How to perform Compression test

Raid3n · 02-25-2010, 08:51 AM

It's quite simple and there are a few ways to do it that might differ a bit from what i describe. So your best option would be to get a Haynes, or some kind of name brand repair manual, that will have step by step instructions.

First of all you need the "compression tester" tool, that you can find at any auto parts stores for around $30. Don't pay more for the most expensive ones, its not necessary. unless you are ballin and want to buy this for $100+

Then you need to find out the factory compression specs for your paticular motor, optimal (standard) and the minimum allowed before rebuild. Lastly your motor needs to be at operating temperature and have a good battery as you will be cranking the motor a bit.

Basically what a compression check will tell you is what mechanical condition the upper end, ie pistions rings, valves, head gaskets are in. More specifically if there is leakage from the piston rings, defective valves and seats or a blown head gasket.

Begin by cleaning up a bit around your spark plugs to prevent crap from falling down into your cylinders when you take the plugs out. Dirt getting into your motor is not a good thing.

Many manuals will tell you to take out all your spark plugs all at once but I believe this is unnecessary and increases your chances of mismatching your plugs when you're finished. But what ever you wanna do or find easier.

Block the throttle wide open, which i didnt do just floor the gas pedal when cranking.

Disable the fuel system by removing the fuel pump fuse, and disable the ignition system by detaching the primary low voltage wires from the ignition coil. Or like me and just disconnect the coil spark wire from the coil, your call on this one.

Install your compression gauge on which ever cylinder your heart desires, I started on number 4 and worked my way to number 1. The guage installs very easy just like a spark plug except you dont tighten it down more than hand tight.

Crank the motor over several times, I cranked for about 4 seconds per test and watch the guage climb. In a healthy motor, compression should build up rather quickly. Low compression reading on the first stroke, followed by very gradually increasing compression on successive strokes, indicated work pistion rings. A low compression reading on the first stroke, which doesnt build up during successive strokes, indicated leaking valves or a blown head gasket, which also could be a cracked head. Heavy carbon deposits on the valves could also cause lower than average reading as well as a possibly higher than average reading. Record the highest reading that the guage stops at after cranking for a bit.

Repeat this process on the remaining cylinders

If the readings are much below normal (standard) readings, try and add some oil, about a teaspoon's worth, directly in the the spark plug hole and repeat the test.

If the compression increases significantly after the oil is added, the pistion rings are definitely worn. If the compression doesn't increase significantly, the leakage is occuring at the valves or a head gasket leak is to blame.

If two adjacent cylinders have equally low compression, theres a strong possibility the head gasket between them is blown. The appearance of coolant in the combustion chambers or the crankcase would verify this condition.

If one cylinder is about 20 percent lower than the others, and the engine has a slightly rough idle, a worn exhaust lobe on the camshaft could be the cause.

If the compression is unusally high, the combustion chambers are probably coated with carbon deposits. If thats the case the cylinder heads can be removed and decarbonized. Or my way to fix that if it's not too severe is use some bad ass fuel system cleaner and drive that ish the way it was ment to be driven, that usually cleans out the majority of the bad carbon.

If compression is way down or varies greatly between cylinders, it would be a good idea to have a leak-down test performed by a automotive repair shop. This test can pin-point exactly where the leakage is occuring and how severe it is.

Lastly reattach everything the way you found it.

Note: I am not resposible for any mishaps or if your car explodes or what ever the case maybe. so GOOD LUCK

IF you have any question feel free to ask.

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