Peugeot 207 sporadic, sudden and very short loss of power

Question

Car: Peugeot 207 2007, 1.4 HDI engine 8HZ 1398cmc, 50kW

Symptoms: everything (cold/warm start, idle, accelerating with no load/some load, etc) works as expected, except that sometimes there's an abrupt and very short (like half a second) jolt from, what I assume, is some sort of loss of power. I would describe it as if the fuel was cutoff for a very short amount of time and then recovers instantly, but I have no idea if that's what really happens.

When does it happen? I really cannot find a pattern, as it happened in all sorts of conditions, but it seems to happen more frequently when driven at higher revs and on terrain which isn't flat.

What I've tried:

• taken to a mechanic: they said to come back when it's reproducible, as there's no faults and they couldn't trigger it on a short drive
• changed the intake air temperature (IAT) sensor: I've used an OBD logger to try and find anything strange, and I found it strange that the IAT sensor reports temperatures between ambient and 130°C and also some black debris on parts of it -- so I've replaced it, but that didn't help

• more expensive fuel
• italian tune-up

Extra pictures:

view under the hood:

old IAT sensor with some black debris:

FLIR confirming the high IAT temps:

Thank you for reading and any ideas/leads very much appreciated 🙏

Answer 1

To give some closure here: the issue started manifesting a lot more often so I tried my luck once more at the mechanic.

And lucky I was: there was a fault logged (which interestingly, didn't appear on the cheap OBD adapter) that guided them to the crankshaft sensor. After just redoing the wiring, without even changing the sensor itself, the issue is (amazingly) gone and the car feels "like new".

Tl;dr bad electrical connection to the crankshaft sensor

Answer 2

If your fuel supply line has a loose connection, a crack or a break somewhere, it could be letting air into the fuel line on the way to the injector pump. Diesels that I've known are pretty good at eliminating random bubbles from the fuel supply, but if operating at higher speed and under greater load, you may be momentarily exceeding the injector pump's ability to bypass air bubbles, leading to a momentary fuel starvation.

I've never been under the hood of a Peugeot, but this looks like the fuel supply and return lines:

If so, you might unclamp the fuel lines there and temporarily insert a short loop of clear hose so you can watch the fuel travel to the engine and back to the tank. You shouldn't see much air in the fuel going to the engine except an occasional small bubble. If you do, this may be the problem. Look for a break between there and the tank, or in the tank itself.

If there are no bubbles going to the engine but a lot of bubbles and perhaps long gaps of air returning to the tank, look for a loose fuel fitting at the fuel filter, the filter's water drain if there is one, or at the injector pump itself.