How To Trace A Fuel Line Fault On A Classic Car

How To Trace A Fuel Line Fault On A Classic Car

This guide captures ‘bygone’ advice on how to How To Trace A Fuel Line Fault on a Classic Car

To see all my ‘bygone’ advice guides please visit Classic Car Maintenance DIY Guides.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.

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An engine that coughs and splutters, peters out regularly and lacks power probably has a fault in its fuel line.

Beginning at the filler cap, the fuel line includes the fuel tank, pump, carburettor and lengths of connecting pipe, wirh the possible addition of a line filter.

How To Trace Fuel Line Faults
A typical fuel line on a petrol engine car.

When dealing with a suspected fuel line fault it is best to work methodically through the system from tank to carburettor. This means that you reach first the faults that are most common and simpler to rectify.

Possible Causes

  • No Petrol In Tank
  • Partial Vacuum In Tank
  • Fuel Contaminated
  • Fuel Pipe Blocked
  • Fuel Pump Faulty
  • Carburettor Blocked

Advice in addressing all the above can be found below.

At all stages of work on the fuel line remember that petrol is flammable and keep lighted cigarettes or naked flames well away. If petrol is spilt on hands or clothing these should be washed. Even if they appear to have dried the danger may still be present.

Fuel Tank Checks On A Classic Car

The first check – so obvious that it is often overlooked – is to check that there is fuel in the tank.

The next is to see that it can get out again.

As fuel is used up the space left has to be filled. If it were not a vacuum would begin to form in the tank and eventually the fuel pump would be unable to suck fuel out. In an extreme case the end result could be the collapse of a fuel pipe or even the tank.

In older cars this problem is overcome by the fitting of a breather to the tank – usually a filler cap which does nor make an air-tight seal.

The most common type of filler cap is the push-on-and twist variety, which has a valve that allows air to be drawn into the tank. If your car has a cap of this type check it for blockages and clean out any dirt by blowing through the valve and swilling it in petrol.

Many later cars have sealed tanks which offer a greater deal of safety if the car is involved in an accident and eliminates petrol fume emission. Some sealed systems have plastic tubes paralleled to the filler pipe to the tank. The tubes are filled with air and as fuel is used they compensate by collapsing progressively.

But if the tank is overfilled the plastic tubes can become filled with petrol. In this case the vehicle will travel for only a few miles before it stops.

If you suspect that an over-filled tank is the cause of the problem remove the filler cap and run the car for a couple of miles, with the filler pipe plugged with a rag, until enough petrol is used to clear the tubes. As soon as this has been done the rag should be removed and the filler cap replaced.

How To Investigate Fuel Contamination

If somehow the fuel has been contaminated – for ex­ample water added to the petrol – the whole system will have to be dismantled and flushed through with fresh fuel.

If you suspect contamination the easiest place to check is at the carburettor. Remove the top of the float chamber dip a rag in the fuel and withdraw it.

Because the petrol and water should remain separate, you will be able to tell whether both are present by looking and sniffing or by setting fire to the rag-one bit will burn, one won’t. (Not sure this is great bygone advice !). I would suggest syringe it out taking the fuel from the bottom of the chamber first and place it in a clear glass container to see if you can observe separation of fuel and water.

Cleaning a contaminated tank usually means wasting a gallon or two of petrol. First remove the drain plug or pipe at the base of the tank. This will clear all the fuel (and contaminant) except the small amount below drain­plug level. To get rid of this you will need to keep re­placing the plug, pouring in a quantity of petrol, and removing the plug again until what emerges is reasonably clean.

If very weak suction or pressure is felt, or none at all, the pump is faulty.

If the cause is a blockage clear it by blowing air through the inlet. If retesting shows that this cure has worked carefully replace or overhaul the pump by the methods detailed later.

How To Investigate A Blocked Fuel Pipe

Fuel pumps are either mechanically or electrically operated. Mechanical pumps are the most common and are the easier to service. Electric pumps, however, seldom go wrong.

Mechanical pumps are always located on the engine block and thus are quite easy to find.

Mechanical Fuel Pump
This SU mechanical pump is an example of a pump which cannot be dismantled. If faulty a pump of this type must be replaced.

Electric pumps are more elusive. Usually they are located near, or even recessed into, the fuel tank. If one cannot be found and its location is not given in the owner’s handbook, trace a fuel line either forward from the tank or back from the carburenor until you find it. The pump will probably be black, and cylindrical in shape.
Once the pump has been found check the inlet pipe for blockages or leaks.

Electrical Fuel Pump
Electrical Fuel Pump

If a pipe is blocked it will have to be removed from both the fuel tank and the pump. This is best done after the tank has been drained as in most cases the tank outlet is difficult to plug quickly once the pipe has been removed. On cars with reserve tanks, if the tap is left on main supply, fuel only needs to drained until the reserve is reached.

A blocked pipe can often be cleared with a blast of air from a foot pump or bicycle pump.

A leaking pipe will let the pump suck air rather than fuel, resulting in airlocks passing along the system. So check for holes in the pipes or loose unions.

If this fails to find the blockage, the pump itself should be examined.

Fuel Vapourisation

Another-rare-cause of air locks is fuel vaporization in the line. This is caused by high engine temperatures or driving at altitude, and most often by a combination of both.

Apart from allowing a hot engine to cool, not much can be done about this fault. It is seldom associated with electric fuel pumps, as they are usually located near the fuel tank and pressurize the whole line.

Investigating Fuel Pumps Problems

If you have reached this stage and its time to investigate the fuel pump. mechanical or electrical then you best advised to visit the following links where these subjects are covered in detail.

Checking For Fuel At The Carburettor

Once the fuel pump is working, the final checks should be made at the carburettor.

The first check is that fuel is reaching the carburettor. This can be done by removing the inlet pipe and holding it over a suitable receptacle while you operate the fuel pump. In the case of an electric pump this is done simply by switching on the ignition. Mechanical pumps can be operated by the hand priming lever if one is fitted if there is no lever, have someone turn the engine over a few times for you.

If the pipe is blocked it should be removed and blown through.

If all the tests above show that fuel is reaching the carburettor then the trouble must be inside the car­burettor itself.

You will need to dismantle and clean it

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