How do you fix low compression in one cylinder

Low compression is normally worn or badly fitting valves, or worn/broken piston ring(s) or a combination of both. A compression test of all cylinders will help point to what is needed to fix it. You will need a cylinder head overhaul or an engine block overhaul, or a complete engine rebuild. Depending on age, mileage etc, you maybe able to replace broken rings or re-fit burnt valves and get better compression. But start with a full compression test and get a good reliable engine man to interpret it, maybe run a couple of other tests and advise you accordingly.

Well this can be a very costly situation, first thing to do is remove all spark plugs, do a compression test on each cylinder, the lowest cylinder will indicate, either a valve broken or a compression ring broke, if a ring broke you got more problems than it’s worth, rebuild completely or invest a new vehicle.As a rule, most engines should have 140 to 160 lbs. of cranking compression with no more than 10% difference between any of the cylinders.

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Finally low compression in one cylinder is never good and will need at least a valve job or a top-end job to repair. There is also the possibility that a cylinder could be scored, making a rebuild questionable. You need to determine if the car is worth the cost of the repair.