Don't drive with less than a 1/4 tank!

It has happened to me and I am sure, at some stage in life, it has happened to many of you. Despite the fact I consider myself a responsible motorist – my petrol light suddenly  came on and I frantically found myself searching for the nearest petrol station to fill up (Engen and Ebucks be damned!). As Murphy’s Law would have it, this happened to me whilst travelling on a particularly lonely stretch of road in the North West province, who-knows how many kilometres to the next service station. Luckily, I managed to nurse the vehicle to the next forecourt in the nick of time and I promptly forgot about this seemingly harmless occurrence.

Then you get the other half of the population. These motorists regularly drive their vehicles on empty. When the fuel light comes on, they don’t even blink an eye – they know exactly how many kilometres they can still wring out of their tank. In fact, each successive low fuel warning light inspires the driver to try beat his or her previous record and eke out a few hundred more meters.

While this may seem like a sizeable achievement (and you may even get drinks on the house in some parts of the East Rand), is it really a good idea to drive your vehicle on an empty tank? I am not referring to the odd emergency, but rather to those who habitually drive until the petrol light comes on.

The answer is it is not! Besides the obvious inconvenience of getting stranded and the associated danger of being an easy target for criminals, you can cause irreparable damage to components of your vehicle such as your fuel pump. The rather steep replacement bill of such a component certainly puts a damper on that free drink and the inbox full of fan mail.

What is a fuel pump and what does it do?

Figure 1: The fuel pump assembly is mounted inside the fuel tank. The pump transports the fuel from the tank to the engine via the fuel lines.

The fuel pump in a modern vehicle is usually an electrical pump which sits submerged in the fuel tank. The job of the fuel pump is to move the fuel from the fuel tank to the engine via the fuel lines. The fuel surrounding the pump and running through the system acts as a coolant for the electric fuel pump motor, as well as a lubricant for the internal components.

The very first motor cars did not have fuel pumps. They relied on gravity to get the fuel from the tank to the engine. Examples of this were the Ford Model A and Model T. In the Model T, as some of our older readers may well recall, the fuel tank sat under the drivers seat, a few centimetres above the carburettor, and relied on gravity entirely on gravity to get the fuel from the tank to the Carb. Of course, it did not take long much of an uphill gradient before the carburettor was now higher than the fuel tank and the vehicle cameto a complete standstill. It was for this reason that these vehicles had to negotiate hills in reverse (and not, as many believe, because reverse is the strongest gear in terms of Torque).

Of course, the good people at Ford realised this was a touch inconvenient and moved the fuel tank in the model A to just under the windshield. This placement was a full 12 inches above the carburettor, so the vehicle was able to negotiate all but the steepest hill (and the speedbumps in Parkhurst)in a forwardly direction. In retrospect, the placement was not ideal from a safety standpoint. A head on collision may easily have ruptured the tank and sprayed fuel (which could subsequently ignite) on the driver – but at least he was halfway up the hill and left this world with a rewarding view!  

Why low fuel damages your fuel pump?

The electrical fuel pump of today is still very much assisted by the gravitational force of the fuel. Yes, it has the ability to pump the fuel when needed, such as on uphill’s and when there is little fuel left in the tank. However, with sufficient fuel in the tank, the gravity assists the pump, so that it does not have to work very hard. The less work the motor does, the longer it will last.

1. Overheating due to insufficient cooling and lubrication

Now consider the scenario when you have very little fuel left in your tank. By very little, I mean below a ¼ tank. There is insufficient pressure or gravitational force to feed fuel into the system, so the pump has to work harder and spin faster to suck the fuel up and through the system. By spinning faster, the motor inside the pump becomes hotter. There is no fuel surrounding the motor at this point to help cool the motor down. The combination of an overworked pump and reduced cooling and lubrication will likely damage the fuel pump. Indeed, one of the leading causes of fuel pump failure is running on a low fuel tank.

The above scenario is particularly critical on late model vehicles without a fuel pressure return system. Running such a vehicle out of fuel once can permanently damage the fuel pump.

2. Overheating due to sediment and blockages

Another reason given is due to the sediment which collects at the bottom of your fuel tank. On older vehicles, say pre- 1980 (unless its Russian- or Chinese-built), fuel tanks were made primarily of metal and tended to rust over time. These rust particles caused a variety of problems; clogging the fuel filter or the fuel pump, plugging the fuel lines as well as the fuel filter, and could damage the engine itself.

Modern car manufacturers now produce fuel tanks made of high-density plastic which negates the formation of rust. That doesn't mean that sediment can't still get in your fuel tank. It can, and does, usually from poor fuel. However, much like the dregs which collect at the bottom of a wine vat, this sediment usually sinks to the bottom of the tank. It tends to stay there until your car is scrounging for  every last bit of fuel it can find. When the fuel pump sucks up the last of your fuel, any debris in your tank will go along with it, and the particles can get stuck in your pump or in your fuel filter.

Once again, this sediment blocks the filter, which in turn makes the pump work much harder to try pull fuel through. This too will cause the pump to overheat and fail.

Figure 2: Driving with a little fuel is one of the leading causes of fuel pump failure. The overworked motor overheats and burns out.


Experts recommend that you should always have at least a 1/4tank of fuel in your vehicle to protect your fuel pump and fuel filter. When your car reaches the quarter tank mark - this is when you should put in more fuel. It will cost you the exact same, whether you fill up now at a ¼ tank or in a few kilometres at empty, but it could save you several thousand Rands in repairs!

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