I thought I would add this post to share info with anyone else with the same problem.
My 1997 2.5 DSE Range Rover P38 failed to start one morning. The engine turned over well, some smoke and unburned fuel could be seen/smelled at the exhaust. It seemed that the glow-plugs were not heating. A local garage started the car using brake cleaner at the air inlet manifold and later checked the glow-plugs. Two of six plugs were not heating up correctly and all six were changed.
However, the car failed to start the next day, exhibiting the same symptoms. Since the plugs had been changed, I investigated the relay attched to the ECU. With the ECU & glow plug relay withdrawn from the protective box, 12V could be detected at the glow plug output from the relay and the engine was successfully started. No connectors had been broken down. Just the ECU and relay had been moved ! Presumably at this stage, a wiring fault or loose connector.
Several days went by with the car successfully starting, until again, the same symptoms occurred. Engine turns, smoke from the exhaust, but the engine does not 'fire up'. Each time this has happened, 12V cannot be found at the glow plug output from the relay when there should be. The ECU and relay are then withdrawn, laid on their side, and then 12V is present and the engine starts.
Two attempts to clean and "close-up" the connectors have not been successful in curing the fault. The diagnosis is not helped by the complexity of the four relay control wires (I'm not sure how the relay is switched and monitored). Also not helping is the glow plug symbol on the dash which is just a 'dumb' timer - it does not relate to the power actually going to the plugs. Furthermore, the fault cannot be confirmed on a warm engine. At least half a day is required for the engine to cool down enough to require glow-plug heat.
The glow-plug relay has now been replaced and the car successfully started two days running from cold without needing to disturb the ECU/relay and wiring.
It failed to start again this morning. Arrrgh.
Any advice/suggestions gratefully received.