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Discussion Starter #1
I just did a compression test because my car has been getting P0304 cylinder 4 misfire .. I changed spark plugs and wires.. So I thought a compression test was in order.. here is the results cyl 1 - 105 .. cyl 2 - 125 .. cyl 3 - 145 .. cyl 4 170 .. Now I know they are supposed to be close to in value though this is ridiculous. How is it that I am getting ECU code P0304 cylinder 4 misfire when cylinder 4 has more compression than the other cylinders?


Thanks.
 

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Well, I might be wrong, but as far as I remembered when I learned engines. The more compression there is in a cylinder the more heat inside it, if you there is too much the mixture can explode before it should.

The difference between those values is too big, how many miles does you car have? what's your engine? how much was your compression tester? How many reads did you try per cylinder?
 

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Thats a moot point, considering he has one cylinder thats 65psi lower than the others tells me he needs to rebuild the motor and soon.
 

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how many miles are on that motor? Also my auto teacher in high school taught us to compression test and if it wasn't great you drop a little bit of oil in the cylinder and if the compression grows as you are cranking it is a sign the piston rings are on their way out.
 

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Discussion Starter #6
My zetec has 66,000 or so on it. And thanks a lot for the replies thus far...I am in desperate need of some guidance.
 

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You didn't happen to test them in the following order: 4,3,2,1 ???

If so, I'd say you ran down the battery.............
 

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You didn't happen to test them in the following order: 4,3,2,1 ???

If so, I'd say you ran down the battery.............
Or if he did it that way, and disconnected the fuel pump without purging the lines, the first cranks would have had fuel, and by the time he got to #1, it would not have had fuel, causing the reading to be lower. I just thought that may be a possibility since they are progressively lower...
 

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Makes no difference.... with one being 65psi less he has bad rings or a burnt valve.... fuel makes NO differnce considering the cylinder is NOT firing when you do the test.
 
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