Quote:
Originally Posted by Bleu Omdurman
Can some one refresh me on how to do a compression test? I have access to the compression test tool (screw on type). Takes two people doesn't it?
On the plugs, I'd prefer long life in this car over performance.
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You do not necessarily need 2 people. You can do a compression test alone in one of two ways. Either you need to see the gauge face (see * below) from in the driver's seat as you turn the engine over (pretty hard to do in the mid-engined cars but not so difficult in front engined cars - I place the gauge facing the cockpit and peer under the propped hood) or you need a remote starter. Either way is fine if you can do it.
In detail, to do the compression test you should start by cleaning the area around the plugs before you remove them with compressed air if you have it.
As Tim states, remove
all the plugs.
Either block the throttle wide open if you are using a remote starter or press the go pedal with your foot to the floor if in the cockpit (again, pretty hard to see the gauge face from the mid-engined or rear engined car seat, but front engined is usually doable). The throttle needs to be wide open or you will not get an accurate reading.
Install the compression gauge in the #1 plug hole.
Crank the engine over at least, I say, 7 times while watching the guage. *The reason to do this is to watch how quickly the compression builds up. It should build up quickly in a healthy engine. Low compression on the first stroke, followed by gradually increasing pressure on successive strokes could indicate worn piston rings. A low compression on 1st stroke which doesn't build up during successive strokes may be leaking valves or a blown head gasket (or a cracked head). Deposits on the underside of the valve heads can also cause low compression. If your gauge doesn't hold the highest compression reading automatically (which most do) you will need to watch closely to see that highest reading.
Write down the highest compression reading and your take on the compression build up for each cylinder immediately as you finish it.
Repeat that procedure for each cylinder.
Compare your readings to the specs for your car and to each other (i.e., cylinder 1 to 2, 3, 4 etc.)
If you have low compression, you can add some engine oil (about three squirts from a plunger-type oil can) to each cylinder through the spark plug hole and repeat the testing.
If the compression increases after the oil is added, the piston rings are worn. If the compression doesn't increase significantly leakage is occurring at the valves or head gasket.
If one cylinder is about 20% lower than the others with a rough idle there may be a worn lobe on the camshaft.
If you find that your compression is way down (compared to specs) or varies greatly between cylinders you may want to do a leak down test.
You do not need an expensive Snap-on tester. If your local AutoZone, or Advance Auto doesn't have loaners, try Harbor Freight. They have one for under $10 that is just fine. They also have one that is IIRC about $40 with quick connects, more sizes of plug hole connectors (for 2 stroke engines and the like), etc. The $10 one has the same gauge and works just fine with most cars.
Good luck.
EDIT: And again, if you do put anti-seize on the plugs (I wouldn't) do not torque them to the recommended values as those values will be "too high" unless the spec'ed values are for lubed plugs.