The 1991 4Runner is a solid body-on-frame SUV, but the 3.0L V6 has a notorious head gasket weakness and the automatic transmission can cook itself without proper maintenance. The 22R-E 2.4L four-cylinder is far more reliable but underpowered.
3.0L V6 Head Gasket Failure
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: White smoke from exhaust on startup, Milky oil on dipstick or cap, Coolant loss with no visible leaks, Overheating under load
Fix: Both heads must come off, deck surfaces machined flat, new OEM gaskets and ARP studs recommended. Budget 12-16 hours labor. Many shops won't touch it—owners often do full engine rebuild at this point since you're already in there.
Estimated cost: $1,800-3,200
Automatic Transmission Overheating and Failure
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: Delayed engagement into gear, Slipping between 2nd and 3rd, Burnt ATF smell, No overdrive or erratic shifting
Fix: The factory transmission cooler is undersized, especially for towing. Fluid rarely gets changed on schedule. Rebuild requires 14-18 hours, plus external cooler install (2 hours). Many owners swap to manual or find low-mileage JDM replacements.
Estimated cost: $2,200-3,800
Lower Ball Joint Wear and Failure
Common · high severityTypical onset: 100,000-150,000 mi
Symptoms: Clunking over bumps from front suspension, Loose or wandering steering, Uneven tire wear on inside edges, Visible play when prying on tire at 6 and 12 o'clock
Fix: Both lower ball joints typically fail together. They're pressed into the control arms—some shops replace the whole arm assembly, others press in new joints. Add alignment. 4-6 hours labor for both sides.
Estimated cost: $600-1,100
Timing Chain Guides and Tensioner (3.0L V6)
Occasional · medium severityTypical onset: 150,000-200,000 mi
Symptoms: Rattling noise from front of engine on cold start, Noise disappears after 5-10 seconds, Plastic shavings in oil pan during changes
Fix: The plastic timing chain guides wear and the tensioner can stick. Requires front cover removal, water pump while you're in there. 8-10 hours labor. Not an interference engine, so low immediate danger, but can jump time if neglected.
Estimated cost: $1,200-1,800
Frame Rust (Northern/Coastal Vehicles)
Occasional · high severitySymptoms: Surface rust turning to scaling/flaking on frame rails, Rust perforation near rear leaf spring mounts, Sagging rear end or cracked frame near shackle boxes
Fix: Body-on-frame means the frame is everything. Minor surface rust can be wire-wheeled and treated, but perforation near suspension mounts is structural. Frame replacement or section welding is 20+ hours and often not economical. Inspect thoroughly before purchase.
Estimated cost: $3,000-8,000
Fuel Pump and Sender Unit Failure
Occasional · medium severityTypical onset: 120,000-180,000 mi
Symptoms: No start or long crank before starting, Stalling when fuel tank is below 1/4, Fuel gauge reading erratically or stuck, Whining noise from rear when key is on
Fix: Pump is in-tank, requires dropping the tank (or cutting access panel through bed floor, common DIY shortcut). 3-4 hours labor via tank drop. OE Toyota pumps last, aftermarket often fail early—spend the extra $100.
Estimated cost: $450-750
Exhaust Manifold Cracking (3.0L V6)
Occasional · low severityTypical onset: 100,000+ mi
Symptoms: Ticking noise from engine bay that increases with RPM, Exhaust smell in cabin with heat on, Visible cracks on manifold surface during inspection
Fix: The cast-iron manifolds crack between ports due to heat cycling. Noise is annoying but not dangerous. Aftermarket headers are a popular upgrade. 4-6 hours labor per side, rear bank is harder. Many owners live with it.
Estimated cost: $600-1,200
Buy a 22R-E four-cylinder model with service records and a clean frame; avoid 3.0L V6 autos unless the head gaskets and transmission have already been done.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.