I'm working off a Peugeot XUD engine with a Lucas pump. I think it's the same pump fitted to the 100Td.
The fast idle is controlled slightly different on my engine.
if I pull on the lever it increases revs. It however (on my engine anyway) is not needed unless quite cold.
I suspect that the glow plug circuit is at fault. Initially I was thinking that you had air in the system but Stylers has already advised on diagnosing and solving this issue so I will offer another possibility which is following on from where I think Jug was heading.
When you turn on the ignition and illuminate the glow plug light, the light remains lit for say 5-10 seconds. The glowplugs however remain heating for another 15-20 seconds though. Even though the light has gone off. It is a good practice on cold starts to wait a few seconds after the light has gone off before attempting to start. This will allow better cylinder heating which will allow for easier combustion. (If you listen once you've turned on the ignition powering the glow plugs and once the light has gone out about 15seconds later you can usually hear the glowplug relay click indicating them shutting back down)
Now. On colder starts even after the glow plugs have powered down after their full 30 odd second duration and the engine is running.
The glowplugs remain powered up. Not at full heat but about 50% they remain like this for 5 or so mins.
This is to help aid the engine to get up to heat.
To test that this second cycle is occuring use a volt meter or a cheap screwdriver with a light inside and an earth wire from it. Start the engine normally. wait a few seconds then place the earth on the battery negative and the positive on the glowplug wire. (you will see it goes from one plug to the next and is attached by 8mm bolts) By placing the screwdriver tip on any exposed part of this circuit it will tell you if the plugs are recieving power.
try again a bit later and you should no longer recieve power.
