LEAD REPLACEMENT PETROL (LRP)

Due to global environmental initiatives the removal of lead from components such as fuels has been mandated, with leaded petrol to be phased out by 2002. LRP is a substitute lead free fuel that has been produced to replace leaded petrol or 'super' in our vehicles. LRP has the same octane rating as leaded petrol and is suitable for all vehicles that previously ran on leaded fuel.

Lead was originally added to fuel for two reasons. Firstly, to raise the octane level, and secondly to help protect against valve seat recession (VSR). VSR is where the heat attained by the exhaust valves is high enough to cause them to stick to the valve seat which over time wears away the seat and results in damage and/or loss of compression. Lead overcame this by adding a lubricating layer between the exhaust valve and valve seat. LRP acts in a similar way but uses special additives to help lubricate the valves and protect against VSR. The additive commonly used is MMT which is a combination of phosphorous and manganese, while some companies use a sodium based additive.

Irrespective of which fuel is being used, during combustion deposits can accumulate on the firing end of a spark plug. To help overcome this, spark plugs are designed with a self cleaning ability, whereby they are able to burn these deposits away once they reach a temperature of about 500oC. The additional advantage of using leaded fuel was it aided the spark plugs cleaning ability by helping burn off deposits whilst the engine (and plugs) warmed up. LRP additives are unable to do this.

A further difference is LRP is less volatile (ie. more difficult to vaporise) than leaded fuel which at low temperatures can cause problems with cold starting. This is best overcome by a well maintained fuel system. Chokes where applicable, should be monitored and working efficiently and idle settings may need to be increased to help the engine through this stage when cold.

Depending on the additive used in the LRP, you will notice the spark plugs firing end may have changed in colour from a light gray to a reddish-orange. Advice from the fuel companies is that these deposits do not affect the spark plug's performance and do not point to any engine problems. You should expect this reddish-orange colour under normal usage in the same way you would expect a light ash colour when using leaded fuel. Fig 1 shows a used plug with deposits resulting from normal operation with leaded fuel, and Fig 2 shows how a plug should look under normal operating condition with LRP. :

Fig 1.

Fig 2.

The use of LRP is causing concern amongst some users who are complaining of misfire and poor engine performance. Any problems with LRP can more often than not be traced back to how efficiently the whole fuel system is performing and how accurately the air fuel ratio is set. If the engine is running rich and/or the self cleaning temperature is not reached quickly then fouling may become a problem. Therefore, it is important not let the vehicle idle for too long after a cold start and to ensure the vehicle is driven for a period of time that allows the self cleaning temperature to be reached.

Some users are reporting that increasing the heat range of the spark plug (i.e. going one range hotter) has improved vehicle performance. NGK only recommends the spark plug specified by the vehicle manufacturer. Differences can exist in fuels between oil companies and even between batches from the same company. In addition, leaded engines of the same make and model can vary greatly in performance given the length of time these engines have been driven on our roads. Given these differences, NGK cannot make any general statements and deviate from the component tested and proven by the vehicle manufacturer. Doing this may affect performance or in the worst case, cause engine damage. It is also important to note that changing a spark plug to fix a problem is only a band aid approach and acts only to (at best) reduce the symptoms while the underlying problem remains unattended. It is therefore important that particular attention is paid to the engine condition, including the fuel system. Following this, if excessive reddish-orange deposits are experienced or problems persist, you may need to consult the fuel company for further advice.