Floor pans rot under carpet from cowl leaks and seam-sealer failure. Patch with 18-gauge steel cut to fit, or buy pre-formed XJ floor patches from KEY Parts / Magnum / Tabco.
XJ floor pans rot in three predictable areas:
Driver's footwell, immediately rearward of the firewall, on the right side of the brake/clutch pedal — water from the cowl drain runs here.
Passenger footwell, mirror location.
Rear seat foot area, under rear cargo.
Pre-formed patches are available:
KEY Parts XJ driver/passenger floor patch: ~$80 each, 18-gauge steel, includes flange for spot welding.
Tabco / Magnum: similar quality, varies in shape coverage.
Auto Rust Technicians: also makes XJ-specific patches.
For smaller rust holes (<6" diameter), 18-gauge sheet steel cut to size with a flange beat into it works fine. Larger areas need the full patch.
Process:
Pull carpet (see carpet replacement entry — good time to do both).
Cut out rust with cut-off wheel, going 1" beyond visible damage.
Treat surrounding metal with phosphoric acid.
Fit patch. Mark and cut for tight fit (1/16" gap or less).
Tack weld at corners, then stitch weld around perimeter with MIG. Avoid blow-through with short bursts on thin metal.
Grind welds smooth. Apply seam sealer over weld inside and underside.
Coat underside with rubberized undercoating (3M, Por-15, or Raptor Liner).
Inside floor: lay sound deadener (see Dynamat entry) then carpet.
While carpet is out: address the cowl drains (see cowl repair) — they're WHY the floor rotted.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.