pug 306 2.0 hdi?

If the injector is leaking from its pintle (the nozzle) into the cylinder then there could be a mechanical fault with the injector itself or it could be getting a permanent ground on the ECU injector control circuit for that particular injector. The injector will have 2 wires on it's connector, one will be 12v live with ignition on, the other is a pulsed "earth" controlled by the ecu, this is how the ecu can vary injector opening time.
There are 2 ways to verify if your injector is open due to either an electrical or a mechanical fault.
1: remove the 12v feed, if the injector stops leaking into the cylinder then it was being opened electrically.Some possible causes are short circuit to ground on control circuit or ECU fault (internal short)
2: swap the injector to another cylinder, if the fault moves with it, the injector is at fault.
If there is an excessive leak then this can lower rail pressure to below that required for starting, diagnostic equipment will be able to monitor "live" rail pressure from the pressure sensor.
How do you know that the injector is leaking into the cylinder? Bear in mind that a mechanical engine fault (low compression etc.) could result in unburnt fuel remaining in the cylinder.

Gary Sampson
March 2007