THE PRESSURE-SENSING GLOW PLUG

▶ While diesel glow plugs have had their roles expanded beyond facilitating cold starts, pressure-sensing types see them feed information back to the engine ECU. The system was introduced first by Borgwarner in 2008, through its BERU brand in North America, fitted to the Volkswagen Jetta. Yet, it was not long before European examples caught up. By 2013, Borgwarner had produced three million pressure-sensing glow plugs. In 2017, that figure grew to ten million.

You will tend to find these sensors fitted to post-2010 (ie. Euro V and VI specification) Volkswagen Group cars, BMW/MINIS, Renaults and General Motors-era Vauxhall products. Considering the popularity of these vehicles on UK roads, the chance of you encountering engines with pressure-sensoring glow plugs is fairly high.

Each glow plug contains a piezo-resistive pressure sensor meaning that its electrical resistance alters with pressure. It is used to evaluate the combustion pressures within the cylinder. This data is converted to an electrical signal, using circuitry situated within the plug itself, which is fed back to the engine ECU. After then, the fuel trim and timing are adjusted accordingly. Not only does this improve engine efficiency and performance but it also reduces exhaust emissions, especially particulates. It also permits small-capacity engines to experience periodic higher peak in-cylinder pressures, without pushing them beyond their limits.

Glow plugs are also becoming more engine specific, because they must reach a specific temperature within a very small time window at set voltages. Plugs that are not designed to operate beyond a certain timeframe can be damaged by extended use. Therefore, efficient engine operation means that replacement glow plugs must either meet, or exceed, the original plugs’ specifications.

We recommend strongly that you buy from quality brands, through recognised outlets. Such companies will have catalogues that will match your car with the necessary replacement glow plug specification.

▶ As the pressure-sensing glow plug relays information back to the ECU, a decent diagnostic device should permit you to retrieve the data. The other cylinders record a zero pressure reading, because they lack the intelligent glow plug. As this data is received by the ECU with that incoming from other sensors, you may find that a pressure-sensing glow plug fault is interpreted by the ECU (incorrectly) as an issue with the fuel pump.

Most cars do not have a pressure-sensing glow plug fitted to every cylinder. This Skoda possesses a sole pressure-sensing unit on cylinder 3, which can be identified easily, by the sensor wires that emanate from it.