1.9TD 'revving' at idle

[mrduke]

I did a quick search, but apparently not a common problem!

The last couple of times I've cold-started the Xantia, the engine revs have been rising and falling by around 100-150 rpm at idle - as if I was gently pushing the throttle every second. It happens less when the engine warms up, but still fluctuates enough for the rev needle to move up and down at idle.
I believe the idle control valves are prone to failure, but I would've thought that would just give a constant idle at the wrong speed, rather than a varying one.
It doesn't cause any problems when driving, but I'm just wondering if something impotant is about to fail?!

[Xaccers]

Engine hunting is normally fuel flow related.
So when did you last change the fuel filter?
Have you popped out the fuel sender unit under the rear seat (beneath a round rubber disc) and cleaned the mesh filter on that?
Is air getting into the fuel system? At this age they're prone to leakes so replace as much as you can from the primer bulb to the fuel pump with clear pipes so you can see any bubbles, also replace your leak off pipes including the blank and the one back to the pump.
Can you blow down the fuel return pipe from the pump to the tank?
Are your glow plugs good?

[mrduke]

I changed the fuel filter last week although I haven't checked the sender unit, or the return pipe - I'll have a look when the rain stops. I suspect the glow plugs will need changing soon, as the firing is a little rough on a cold start.
Thanks for the pointers!

[spider]

I'd agree a varying idle is quite likely fuel related, well air related.

New fuel filter a sensible idea always if not sure when last changed. Typically with an XUD/T unit I say every third or fourth oil change pop a new filter in.

Leak off pipes yes, only really seems to afflict Bosch pumps due to their return path I expect although most are Bosch.

If you have a clear piece of fuel pipe between filter and pump examine it for bubbles.

Primer bulb itself can be a source of this

The seal under the fuel filter housing on the thermostat can fail (does not seem that common though) and the heater waxstat (not the fast idle cable fitting if you have one) can take air in too, this is the white "screw" in the side of the fuel filter, its seals harden over time. Some fill with sealant. I filled mine out of course before it decided to fail as sealant won't stick to fuel.