Citroen XM Forum

Would suggest you go backtrack a little, and use contact cleaner and a good few unplugs/replugs on the connectors you worked on.
The trouble with faults that inexplicably disappear is that one day they could just reappear.
If reliability is important, then faults ought to be identified where this is possible.

Yes, that will be done soon when the ignition is modified with "the relay" mod.

Faulty wires rarely heal themselves, so I suspect this is down to an active component being stuck and not passing sufficient current.
There are four active components, Ign switch, starter motor, Start inhibit relay, selector switch.
Ign switch...changed.
Starter motor...proven to work.
Start inhibit relay...believed changed. (might be worth checking it was the correct one)
Selector switch...possibly stuck due to lack of use, now freed off due to heat and vibration.

It would be advisable to check the contact onto the starter motor and the relay Gnd are both sound.

Afaik, it wasn't an issue the previous owner had, so lack of use may well be the reason and it could well now be resolved.

Even though it now works, you could spend a few minutes performing the "test lamp and jumper wire" diagnostics, to gain familiarity.
If you know what it does when it works, you will gain confidence and more readily spot the failure when it doesn't.

On another note, you appear to be cracking on nicely with the old bus.

Battery terminal clamps can limit current draw.
If swapping batteries failure to clean contacts and tighten properly can allow volts checks but not enough amps to run the starter.
Citroen soft metal battery clamps are a particular problem as they tend to split with age and use.
Using a loose battery and Jump leads can give the same volts but next to no amps problem.
It is only the early Mk1 Simplex ignition switches that have very small switch contacts and thinner supply cables.
Switch contact area was doubled in size with thicker cables from the Mk1 facelift Simplex ignition switches.

Moving on now.

I think I have a fuel problem, she runs fine, revs well.....then starts to miss fire and knock....then will settle down on tick over but miss fire/knock if I then try and rev it again.....

(James (mate up the hill Mech by trade, diesel in the blood) thinks the above, I got him to check as I was worried about the diesel knock being something metal....he said deffo knocking)

Just ordered a new fuel filter, will dig into the tank as well....no bubbles in supply/return pipes

That sounds either like air being drawn in under the increased suction, or the timing being advanced.
I assume you did double check the belt timing after winding the engine at least two turns.

Check the fuel supply pipes for restriction and possible air leaks.
Check the fuel pickup in the tank isn't blocked and still has the the spacer pips, to lift it off the base of the tank. They have been known to wear right down.
I have seen that type of filter unit leak at the joint.

If you don't have any air on the return then air isn't the issue.
Try running the pump directly from a can to eliminate the fuel supply.

There is a possibility of a foreign object in a swirl chamber. The tip of a glow plug, for example.
That would cause a misfire.
Does it appear to be a single cylinder, or all four?

If it was a damaged valve the problem would be constant and you would hear and see it firing into the intake, or exhaust.

Cheers.....did not know about "spacers" on the tank filter.

I really does "feel" like fuel starvation.

Yes I checked the pump timing after 5 turns, all was as should be.

You might be able to move the bottom of the tank away from the fuel pickup a little either by clapping on a suction (glass-type) grip, or by adding a good bit of fuel to the tank.
Check also there is no vacuum building up in the tank (i.e. breather is working), by just leaving the filler cap ajar.

The fuel pickup is spring loaded to keep it tight to the base of the tank, but it is possible the tank is simply low on fuel, or there is a pinhole allowing air into the suction pipe above the pickup.

Other favourite air ingress locations are the fuel pipes on the bulkhead at the back of the timing belt cover...Awkward to access and may have been disturbed during the works.

It seems odd that this car was running perfectly prior to the timing belt snapping, but now has starter motor and fuelling issues.
Both systems are electric, so maybe a supply or Gnd issue is present...check Gnd points and main supply cables at the battery clamps.

The fuel pump is mechanical, but fuel delivery is controlled by the spill valve, which has to close and open for every injection pulse. Intermittent power failure would cause havoc with the supply of fuel.

Connect a can of fuel to both supply and return, if the problem persists wiggle the loom and main supply cables while the engine is running.

Will do.....thanks for the extra tips chaps.

Filter change and checking the tank pick-up today.

Cleaning up the front bumper trim in the spare time I have to stay in the house.