Fuel in general

Unleaded fuel

As a general rule, older engines (up to 1987 approximately) are not explicitly designed to run on unleaded fuel. Later engines are capable if the necessary adjustments are made. A few engine models are capable of running on both kinds of fuel without any adjustment. And finally, every car equipped with a catalytic converter, either in the factory or with an aftermarket product, must be run on unleaded fuel only. Lead is poison to the catalytic converter and the oxygen sensor, meaning that even one tankful of leaded fuel can cause serious damage to these delicate and painfully expensive parts.

If your car is equipped with a carburettor, you might not have a catalytic converter (although in some countries, Germany and the Netherlands for instance, late carburettor BXs were fitted with a converter). Fuel injected models, especially newer ones generally have a converter. But in any way, you can check for yourself. Raise the car to the highest position (or stand over an inspection pit, if you have one available) and look beneath it. The exhaust pipe starts from the rear of the engine and goes to the back of the car. The position of the converter would be somewhere beneath the front seats and looks like a cylindrical swelling on the pipe. Then the pipe goes on to two big items located around the spare wheel, the first one (perpendicular) is the expansion chamber, the second one (at the end) is the silencer. The oxygen sensor, if fitted, sits on the pipe before the converter.

You have another option. All cars originally equipped with converters have a tank filler pipe inlet opening of smaller diameter than usual. The filler pistols at the gas station are similarly sized so that you cannot squeeze the leaded pistol into the car by mistake. Thus, if you have the smaller inlet, this is a clear indication that you have to use unleaded fuel. There should be a sticker inside the filler door stating UNLEADED ONLY and the instruction manual of the car should draw your attention to the exclusive use of unleaded fuel, too. However, stickers might be missing (in this case, it is a good idea to buy another sticker, these are sold at gas stations or parts stores for a negligible price), instruction manuals can get lost but the filler pipe hopefully remains in place.

However, if you have the bigger size pipe, you cannot tell for sure. Depending on the engine, it might run on both leaded and unleaded fuel. The original and genuine Citroën specification is as follows (for all models, not only BXs, registered in the European market; the series number can be found at the beginning of the chassis number.) The words yes, no and only speak for themselves, a degree specified means that the engine should only be run on unleaded if the ignition timing advance is set to the value specified.

Brana Radovanović

You will not find leaded fuel anywhere in the European Union after the first day of the year 2000. If you have one of the few models requiring leaded fuel, you will need to use unleaded fuel and add a lead substitute fuel additive. Consult the manufacturer of the additive and your Citroën dealer to find out which one you can use.

Trouble with fuel level meter

Although theoretically this might be a failure of either the meter or the level transmitter, the second one is much more likely and, first of all, much easier to check (to get at the meter you would need to remove and disassemble the instrument panel).

Fold the rear seat forwards, the transmitter is there under the insulator mat, covered by a circular plastic cover. Disconnect the wiring, unscrew the big black plastic nut. Gasoline is highly flammable, take the necessary precautions before you start! Lift the transmitter from the tank, let the fuel flow out from the sensor (the hole is rather small, this might take several minutes especially if the tank is full) and let it dry. Cover the hole in the tank to avoid the ingress of dirt and the danger of fire.

With the transmitter unit in your hands, shake and turn it upside down to hear if the float is sliding up and down freely. Reconnect it to the wiring, hold it horizontally and by gently tapping it with your finger, try to send the float to the other end. The meter reading should be practically proportional to the movement of the float. The warning light should come one near the end. This test reveals if the sensor is completely broken.

To find out more, drive until the warning light is coming on and the meter shows only a little amount left. Observe the meter reading carefully. Remove the sensor and submerge it into a bowl of fuel (highly flammable vapors, again, be careful!) until you get the same readout on the meter. Note the level on the sensor. Repeat this procedure when you have replenished the tank (noting the amount you could put in). Now you have two marks on the sensor and you also know the liter values they correspond to: it's quite simple to measure how the gauge answers anywhere between this extremes. Don't expect a linear relation between the amount of fuel and the meter reading, though. It is supposed to cheat by showing less fuel at the end than actually is in the tank, to help you avoid running out of fuel.

By the way, it is very important to replenish the fuel tank well before it is actually empty. It is advisable to do so as soon as the warning lamp comes on. The fuel pump is both cooled and lubricated by the fuel flowing through it. If the car becomes too dry, the fuel pump has to operate under more stress and this will shorten its life.

The transmitter is submerged in fuel forever and its float constantly scrapes the surface of the resistor track formed on the printed circuit board. After several years of use, the glue between the printing and the board wears out. The contacts for the warning light are independent from the level resistor track and they are bigger, hence the glue holds them much stronger; it is common for the level gauge to fail while the warning lamp still functions correctly. If the transmitter wears out totally, the only solution is to renew it.

Fuel filter

Citroën specifies the following filters: