hunts at idle, very up and down but sort of clears then comes back

Hi I came on here a few months ago about my new pluriel which had a running issue which turned out to be the cambelt had partially slipped. I've since replaced the cambelt and water pump, i also replaced the headgasket as after a closer inspection inside the bores I notieced one of the valves had glanced a piston so it had a new valve. It's now all back together and starts first turn of the key. The problem is that it hunts at idle, very up and down but sort of clears then comes back. I found if I unplug the map sensor it runs clean. I've tried another sensor of a good running car but still have the same issue. Any ideas on the problem?

If you disconnect the MAP sensor the engine ECU "falls back" into using it's maps (a pre-set matrix of rpm and load vs Fuel injection timing).
So I would assume it's something that the MAP sensor is reaction to but is fine if the air fuel ratio is fixed.
The only thing I can think of is maybe a vacuum leak? If the car runs fine at speed, small vacuum leaks will effect idle.
Check that the inlet manifold isn't loose. Check the purge valve operation and the hoses on the manifold side of the valve. Check the line to the brake booster.
Any other clues? Does it do it when the car is cold or all the time?

It's not back on the road as yet but does it on start up. I've replaced all the manifold gaskets when I did the head gasket. Will have a look at what you suffered and see if I find anything.

Mrtom1973 wrote: ↑Tue Sep 22, 2020 9:01 pm It's not back on the road as yet but does it on start up.

So It's fine when warmed up? Or is it every time you start the car (warm or cold)?

It's always on start up. I did try another map sensor as we have one on the road but no different and there's no engine management light on or fault codes.

Mrtom1973 wrote: ↑Wed Sep 23, 2020 6:17 am no engine management light on or fault codes.

What did you read the codes with?

It's an app I have on my phone a nd a Bluetooth obd connector.

How long has your car been off the road? When do you expect to get it back on the road?

There's no rush to get it on the road as of course I want it running properly. I got it off a guy who was selling it for his parents. It had a running problem and apparently the recovery guy said that it was maybe the temp sensor which they replaced and made no difference. I found when I got it home and had a look at the engine the cambelt had stripped 2 teeth so replaced the belt and water pump but still wasn't running correctly. I put a borescope in the cylinders and found one valve had slightly glanced a piston so took the head off put a new valve in and replapped all the rest.

If you have never had it on the road then you don't know how it ran before. Yes? The cam belt being out may have put the fuel trims out causing it to run erratically.
I propose that you might have to "get it on the road" to reset the fuel trims. Idling the car in the garage/drive will not reset the trims (if that is the problem).
Also you might not know the quality and age of the fuel in the tank. Have you changed the fuel?
My current C3 had 2 blocked fuel injectors and barely ran under its own power, however, that didn't stop the owner from driving it!!! The timing belt was out by 3 teeth but I'm not sure how that effected the performance. It had a smashed knock sensor and a whole raft of other small problems which may or may not have contributed to the poor performance. The blocked fuel injectors put the fuel trims completely out of whack and destroyed the catalytic converter. When I changed the fuel injectors and fixed the other stuff, it took a while for the engine to "settle down". It did not run perfectly straight after fixing the problems with the engine and did not idle or run well for the equivalent of a few tanks of fuel.
I have documented some of my adventures here;
the-citroen-c3-club-area-whats-great-ab ... t3521.html
My point is; if you cant find anything wrong with the engine, perhaps there isn't anything wrong?