Once the boot cracks, there are a few scenarios:

1. Minor grease leak and no dirt ingress into the CV joint:
- Disassemble the CV joint (info in forum), clean it with solvent such as mineral spirits etc. Clean all the balls well.
- Let them dry, then
- Re-pack with grease
- New rubber boot, use only GKN or BMW dealer boot.

2. Major grease leak and dirt has entered the joint:
- Sometimes you are lucky because if you catch it early w/o CV joint damage (no clunking sound during right/left turns), then do #1 above.
- If you have clicking sound during right/left turns, time for new axle.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT