How to do a compression test on your engine

Author:

Roadshow

Keywords:

Roadshow,Cooley On Cars,Brian Cooley,Kelly Hendricks

Subtitles:

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 a elaborate pressure chamber as long as it seals everything is going well if it's not sealing well it all goes to hell you're gonna be losing power you're gonna be burning a lot of gas for no reason you're gonna 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 where you lose compression because it isn't sealing well up here by the valves well 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 as 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 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 might be two different fuses that way it's not pumping gas in that you don't want to burn anyway right now step two you also want a 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 and when I 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 going to ground I'm not gonna arc out next up you want to pull your spark plug or you can pull them all at once but make sure you keep track of where all the plug wires goes he can put it back together right I don't see any problem just doing one of them at a time and that's what I'm this is what you want to use a special spark plug socket that's deep and the right size and has this rubber insulator in it and it helps to have a universal joint because some of this can be pretty tight now you attach your compression tester these cost a few tens of dollars are 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'll 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 Frank's and keep doing that until the needle has peaked and isn't climbing anymore now take that reading I'm looking at about 130 psi there on hole 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 generally varies by car and you also want to see them all be fairly similar with no more than 10 or maybe 15% variants across all the cylinders if that's the case on both fronts you've got a pretty healthy engine if one of them is very low you can try squirting some motor oil into that particular cylinder and reattach your compression tester run the burp 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 it'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 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