Safety: Do not keep driving with coolant mixed into the transmission fluid. Tow it if the fluid is pink or foamy.
Checks, in order
- Dipstick and coolant checkCheck the transmission fluid color and smell. Also look at the radiator coolant for an oily film.easy $ · under about $50
- Pressure test the radiator coolerA shop can test the cooler passages. Many owners replace the radiator whenever this is suspected.medium $$ · about $50 to $500
- Replace radiator and linesReplace the radiator and inspect the cooler lines for cracked hoses. Flush the cooler circuit to get the old fluid out.medium $$ · about $50 to $500
- Transmission serviceAfter the leak is fixed, do a full fluid exchange and watch shift quality. Slipping or a burnt smell afterward means a rebuild may be needed.hard $$$ · shop-level job, $500+
Applies to: 2003 to 2009 4Runner with the 4.0L 1GR-FE V6 and the 4.7L 2UZ-FE V8, 2WD and 4WD. Engine specific items are labeled, and some issues apply mainly to early model years.
Tools and parts for these checks
Links below are Amazon affiliate links: as an Amazon Associate, toyotadata earns from qualifying purchases, at no extra cost to you. Confirm fitment for your exact truck before you order.
| ITEM | USED FOR | BUY |
|---|---|---|
| Radiator for your truck | Replacing a radiator whose internal trans cooler has failed | Shop on Amazon |
| Automatic transmission fluid for your truck | Flushing coolant-contaminated fluid out of the transmission | Shop on Amazon |
| Fluid transfer pump | Filling through the fill or check plug | Shop on Amazon |
| Cooling system pressure tester | Finding coolant leaks and testing the radiator cap | Shop on Amazon |
The same problem on other Toyotas
More 4th Gen 4Runner problems
- V6 losing coolant, cold start misfire or overheating (early 4.0L) (Engine)
- Frame rust or rusted suspension mounts (Frame & Body)
- Clunk, knock or wandering from the front suspension (Steering & Suspension)
- Sticky, melting or cracked dashboard (Body & Electrical)
- V8 timing belt service: when is it due? (Engine)
- Ticking or diesel-like noise on cold start (Engine)
All 16 4th Gen 4Runner symptoms in the interactive troubleshooter →
General guidance from owner experience. Confirm torque specs, fluid capacities and procedures in the factory service manual. Brakes, steering and anything structural: if in doubt, have a qualified shop inspect it before driving.