Dinsen > Matra Murena > Electrics > Electrical fuel pump

Electrical fuel pump

An electrical fuel pump is an interesting alternative to the mechanical fuel pump for several reasons. This note discusses my experience with installing such a device.

An electrical fuel pump can be fitted to replace the mechanical fuel pump on the Murena 2.2 engine. This can help starting in cold weather, reduce fire risk as the fuel lines can be kept away from the exhaust manifold, and be used to cut off fuel supply as a theft protection.

Different pumps

Various fuel pumps are available, e.g. the uniqtous Facet pumps, Pierburg pumps, or the HÜCO electronic pumps. Basically, what you want in the Murena is a pump that can pump the fuel out of the tall fuel tank, pressurize the fuel supply lightly to the carburettor(s) properly, and supply the needed amount of fuel - even when the car is running full throttle long enough to empty the float chamber. These are some pictures of different fuel pump installations in Murena's:

Facet fuel pump in Murena
Facet fuel pump in Murena

Pierburg fuel pump in Murena
Pierburg fuel pump in Murena

My own Biltema fuel pump installation
My own Biltema fuel pump installation

Choosing the right pump

The mechanical fuel pump is specified to supply a static fuel pressure of up to 200 - 300 mbar. This fuel pressure is limited by a spring in the shaft that operates the membrane in the pump, and is measured without or with only very little any flow in the system - the pressure will drop below that value when fuel starts flowing. A return line in the system with a restricted opening ensures that excess flow can get away from the float valve, avoiding a pressure build up.

The supply pressure to the caburettor must be kept below these values.

Source: HÜCO www.hueco.com/html/ae_downloadsen.html

The diagram above shows the characteristics of a HÜCO suction pump. It shows the supplied fuel pressure as a function of the flow of fuel through the pump (in litres per hour), as well as the suction of fuel from the tank. The suction detemines how far up the pump can lift the fuel.

In the Murena, the pump will generally sit about 50-70 cm above the bottom of the fuel tank. This is how far the pump must lift the fuel in an almost empty tank. Petrol has a lift ratio of 1.3 m per 0.1 bar, so the suction must be 0.05 bar or more to empty the tank. Assuming that the feed pressure only needs to be marginally over 0, this pump can supply up to 110 litres/hour before starvation sets in on a Murena (note that this is an average value, not a peak - the float chamber alone contains enough fuel to run the car well through a short acceleration, and the pump only needs to be able to fill up the chamber again before the next period of fuel consumption sets in).

The similar pressure pump has the following diagram, and except that the suction it can provide is only up to 0.1 bar, the diagram below is actually only an extrapolation of the above. This shows that the difference between the two pumps is not that great, so they can be interchanged rather safely, as the Murena carburettor should be able to cope with up to 0.3 bar fuel pressure:

Source: HÜCO www.hueco.com/html/ae_downloadsen.html

The HÜCO pumps are electronically pressure regulated and seems to be good and modern choise of pump for a carburettor car. The pumps are diaphragm pumps driven by a solenoid in the A part of the pump in the drawing below. The diaphragm is in the B-part along with pressure and suctions valves.

As soon as the ignition switch is in ’on’-position, voltage is supplied and the plunger of solenoid moves upwards and, consequently, fuel is sucked into the valve housing. While the plunger moves upwards, it triggers a sensorcontact and the current flows through the coil of solenoid will be cut. A spring mounted beneath the diaphragm will now push the diaphragm in its starting position and the fuel is being pushed through the pressure valve. By reaching the initial position the plunger triggers the sensorcontact and the whole workcycle starts over again.

Some pumps available on the market are not pressure regulated, and particularly the cylindrical type Facet pump is known to supply a fuel pressure that is too high for the carburettor, and in this case a pressure regulator is needed.

A Filter King pressure regulator with glass bowl and built in fuel filter element. This device reduces the fuel pressure to a given value which is adjustable by the screw on top of the unit.

My installation

I have chosen to use a HÜCO type pressure pump from Biltema and fit this above the tank. The specifications of it matches those of the original pump and with a maximum suction of 0.1 bar and short fuel lines, it should not have problems coping with the fitment above the tank.

I have fitted an ordinary in-line filter in front of it. A problem with the Biltema pumps is that they are only supplied with 8 mm connections, and these don't match well with the 6 mm fuel lines of the Murena. These need to be converted, and one might think that this could affect fuel pressure in the system. It does not, however, which is because we have a static regulated pressure in the system - if the pressure was induced by the fuel flow it would have been problematic to change the inner diameter of the pipes.

Mechanical fitment is problematic, however, as there is very little space in the engine room. I have chosen to fit the pump in the hollow above the fuel tank under the left wing, but found that I had to construct a small aluminum bracket and bolt the pump on that. This displaces the pump backwards and upwards compared to the holes I made. Drilling holes in the chassis there was barely possible with a 90-degrees attachment to my electric hand drill, which is why I could not get holes drilled further back. It may be possible to drille better holes from the wheel arch if the suspension strut is removed, but as this is a rather difficult process, I have not considered that option. Note on the picture below that the bracket is bent about 5 degrees to angle the pump correctly.

Safety and pump cut-off

The mechanical pump has a definitive advantage over the electrical one on the safety side: It stops supplying fuel as soon as the engine stops. With an electrical pump connected directly to the ignition, fuel will continue to flow as long as the ignition is on. In case of an accident, this can be very dangerous.

Being an engineer, I decided to design a circuit to control the fuel pump safely myself.

These were my requirements:

  1. The fuel pump must run reliably while the engine is running.
  2. When engine stalls, the fuel pump should stop too.
  3. When the engine is cranked, the fuel pump should run.
  4. When ignition is turned on, the fuel pump should run for a few seconds regardless of whether the engine is running, to prime the system.
Design question 1 is to find out how we are going to detect if the engine has stalled? There are several solutions to this problem:
  • Install an oil-pressure switch to power the fuel pump through. As long as the engine runs, the oil pressure switch will be engaged and the pump will run.
  • Check the carge-light output from the generator. As long as it is off, the engine can be assumed to be running. When it turns on, the engine has stopped.
  • Detect ignition. As long as ignition pulses continue to come with small intervals, the engine must be running.
I have chosen the last option. The first (fitting an oil pressure switch) is ruled out as it will require that a proper place is found to fit one. The second is an all-electric modification and therefore simpler, but if the generator wears out, the fuel pump will stop. This is not ideal as I need a way to "get home" in case of a generator failure. So this leaves the ignition pulse sensing.

This is the circuit I have come up with:

fuel pump relay circuit

The circuit based on the information found in a web page by a guy named Scott Fraser who has reverse engineered a VW/Bosch fuel pump relay (link does not work any more but the schematics are available on think link (jpg picture).

Central in the circuit is the 555 timer, wired as a monostable. The circuit senses on the ignition coil and triggers on the ignition voltages. Note that it needs to be driven quite a lot above 12V to trigger which makes it immune to noise.

Compared to the original circuit, an additional CR-network (the 270K resistor and 47uF capacitor) is fitted to trigger the 555: This is to ensure that right after ignition is put on, the fuel pump runs for about 10 seconds. This primes the fuel system without the need for cranking the engine.

A relay drives the pump. The VW/Bosch relay has an integral fuse fitted, but in this design it is fitted externally.

No additional circuitry is necessary to run the fuel pump while cranking as the time constant of the 500K/4.5uF RC network on the 555 is about 2 seconds. This is enough to keep the pump running.

The "-ENABLE" input has to be connected to ground for the pump to run. This is driven by the immobilizer in my car.


Components with extended temperature range (-40 - 125 deg C) should be used. The power to the relay (and pump) should be fused properly.

Building the fuel pump relay on a piece of strip board
The still-not-complete relay circuit being built on a piece of strip board.

This photo shows my completed relay circuit ready to be fitted in the car and to replace the simple relay I have been using (without ignition sense). The box isn't sealed since it is fitted in the cabin.

Completed fuel pump relay in box

A crash sensor could optionally be fitted in the power line to the fuel pump or to the ignition wire. This will stop the fuel pump in the event of a crash.

Alternative solutions

  • One guy mentions that he has connected the pump directly to the ignition. This is simple and reliable.
  • Bernhard discusses the fitting of a Pierburg relay with the same features and my solution, except the start-run mode on this page.
  • Jean-Yves has connected his pump directly to the ignition line. His pump installation is described on this page
  • Roy Gillard suggests using an in-tank pump from a TR8 www.rimmerbros.co.uk/rimmer/rover/v8engine/pumps#Fuel%20Pump.
  • Finallym, there's the option of using the VW fuel pump relay p/n 321 906 059C as described here.

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Page updated: 2012-01-11 11:24:43