Fuel pump issues: Why does my car have two fuel pumps? Is my fuel pump bad?

One symptom of a bad pump, broken hose, and/or clogged screen is engine misses at high power under severe cornering at progressively higher fuel quantities. A good system will run on a track all the way to 'empty' without problem. If the problem is not tied to cornering or gas level, there’s a possibility of simply a clogged fuel filter. The fuel filter is located in the engine compartment near the air filter; the pump is bolted to the rear fender behind the right rear wheel. Later cars have an additional fuel pump inside the tank.

This was part of the continuous improvement to correct fuel supply inadequacies. The in-tank pump is low pressure and simply provides a pressure 'head' to the high pressure pump. This is common practice, for example, in aircraft turbine engines that use high pressure (HP) pumps which rely on the fuel for lubrication of the HP pump. Exactly the case in the 924.

Before replacing the fuel pump, however, consider replacing the fuel pump relay and fuse. These are much less expensive, and are also subject to deterioration too. Just check to make sure you can feel the relay “click” as it closes the contacts when the engine is switched to start. You can also use a voltmeter at the fuel pump contacts to check for power back there – a break in the wiring is possible, though unlikely.

Anyone with in-tank pumps should take the time to pull the unit for inspection. The pump is connected to the fitting by a short section of 'hose'. After all these years you may find the hose is cracked from vibration. If so the system is essentially gravity feed to the HP pump intake. A separated hose leaves the pump connected to the tank fitting by only the 2 electrical wires, so remove the unit carefully.

On the inlet side of the submerged pump is a fine mesh screen. It also, has been in there for some 20 years and WILL be at least partially clogged. 'Slosh' it in clean fuel and use a SOFT toothbrush to dislodge the crud. You do not want to force crud into the screen where it will eventually make its way to the expensive main HP pump.

With the pump assembly DRY of GASOLINE!! Hook it up to a 12 volt battery. Use wires long enough that any spark will be removed from the pump! If it spins (makes noise is the technical term) then it works. For more detailed testing of the fuel pump, to check on how well the pump is performing or how likely it is do fail, see the Technical Section about the Fuel Delivery System.

What kills the pump(s) is bad quality gas, crud in the gas tank, or repeatedly running the tank dry/nearly dry. These pumps rely on the gas to cool the motor, so they can quickly overheat and seize if run dry. The pump element is a roller pump design. The original Bosch pumps used and still use metal roller elements. Consult the Haynes manual for a diagram of what this looks like. What it means to you is that there are very close tolerances in the pump element that do not tolerate dirt. If they get dirty, they will jam. Similarly, if they sit with watery fuel, the water will corrode the pump elements and cause them to stick. Pierburg, a subsidiary of Hella, makes a better pump - they have plastic roller elements which can tolerate the dirt, and won't corrode in water. This also supports the recommendation that you drive your car regularly! If you keep passing fuel through the system, water in your fuel will not have time to settle and cause corrosion, and dirt build-up will be kept to a minimum. See the Technical Section on Fuel Pumps for more info and part numbers.

If you have an early '77 924, and are shocked by the cost of the replacement fuel pump!!! Read the Technical Section on Fuel Pumps for information about how to update to a later-style pump for a fraction of the cost of the old style.