Yeah, I noticed that that spot was soft last November when I noticed one of my control arms had broken. I didn't have 220V in the garage at the time or a heater so I waited until spring to fix the drivers side (which had the broken arm). I booger welded the arm as a quick fix. I poked at it wit the screwdriver and it literally just crumbled to nothing.
From what I could tell sand had fallen in the the bottom U shape of the mount, got wet, and never dried. It basically rotted at a horizontal line on the outside in line with the top of the U shape and the inside rotted out from stuff being packed in between the control arm and the frame. I never noticed it until it got this bad.
Oddly enough these GMT360's don't visibly rust like a GMT800 since the doors have plastic cladding over them, the rockers have a cover, and the plastic rear bumpers go up pretty high so the only visible rust you get is over the rear wheels (which I already fixed).
265k on it now. Shooting for 300k? My oil pump pickup oring is cracked I think but its not worth fixing it. If that goes I'm shutting it off and keeping the LM4 for a project and finally replacing it.
This is the drivers side last may before I fixed it. The passenger side was worse.
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