An extremely common issue with the TD5 engine, we've lost count of the amount of these we have done to customer's and our own vehicles.
The issue effects both the Discovery and Defender models with this engine, basically the injector loom passes through the underside of the rocker cover of the TD5 engine. In normal operation this part works fine but as it deteriorates, the loom lets oil pass through it, into the wiring and eventually back to the red plug on the TD5 ecu (more common on the Discovery as the ecu is closer but we have had oil at the ECU end on both models)........in one extreme case we've had the underseat box on the Defender TD5 sitting an inch deep in engine oil!
The side effects of all this can present in various ways as it contaminates the ECU but normally it presents with poor running, misfires and lack of power.
We stock 2 different types of loom and we've used both with good results.
The cheaper loom is an "Allmakes" loom, it's actually a good quality loom but essentially classed as a "budget" loom.
The dearer loom is an OE loom and is covered by a 2 year warranty.
Whilst changing the injector loom it is advised to change the rocker gasket unless you know it has been changed recently.....click here for the rocker gasket product page.