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This is not on an MN12 but rather my daily driver. I did a compression test on my 97 dodge avenger tonight and got a consistent 180 to 185 across the first three cylinders. However cylinder number 4 was 270, yes that's not a typo 270. I even checked it with another compression tester. I doubt this has anything to do with it but I will add anyway. It is a dohc 2.0 mitsubishi motor that has the rubber valve cover gasket with the spark plug hole gaskets as well. The number 4 spark plug hole was full of oil so I know I need to change the gaskets but oil running down into the cylinder just prior to the compression test couldn't cause higher compression could it? At least not to that extent I would think. What could possibly cause this? The car runs fine so I am baffled here. Thanks for your help.
 

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I doubt this has anything to do with it but I will add anyway. It is a dohc 2.0 mitsubishi motor that has the rubber valve cover gasket with the spark plug hole gaskets as well. The number 4 spark plug hole was full of oil so I know I need to change the gaskets but oil running down into the cylinder just prior to the compression test couldn't cause higher compression could it?
Of course it's relevant. General compression tests are sometimes done dry and 'wet'. The wet test is after a teaspoon of oil is put in each cylinder.
 

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Wouldn't a reading of 270 be a 18:1 compression ratio? (270/15psi=18... Yeah, I know STP isn't 15psi, but it's close enough, lol.)

Something isn't good; could the cylinder have filled most of the way up with oil?
 

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Wouldn't a reading of 270 be a 18:1 compression ratio? (270/15psi=18... Yeah, I know STP isn't 15psi, but it's close enough, lol.)

Something isn't good; could the cylinder have filled most of the way up with oil?
Not sure how much oil fell in as the spark plug hole was full when I pulled the plug but number 4 was the last cylinder I tested shouldn't most of the oil been gone by then? Could the oil make that much difference? I guess I should pull the plugs again tomorrow and check cause I put the plugs back in when I was done and started the car so any oil should definitely be gone now I would think.
 

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The oil causes the rings to seal better, hence the higher reading.

If you want, you could repeat the compression test, squirting oil in each cylinder and note the difference between the dry and wet readings.

The only other explanation would be extreme carbon buildup on the one cylinder. Rare but possible.
 

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Discussion Starter #6
Oil makes that big of a difference?
 

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Oil filling the part of the volume of the combustion chamber is how you get 18:1 compression readings...

Your max reading shouldn't be more than your compression ratio X air pressure.
 

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Oil filling the part of the volume of the combustion chamber is how you get 18:1 compression readings...
This makes sense as the spark plug hole is about 6 inches deep and as big around as a 5/8 socket (roughly) so needless to say a fair amount of oil poured into the cylinder. I let it run for a few minutes then pulled the plugs and checked again tonight and got 185 lbs on the cylinder in question. Thanks for your input guys, situation resolved. Now I just need to change the valve cover gaskets.
 
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