What Is a Car Fuel Tank?
The fuel tank stores your vehicle's petrol or diesel and feeds it to the engine on demand. In most modern cars it's a sealed plastic or steel reservoir mounted under the rear of the vehicle, typically holding between 40 and 70 litres.
The tank isn't just a passive container — it's an integrated system that includes the fuel pump, fuel sender unit (the float that measures fuel level), fuel filler neck, evaporative emissions canister (EVAP), and various venting valves. Together these components ensure fuel reaches the engine cleanly and that fuel vapour doesn't escape into the atmosphere.
Key Components Inside and Around the Tank
- Fuel pump: Submerged inside the tank on most modern vehicles. Pressurises fuel and pushes it to the injectors. See our full article on fuel pumps.
- Fuel sender unit: A float connected to a variable resistor. As fuel level drops, the float lowers and changes the electrical resistance, which moves your fuel gauge needle.
- EVAP canister: Captures fuel vapour and routes it to the engine to be burned, reducing emissions.
- Fuel filler neck: The pipe you insert the pump nozzle into. Has a rollover valve to prevent fuel spillage if the car tips.
- Petrol cap: Seals the filler neck. A missing or faulty petrol cap can trigger the check engine light on modern vehicles.
What Causes Fuel Gauge Problems?
An inaccurate fuel gauge is one of the most common fuel tank complaints. Causes include:
| Problem | Symptom |
|---|---|
| Faulty sender unit float | Gauge reads incorrectly or swings erratically |
| Corroded sender unit resistor | Gauge sticks at a fixed reading |
| Loose wiring connector | Gauge drops to empty unexpectedly |
| Faulty instrument cluster | Gauge reads wrong across all conditions |
| Damaged fuel pump module | Gauge and pump fail together |
In older Japanese imports common in NZ — Mazda Demios, Honda Jazzes, Toyota Corollas — the sender unit can degrade with age, especially if the tank has occasionally run very low (sediment from the bottom can damage the float mechanism).
Fuel Tank Leaks and Rust
Steel tanks on older vehicles can rust from the outside in, leading to visible fuel stains or a petrol smell. Plastic tanks are corrosion-proof but can crack after impacts. Warning signs of a leak:
- Persistent petrol smell inside or around the car
- Visible fuel stain under the parked car
- Fuel level dropping faster than expected
A leaking fuel tank is a fire hazard and should be assessed immediately.
WoF Implications
NZTA WoF inspectors check for:
- Fuel leaks — any leak is an automatic WoF failure
- A missing or loose petrol cap — this may be flagged
- Damaged fuel lines running from the tank to the engine
NZ Cost Estimates
| Repair | Estimated Cost (NZD) |
|---|---|
| Fuel sender unit replacement | $200–$450 |
| Fuel pump + sender module (combined) | $350–$700 |
| Petrol cap replacement | $25–$80 |
| Fuel tank replacement (small car) | $500–$1,200 |
| Fuel tank repair (welding/patch, steel tank) | $200–$500 |
When to Book a Mechanic
Book a mechanic if:
- Your fuel gauge is consistently inaccurate or erratic
- You smell petrol inside or underneath the car
- You see a fuel stain under your parked vehicle
- Your check engine light is on with codes related to the EVAP system (e.g. P0440–P0457)
- You're running out of fuel despite the gauge showing a quarter tank or more