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.

Note that this is of utmost importance! The use of leaded fuel in a catalyst-equipped car will inevitably (and immediately) lead to very serious damage of the converter!

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.