MOTOR CLINIC

How long can you drive on an empty fuel tank?

In Summary

• The fuel pump must be fully submerged in fuel for it to work efficiently. 

• When the pump is working, it compresses fuel thus giving it high pressure, the energy involved from the rotating parts in the pump produces heat and must be cooled. 

Fuel gauge.
Fuel gauge.
Image: BOOSTED AUTO

All vehicles are equipped with fuel gauges to monitor how much fuel is left in the tank. 

The construction differs from analogue gauges with a pointer varying between empty "E" and full "F" while most modern ones have digital gauges with small bellows or bars used to give an approximate indication of fuel capacity in the tank. Some can calculate the approximate distance for the fuel in the tank 

  Unlike older cars that used a mechanical fuel pump, modern cars are equipped with an electric fuel pump sitting at the lowest part of the fuel tank to supply fuel to the engine 

 All the safety measures i.e proper insulation and lack of oxygen ensure minimum fire hazards can occur as a result of the electric motor inside the fuel tank 

FUEL PUMP OPERATION 

The fuel pump MUST be fully submerged in fuel for it to work efficiently. 

When the pump is working, it compresses fuel thus giving it high pressure, the energy involved from the rotating parts in the pump produces heat and must be cooled. 

It's only possible that the cooling effect is achieved by the cold fuel passing through the pump. 

 Whenever the fuel light illuminates on the dashboard, it means that you are driving on reserve fuel. Some drivers have mastered the art of tempting fate with the fuel light. They will drive until the car is just about to stall before refuelling.

Others prefer refuelling in specific petrol stations hence the main reason for driving on fuel light for long periods trying to navigate to the preferred petrol stations.  As all this is happening the fuel pump is not able to be fully submerged in the reserve fuel. The operation of the pump under this condition generates more heat without a cooling effect taking place. The effects of too much heat on the pump are:

  •   Low pressure in fuel lines
  • Fuel pump failure
  • Rusty fuel tank 

LOW PRESSURE IN FUEL LINES 

Too much heat causes a lot of friction within the pump thus causing wear which affects its tolerances. This may result in a vehicle having hard starts or lack of enough power especially on cruising. In short, this is a sure recipe for a failed pump. Whenever this happens it won't take long before the pump fails completely 

FAILED PUMP 

Subjecting the pump to prolonged overheating due to less fuel in the tank or clogged fuel filter reduces its lifespan and prone to overheating even when the tank is full of fuel. As a  fail-safe mechanism, when the pump becomes too hot, the engine control unit cuts off the power supply to the pump to protect it from any chances of explosion. Once the power supply is shut, the engine then stops. After some time (5min and above) the engine starts up and drives ok. This is mainly because the pump has cooled down and is back to operation for some time before the same cycle repeats. The best solution is usually to replace the pump with a new one.

RUSTY FUEL TANK 

Inside the fuel tank, a special anti-corrosive protective compound is usually applied to prevent rusting. 

When fuel levels remain extremely low, oxidation takes place and rust forms and is eventually syphoned by the fuel pump which causes blockage and rust in the fuel system. 

Keeping fuel above quarter minimises chances of oxidation and keeps the fuel  system clean.

The writer is the owner of Boosted Auto car repair shop