I have a 2008 Chrysler Town and Country 4.0

Two years ago in the spring I started getting a misfire on cylinder #5 (code P0305) The head was removed and carbon deposits cleaned. 60k miles and 2 years later this spring I started having the same misfire on cylinder #5. There is only 60psi compression, the rest are 180 and doing a leak down test I can hear a leak through the exhaust. I have come across a lot about Carbon build up on the exhaust valve stems. Anyone have any advice on how to fix this without having to tear the whole engine apart again? I heard a suggestion of slowly sucking five ounces or so of a "top engine cleaner" through a vacuum line while it is running to break up carbon deposits. Does anyone have any idea why this engine would have rebuilt carbon deposits. Dealer said the previous owners were probably town drivers and used low grade fuel but I have been using mid grade gas or better and regularly get the engine revved up. Either carbon builds up and and now I am loosing compression causing a misfire or something else is causing a misfire causing carbon deposits which now has me experienced low compression... Any help would be greatly appreciated.