that cable off. And put it somewhere where it's not gonna ground and not gonna arc out. Next up you wanna pull your spark plug or you can pull them all at once but make make sure you keep track of where all the plug wires go so you can put it back together right. I don't see any problem with this doing one of them at a time and that's what I'm gonna do. [BLANK_AUDIO] This is where you wanna use a special spark plug socket that's deep on the right sides and has this rubber insulator in it. And it helps to have a universal joint cuz some of this can be pretty tight. Now you attach your compression tester. These cost a few tens of dollars, they're not terribly expensive. It's a thing that goes in your spark plug hole with the right thread and size, a gauge on the other end in PSI, and a rubber hose between them. You just put it in. Where the plug you just removed used to live. Finger tight. Hand tight. Firm, but no wrenches on this. Okay, now this is a remote engine starter. This is usable for an older car. Let's say, mid-80s or earlier. If you've got a very late model car, this isn't gonna really make any sense. I wouldn't advise trying to figure this out. You just have to jump in and out of the car and turn the key, but that's okay because you've got ignition and fuel flow disabled, it's not gonna start, it's just gonna crank. But I'm gonna use one of these, so I can do it all right here. Now give it a few cranks. [NOISE] And keep doing that until the needle has peaked. It isn't climbing anymore. Now take that reading. I'm looking at about, about 130 psi there on hold number one. Now write that down. What you're looking for overall is for each cylinder to be somewhere in the low 100's to mid 100's psi, let's say 100 to 150. the [UNKNOWN] varies by car and you also want to see them all be fairly similar with no more than 10 or maybe fifteen percent variants across all the cylinders, if that's the case, on both runs you got pretty healthy engine. If one of then is very low you can try squirting some motor oil into that Particular cylinder and reattach your compression tester, run the [UNKNOWN] test again. If suddenly it's got better pressure, you probably have a worn out interface of your piston rings to the cylinder wall. If it didn't help, you probably have an issue with your valve and the valve seat that it closes on. Now this is not a test for compression ratio which we talk about sometimes in our car reviews. That's a whole different thing. And if you want to go to master class on this, you can do another version of this test called a leak down. That uses a different tool related to our compression tester, but a little more elaborate, and it hooks up to an air compressor. And fills the cylinder that way and measures how long it takes for that pressure To then leak out or leak down. We'll cover that in a different how to. I hope if you do a compression test you find your cylinders are all high and happy. [MUSIC]