Plastic fuel tank – Audi 100 4X4 quattro C3 S6 S4 C4 V8

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.