No response yet on the Defender Forum, so opening to a more specialist audience.

My son's D90 TD5 has just developed a very lumpy tickover. Any attempt to drive it more than 10 yards and very lumpy graduates to really, really unpleasant and a slow limp back. It starts first time and sometimes seems fine until you try to rev the engine at which point it goes back to a lumpy idle.

Their are no DTCs. There is no oil in the ECU red plug, and changing the injector loom this afternoon made no difference. As an aside the outgoing loom was a Bearmach one fitted 3 years ago and the plug under the rocker cover was dry as a bone. I might clean it and keep as a spare.

When it starts and runs fine the idle is solid at 745-750 and the roughness on all 5 injectors is no more than +/- 2. As soon as it goes lumpy the idle is a bit less stable and individual injectors can be up to +/- 20.

I have not yet checked the fuel pressure, but the fact that the engine starts means that the fuel pump is still delivering 4 bar at idle. However, it can no longer be heard. It cannot be heard before start even if running the auto priming system. The MIL flashes correctly, but no whine at all from the pump - nor any sound of air in the fuel.

The obvious conclusion is that the fuel pump is barely coping and needs to be replaced. Does the collective agree, or is there another possibility I have not thought of?

if you remove filler cap you should be able to hear the pump,

I'll give that a go, thanks. But is it normal for a fully functioning pump to suddenly become much quieter? It seems a big coincidence that this happened at the same time the engine became unusable.

its not uncommon for relay/fuse to fail

We have checked the fuse and the relay still clicks. With the engine running, pulling the relay immediately stops the engine, so the pump must be running well enough to deliver the minimum pressure to permit start and idle.

I’d be looking at the fuel pump for sure. Check it with a multimeter. If I were to buy a td5 again now, I’d actually change what you’ve changed and the pump as preventative maintenance! Have you checked the regulator isn’t leaking?

Measure mV across the pump's fuse as it is while the engine is running, the same principle like for the D2 addapted to Defender and to 20A fuse instead of 30A https://www.landyzone.co.uk/land-rover/check-td5-fuel-pump-hp.358086/ so below 10mV the pump is fubar

The problem appears to be electrical.
Focused on the suddenly rough running engine, until this morning neither my son nor I noticed that the nearside rear lights are now not working. I assumed this was an unrelated common earth issue, and sure enough when providing a known earth to the brake/tail light and with the brakes and side lights on, I get a solid 12V on either live terminal at the light connector. With everything turned on (left indicator and reverse light) when I start moving the wiring to trace a common earth, all the lights come on correctly........and so does the fuel pump. There is a normal pump whine that comes and goes with the lights as the poor connection is moved.
Now I am confused. Because with the lights not working and the pump quiet, the engine will start but runs very poorly. With the lights working and the pump giving a quick whine, the engine starts and purrs (OK, TD5 so maybe not purring, but runs smoothly). It is a two stage pump, but there is only one power feed and both stages are needed to provide the 4 bar required to start. So either the pump is powered or it is not. How do you get a situation where it is part-powered and delivering only minimal flow?
The connector with the dodgy connection is the one that connects the number plate light. What we found was the male half of a translucent connector with two bare female terminals pugged into it and secured with insulating tape. It was hot to the touch and there was visible arcing that coincided with the lights working. This is not the first time we have found some pretty impressive electrical bodging in this LR, but how a loose connection to the rear number plate light can affect a critical item like the fuel pump is a new level of BS.
Time for a complete strip down and assessment before the whole thing goes up in flames.
Thanks for the help everyone!

Si Click said:

So either the pump is powered or it is not. How do you get a situation where it is part-powered and delivering only minimal flow?

Click to expand...

If due to a bad contact or earth the voltage drops much bekow 12V cos the pump runs even with 6-7V with lower rpm, that's how you can control the rpm of a DC motor by modifying the voltage. The LH brake, nr plate and tail lamps are sharing the same earth point with the pump, unfortunately there's a missmatch in the diagrams cos in some it's C0558 in others is C0556 eventually you'll have to follow the black wire from the pump untill the earth point, for me C0556 seems the correct one but not 100% sure