Do you tend to avoid the service station until your vehicle’s fuel light has been on for ages and the engine is running on fumes? It’s a very bad idea to do this, since it can damage your vehicle quite badly, says Jake Venter.
I hate driving my wife’s car. She often fills up the petrol tank only after the red warning light has come on. I’ve warned her that when you drive with very little fuel in the tank, the petrol pump inhales some of the sediment that is always present in an older car. She obviously believes that it will not happen to her.
This practice is bad enough for the old-fashioned low pressure sucker pumps, but it just about kills the modern tank-mounted high pressure pusher pumps. In fact, experts warn that one should not drive with less than 20% of the capacity in the tank.
Mechanical pumps feeding carburettors
Feeding one or more carburettors is an easy task for a pump, and the pressure is not critical because the fuel goes into a float chamber, where it is under atmospheric pressure. As a result, the pump delivery pressure of 28 to 41 kPa (about 0,28 to 0,41 bar, or 4 to 6 psi) is able to vary by about 20% without the mixture being affected, except perhaps at maximum speed.
The pump has to suck the fuel from the tank, and most pump problems occur on the suction side of the unit. A leak or blockage on this side may not be easy to spot, but it will impair the pump’s action.
Just about the only thing that can happen on the pressure side is a leak, which can usually be seen with the naked eye.
Check for fuel delivery by loosening the fuel pipe union at the carburettor a turn or two while the pump is working. There’s no need to undo it completely. As soon as the fuel starts to squirt out under pressure you can close it again, hopefully without spraying fuel all over the engine.
If the fuel doesn’t squirt out, it may mean that the pressure is too low, or that there is very little, or no, delivery. This test should preferably be done with a cold engine, or with a cloth around the carburettor so that fuel doesn’t get a chance to ignite.
Should there be a lack of fuel at the carburettor, then the in-line filter between the tank and the carburettor should be examined. If the filter is transparent, one can see if fuel is flowing, but an opaque unit should be replaced if it has seen some mileage.
Fuel filters are not expensive, and often get blocked by an accumulation of dirt. In fact, it’s a good idea to always carry a spare filter.
Many cars have a coarser filter inside the fuel tank that can also get blocked, but they are often difficult to get at. The pump itself can also become very temperamental when dirt gets into the spring-loaded valves, or when there is a small tear in the diaphragm.
Total pump failure will result in the car refusing to start or cause the engine to die suddenly. A partial failure may only show up under heavy acceleration or at high speed.
Fuel pump repair kits are no longer available, but complete pumps are usually not too expensive, and replacement is easy.
About the only precaution you have to take is to make sure that the pump operating arm fits on the correct side of the camshaft. Some arms go under the camshaft and others go on top.
It’s also a good idea to prime the pump by hand if possible, before fitting it in position, otherwise the car might take a long time to start due to the pump sucking pure air for a while until the fuel arrives, and it’s not designed to do that.
A useful mechanic’s trick to employ if the engine refuses to start is to pour a teaspoon of petrol down the carburettor’s throat, after removing the air cleaner. This will cause the engine to start, thus forcing the pump to move fast and start supplying fuel.
Electric pumps feeding carburettors
Most of the remarks concerning mechanical pumps also apply to low pressure electric pumps. The best-known pump of this kind is the SU, which is fitted to many post-war British cars. This has a diaphragm energised by a solenoid. A set of points, rather like the points in a distributor, provides the make-and-break that controls the solenoid.
There can be few mechanical objects that have been cursed more often than these pumps. The points respond well to frequent cleaning, but if neglected the pump will stop. Often it just needs to be shocked into life. If you ever go to a vintage car meeting, you’re bound to see somebody attacking his SU pump with a rubber hammer!