Ok I have a '98 Subaru forester. I got all plugs tested an it only said #3 & #4 was bad, I changed all 4 to be on the safe side. When I run the new #4 plug it smokes like its on fire but when I run the old one (which doesn't work, another words just runnin it on 3 cylinders) it doesn't smoke. Could this be because the knock sensor is bad or is it the pistons? Any advice would be greatly appreciated!
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