GMT400 Rust Areas and Frame Inspection Checklist

GMT400 (1988–1998) – OBS Discussions Exterior
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Use this checklist when inspecting a GMT400 (1988–1998 Chevy/GMC C/K, Suburban, Tahoe, Yukon) for rust and frame integrity.

  1. General Preparation
  • Bring flashlight or headlamp
  • Bring small inspection mirror
  • Bring flathead screwdriver or pick (for probing flaky metal)
  • If possible, bring a floor jack and jack stands for better access
  • Wear eye protection and clothes you don’t mind getting dirty
  1. Frame – Critical Structural Areas
    2.1 Front Frame Sections
  • Front frame horns (ahead of front wheels)
    • Check for bends, crush damage, or patch repairs
    • Look for heavy scaling around tow hook mounts
  • Steering box area (driver side frame rail)
    • Inspect around steering box mounting bolts for cracks or deformation
    • Look for spreading, rippling, or plated-over repairs
  • Front crossmember (under engine)
    • Check bottom and sides for deep pitting or scaling
    • Probe any soft or bulged areas

2.2 Mid-Frame – Under Cab

  • Frame rails under cab
    • Look at inside and outside surfaces for rust scale and flaking
    • Pay special attention to areas near body mounts and fuel/brake line clips
  • Body mounts
    • Inspect mount brackets welded to frame for separation or cracks
    • Check for crushed, rotted, or missing rubber bushings
  • Transmission crossmember
    • Check mounting ears and bolt holes for elongation or cracking
    • Look at where it meets the frame rails for rust and stress cracks

2.3 Rear Frame – Under Bed and Rear Axle

  • Frame over rear axle (arch area)
    • Look at bottom, sides, and inside of the C-channel
    • Pay attention to heavily scaled or swollen areas where metal may be separating in layers
  • Rear spring hanger areas
    • Inspect both front and rear leaf spring hangers
    • Look for cracks around rivets/bolts, rust-through, or repair plates
  • Shackle mounts
    • Check where shackles bolt to frame or hanger for severe rust and cracks
  • Rear crossmembers
    • Rear-most crossmember and any intermediate crossmembers
    • Check for missing sections, plate repairs, or deep pits

2.4 Frame Interior & Hidden Points

  • Inside of C-channel
    • Use flashlight and mirror to look inside frame rails where possible
    • Check near fuel tank, rear bumper, and spring hangers
  • Bumper mounting brackets
    • Inspect where bumpers bolt to frame for rot and cracks
  1. Suspension & Steering Components (Rust & Safety)
  • Control arms (front)
    • Check for rot at spring pockets and bushing eyes
  • Torsion bar crossmember (if equipped)
    • Look for heavy scaling, cracked mount areas, or evidence of movement
  • Leaf springs
    • Check for broken leaves, heavy rust between leaves, and thinning
  • U-bolts and spring plates
    • Inspect for deep corrosion and loss of metal section
  • Steering linkage (pitman arm, idler arm, tie rods)
    • Inspect for excessive rust that might compromise strength
  1. Body – Common Rust Zones
    4.1 Cab Exterior
  • Rocker panels
    • Check outer rockers for bubbling paint, soft spots, or holes
    • Look under door sills; tap and probe suspicious areas
  • Cab corners (behind doors)
    • Inspect visually and feel with hand for thin or crunchy metal
  • Lower door skins
    • Look along bottom seam inside and outside of door
    • Check for rust bubbles near drain holes
  • Fenders (front)
    • Inspect inner lip near wheel arch
    • Check lower rear section behind front wheel where mud accumulates
  • A-pillars and windshield frame
    • Look around windshield seal for bubbling
    • Check door hinges areas for cracks/rust

4.2 Cab Interior & Floor

  • Floor pans (driver and passenger)
    • Lift floor mats/carpet if possible; look for holes, patches, or soft areas
    • Check under pedals, seat mounts, and kick panels
  • Seat mounts and seat belt anchor points
    • Inspect for rust weakening around bolts
  • Inner rockers
    • Look along the interior pinch weld and inner rocker area for crusty seams

4.3 Bed (Pickup)

  • Bed floor
    • Inspect from inside bed for soft spots, holes, or poorly done patch panels
    • Pay attention to around wheel tubs and over frame rails
  • Bed cross sills (under bed)
    • Crawl under and inspect the bed supports that run across the frame
    • Check for separation, rot at mounting points, and missing metal
  • Wheel arches (bed sides)
    • Look around the lips from inside and outside the wheel well
  • Stake pockets and top rails
    • Check for bubbling and rust coming through seams
  • Bed-to-cab gap area
    • Look at front bed panel (closest to cab) for rot along bottom seam

4.4 SUV/Blazer/Suburban Specific

  • Rear cargo floor
    • Lift carpet if possible; inspect seams and under rear seats
  • Rear wheel wells (inner)
    • Look where inner wells meet outer quarter panels
  • Tailgate/liftgate lower edge
    • Check seam at bottom, inside and out
  1. Underbody Attachments & Traps
  • Fuel tank straps and mounts
    • Inspect straps for thinning, flaking, or complete separation
    • Check mounting points on frame for rust-through
  • Brake line brackets and lines
    • Look for rusted-through brackets and severely pitted lines
  • Fuel lines
    • Inspect for heavy scaling and damp spots indicating leaks
  • Exhaust hangers and mounting points
    • Check where exhaust hangers weld/bolt to frame or body
  • Heat shields
    • Inspect around any heat shield mounting points for trapped moisture rust
  1. Front & Rear Inner Structures
  • Inner fenders (front)
    • Check metal inner fenders (if present) for rust at mounting points, lip edges
    • Look where inner fender meets firewall and core support
  • Core support / radiator support
    • Inspect lower sections, especially under battery and at frame mounts
    • Pay attention to battery tray area for acid-related rust
  • Rear body mounts (SUV/4-door trucks)
    • Inspect mounts by rear doors and rear-most body mounts for rot and separation
  1. Visual Signs of Previous Rust Repair
  • Mismatched undercoating or fresh black paint on specific sections only
  • Overlapping patch panels instead of butt welds
  • Excessive seam sealer or body filler over suspected rust areas
  • Plate patches welded to outside of frame rails (especially near spring hangers and mid-frame)
  • Inconsistent weld quality or welds over rust scale
  1. Severity Assessment Guide
  • Surface rust only
    • Light orange/brown discoloration, no metal flaking, still solid to hammer/pick
  • Moderate rust
    • Flaking scale, pitting visible, but no through-holes in structural areas
  • Severe rust
    • Holes, deep pits, significant metal loss, or frame sections that flex when probed
  • Structural compromise (walk away or plan on major repair)
    • Frame cracks, broken mounts, or rotted-through spring hanger/frame areas
  1. Safety & Roadworthiness Checks
  • Verify frame is straight (look down length of frame rails for waves or kinks)
  • Check for any frame patches near steering or suspension mount points
  • Confirm all suspension mounting points are solid (no movement when pried)
  • Ensure no brake/fuel lines are dangerously corroded or leaking
  1. Documentation & Photos
  • Take wide shots of:
    • Full frame from each side
    • Under-cab area
    • Under-bed/over-axle area
  • Take close-ups of:
    • Any rust-through
    • Frame patches or weld repairs
    • Spring hangers and body mounts
  • Note locations of any structural concerns for later evaluation/quotes

Use this checklist to compare multiple trucks and to estimate potential repair work before buying or restoring a GMT400.