What can cause a compression test to show low compression on any cylinder?

About half a year ago I went to check the compression at a local Honda Dealer, and they said 3 cylinders were OK and one was very bad. They said they poured some oil inside each cylinder and I got results of about 11:1 on 3 cylinders and about 8:1 on the 4th. (I still have the results card)

Since then I read a lot about cars and how engines work and all the different things under the hood, and it mostly said: bad compression is mostly caused by worn piston rings. But wouldn't that cause oil to enter the combustion chamber, and wouldn't that cause my spark plugs to be oil fouled? I chanegd spark plugs about 2 months ago (after about 4-5 years that my brother never bothered to change them) and they turned out to be looking quite well, except for their excessive wear (there are sites out there with pictures of spark plugs and how they should look if this or that is happening inside, like oil leaking... ).

Anyway, what happens to the car is - it lacks a LOT of power (I guess it should be a clogged catalytic converter), and it idles a bit rough, rpms going up and down all the time (I don't have a tachometer, but you can notice it not idling steady), for which I guess the reason should be clogged PCV and EGR valves, which I hope to clean in the next week.

Well, sorry for the long question, but I'd just like some of your opinions on what's going on. Oh, and I read somewhere that non-adjusted valves could cause compression to be low (valves have NEVER been adjusted).

Thanks, Kresho

There are 3 causes of loss of compression. 1 Worn or sticking rings. 2 Burned or miss adjusted valves. 3 Leaky head gasket. If the compression was higher after adding oil than before, that indicates problems with the rings. I am guessing your Civic has hydraulic lifters or whatever, and does not need adjusting. A leaking head gasket will often show low compression in 2 adjacent cylinders. There may also be coolant, oil, or exhaust in places they don't belong.

Go to an auto parts store and pick up an engine cleaner. Ones like Auto Rx that you run from one oil change to another work better than the flushes you add just before changing the oil. They can be added later as long as you aren't too close to an oil change. Follow the directions carefully. It might free up a stuck ring and help some other problems. Take the valve cover off or even try to look in through the oil filler if it is there. You should be able to see the tops of the valves and other parts. If you don't see any like screw head like things surrounded by an nut, your valves are not adjustable. If you do see them, check the valves on the bad cylinder. They should be able to move a little. Compare them to the others. It the others move, but one or both of them don't, loosen the nut and back the screw out until it has about as much as the others, then tighten up the nut. Unfortunately, if the car was run much that way, the valves may be burned. Both burned valves and leaky head gaskets require removing the head, a big job, not your first DIY project.

Good luck. Others here, more familiar with a Civic, may give more details than I can.

Thanks for the info.

Unfortunately, I knew absolutely nothing at the time when compression test was made, only thing I remember is they mentioned something about "pouring some oil" in conjunction with "rings" which I now understand. They only did one test, since I heard them cranking the engine only 4 times (in a full test), which was obviously a wet compression test.

I think a head gasket problem could be eliminated since 3 cylinders did well, and only 1 was bad.

I've got a service manual for my car, and I've read carefully a part about adjusting the valves (they are adjustable), and it doesn't seem as such a hard job to do. I also already have the "feelers" and the clearance specifications, and was hoping to do the adjusting in a week or two, since it is explained in quite a detail.

I'll definitely try the engine cleaner, will look it up together with the "carbon cleaner" I'll be needing to clean the EGR and PCV valves.

Anyway, thank you for your answer.

Kresho