Add a tea spoon of engine oil to the #5 cal through the plug hole and see if the compression does increase (rings).
Next up: leak down test. Tons of internet videos on how to. All you need is access to a air compressor and one of the leakdown testers. Real inexpensive as tools go.
This test will determine if the cyl seals, valves, etc.
there are many reasons for poor compression readings in one hole. It could be something simple like rings washed down by too much fuel. Does the oil off the dip stick smell like gas?
Other thoughts.....
if the Disa valve has been replaced in the past, there isn't the possibility of the shaft coming loose and making its way down the intake to one of the cycles.
Valve Burt/ bent not closing due to a valve spring, etc.
Here is hoping all end up simple.
Install a new/ known good injector and see if your issues go away.
__________________
2005 X5 4.8IS
The
Blue ones are always FASTER....
Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green