I have a 2002 Ford Focus with a 16 Valve Zetec 2.0l engine. I checked using a Compression tester all four cylinders and I have excellent pressure in the first 3 and only half as much in the 4th.
This is likely one of two things: worn rings on the piston, or a slightly poorly-fitting or dirty valve (one or more of the four).
You might, before digging into this, consider running a can or two of a good 'over-the-counter' engine 'restorer' at your next oil change. This may help dissolve any gum or carbon making rings or valves stick. Some fuel treatments may help if there is physical gum or carbon holding a valve slightly off its seat when closed. There are more involved treatments for cleaning valves and upper cylinder 'in situ' -- some of which are somewhat snake-oily.
Note that if rings are causing the problem, you will get 'fluttery' pressurization at the engine-oil fill hole with the engine running, as the 'lost compression' will be blowing into the crankcase volume. You might check by putting a small quantity of non-detergent oil (or diesel fuel) into #4 through the spark-plug hole and bar the engine over a few times to ensure no hydraulic lock. This should improve seal long enough for a compression test. (It will smoke while the oil burns off, but this should not hurt the cat).
On road testing, light haze at start that soon clears up indicates worn valve guides, which might be related to low compression if the bypassed oil has carbonized to hold the affected valve off its seat. Rings might produce haze after high-speed running or heavy acceleration, or produce heavier oil blow into the PCV system (as the oil area is now being pressurized with relatively hot combustion gas).
If it is not rings, and an OTC treatment doesn't solve the problem well, you can take the valve cover off and inspect the valvetrain, stems, and cams to be sure all the valves are working freely, there is no binding holding any one of them slightly off its seat, etc. I suspect that if there is anything going on with valves that a cleaning treatment doesn't fix, you're looking at pulling the head, in which case you should plan for a full valve job (as its costs would be relatively slight in comparison with re-timing, replacing the head gasket, etc.)
Ford Focus Cars...
on Feb 01, 2019