Step-by-Step Guide — Replacing the Fuel Tank on an Audi 100 (quattro / 4×4 — C3 / C4 / S-series / V8)
Difficulty: Moderate ® Advanced (requires lifting the car, handling fuel, working under the vehicle).
Safety first: If you’re not comfortable working with fuel systems or heavy lifting, hire a professional.
WHAT YOU’LL NEED
Tools
- Floor jack + good jack stands (rated for vehicle weight)
- Small transmission / bottle jack or a second jack to support tank
- Socket set (metric), ratchet, extension
- Screwdrivers, pliers, pry bar
- Fuel-line quick-disconnect tool set (for the OEM connectors)
- Lock-ring tool / large screwdrivers and hammer (for fuel module lock ring) OR the correct factory
tool
- Utility container / approved fuel jerry can
- Fuel siphon or hand pump
- Torque wrench (for critical fasteners; consult manual)
- Wire brushes, rags, drain pan
- Replacement hose clamps, new O-rings/seals, rubber straps or hangers if rusty
- Safety equipment: gloves, eye protection, respirator if needed, fire extinguisher nearby
Parts & supplies
- New fuel tank (or cleaned/repaired used tank)
- New sender/fuel pump module gasket / lock ring O-ring (Engine Gasket Sealant)
- New fuel line clamps or OEM quick-connect fittings
- New filler neck seal (if applicable) and vent hoses if brittle
- New rubber tank straps/isolation pads if corroded
- New fuel filter (if in-line)
SAFETY & PREP
- Work outdoors or in a very well ventilated area. No open flames, sparks, smoking.
- Put a suitable fire extinguisher rated for flammable liquids within reach.
- Relieve fuel system pressure before disconnecting lines.
- Disconnect the negative battery terminal after relieving pressure.
- Use jack stands — never rely on a jack alone. Chock front wheels.
- Drain fuel into approved, grounded containers and dispose legally.
STEP-BY-STEP PROCEDURE
1 — Prepare vehicle
- Park level, engage parking brake, chock front wheels.
- Remove fuel pump relay/fuse and run engine until it stalls to relieve fuel pressure.
- Disconnect the negative battery terminal.
2 — Drain fuel
- Siphon as much fuel as possible into an approved container.
3 — Raise vehicle
- Jack up and support with jack stands. Support tank with second jack.
4 — Disconnect connections
- Filler neck, vent hoses, pump/sender wiring, fuel lines (use proper tools).
5 — Remove straps/mounts
- With tank supported, unbolt straps. Lower slightly, check for any lines still attached.
6 — Lower & remove tank
- Lower carefully, pull tank out. Two people recommended.
7 — Transfer pump/sender
- Replace gasket/O-ring. Ensure float/sender arm moves freely.
8 — Inspect/replace ancillaries
- Replace filler neck rubber, vent hoses, straps if rusty.
9 — Install new tank
- Raise into place, connect filler, vents, lines, wiring. Reinstall straps.
10 — Prime & test
- Reconnect battery, prime fuel pump by cycling ignition ON/OFF. Check for leaks.
11 — Final inspection
- Start engine, check gauge, inspect again after short test drive.
TROUBLESHOOTING
- Fuel smell ® Check clamps and seals.
- Filler won’t align ® Adjust filler neck.
- Engine won’t start ® Check pump wiring and prime system.
- Fuel gauge wrong ® Reinstall sender correctly.
DISPOSAL
Dispose old fuel and parts legally at recycling or auto shop.
WHEN TO CALL A PRO
- Heavy rust or seized fasteners.
- If driveshaft/exhaust must be removed.
- Unfamiliar with depressurizing fuel system.
