Get some form if scan tool hooked up to the car. Even Torque on your phone with a decent Bluetooth adaptor should be able to give you live info on lambda and boost pressure etc. You’re trying to find a fairly minor sounding driveability fault, so something is probably going slightly wrong intermittently. A decent scan tool can monitor the engine’s sensors and give you a good steer as to what’s going on. A fault code reader will only pull up problems that are enough to properly upset the car.

And stop changing bits at random and thinking the ECU might have corrupted. It’s French, but even then, computers are pretty reliable things. You’re far more likely to find a simple mechanical fault like a boost leak of dodgy filter than you are to find that your ECU has gone mad. And if your timing belt was flapping about, it would land on the road pretty sharpish I reckon.

Just had a quick scan of the thread.

My first guess given the symptoms would be EGR valve, however since blanking that didn’t change a thing, that’s ruled out.

However without access to live data, everything else is pure guess work. General things I’d be checking would be boost pressure (i.e. is the wastegate actuator/control solenoid working), and a general check of the fuel system (i.e. injector balance and fuel pressure).

My gut feeling would actually be a fuel system issue, given the specific RPM range it happens. IIRC the injectors on a diesel of that era reduce the number of injection phases at 1500RPM. An injector balance check via live data, or a leak off test may show up something, but not always.
There is also the classic check of the fuel filter housing for any metal bits lying in the bottom, which is a potential sign of something breaking up in the fuel system.

Find a new technician.
Not having a fault stored, just means something hasn’t exceeded a pre-programmed limit, not that that specific item isn’t the cause of the problem.
Spikey fuel pressure could easily be the problem, it’s just a case of finding out the cause. Given the longetivity of the problem i.e. it’s not failed completely, it’s obviously not something overly critical that’s failing.
Again live data is your friend. When the fault is happening, what is the fuel pressure regulator PWM? Surging i.e. pressure goes high, PWM drops with no related drop in pressure, on that reading would indicate the valve sticking. Again checking how the 3rd piston cut-off is reacting would help point you in the right direction. If there is no corresponding response in any of the actuators/sensors, then it could be an actual mechanical pump fault, as the 3rd piston cut-out does operate around that speed, however I would expect that to either work or not work (if it did fail to cut-off, I’d expect over pressure faults, and if it failed cut-off, I’d expect poor response at low speed due to low fuel pressure).
Everything else I can think of would normallu cause issues under high load, during starting, or at low speed.

sounds the same as my missus 307, after trying ALL of the above it turned out to be a blocked cat.

A blocked cat with surging, should cause a corresponding surging in boost pressure, plus I’d expect a lack of power at higher revs, possibly combined with more smoke than normal.

Again, somebody competent looking at live data should be able to highlight the area of the problem.

Depending how mechanically minded you are, the fuel pump isn’t that difficult to change. I did mine on the drive just following the Haynes manual. Hardest bit was locking the flywheel to get the nut holding the auxillary pully on undone. Once you have that off, the timeing belt comes off easily enough and the rest is simple.
Just take your time 😆

I had a golf do this to me. Loved that car but the day I part exed it was an huge weight off my shoulders. Multiple garage trips and fees for fault reading, changing numerous components, never got to the bottom of it.

At what point does the aggravation stop being worth it?

on the other hand – its a slight dead spot. just drive slightly slower or faster till it gives up totally.

as above i doubt i would be sticking a new fuel pump in.its well known that once they start to break down they like to enter the engine and cause all sorts of trouble – if i could find a known good engine im more likely to drop one of them in if its a tidy body and all the suspension/gearbox is good.

you paid 150 quid for diagnostics ? HOLY SHIT.

i bought lexia and peugeot planet plus the box for 60 quid to program some keys after being quoted 128 quid plus vat each and i wanted to do 3 for one car and 2 for another. – still my u480 picks up the codes in the injection system better than peugeot planet or my mates modus – he was a bit miffed that my 12 quid machine could see stuff his 3grand one didnt 😀

Theoretically it can come from the HP pump as the fuel goes back into the tank from the fuel rail (through a cooler) and they could then pass through the tank pump back up to the filter.

But I would normally go with the tank pump being the suspect one first and I don’t know if there is any brass bits in the HP pump.

you could put a small clear inline filter on the return pipe just before the tank to see if it’s the HP pump or just swap out the tank pump for a new one and see if that does the trick. Fully clean out the diesel filter too (carefully).

In case i missed it in an earlier post, have you hade a look at the c4 owners forum on the net ? they mention similar faults on the c4 1.6 hdi and some of the other Citroens too. The C4 diesels sometimes get problems with blocked fuel filters whixh can cause the on-board diagnostics to throw up an “anti-pollution system faulty” message, which sounds like the c4 equivalent of your problem….the filter about £40 to change so it may be a good place to start. Hope this helps. Good luck.

I found when I have done this before the best way is to get a piece of stiff metal rod about 18″ long bend a right angle on it about 3/4″ long and file a flat edge on it to get underneath the clip then push it down this will also help pulling the wiring back up through as you know access is a little restricted!it pays also to put electrical grease on the connectors before refitting and make sure the rubber gaskets on the connectors are in good condition as both the grease and the gaskets help prevent micro vibrations opening up the electrical connectors.

Good result!