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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

OBD logger of IAT and coolant

  • more expensive fuel
  • italian tune-up

Extra pictures:

view under the hood:

view under the hood

old IAT sensor with some black debris:

old IAT sensor

FLIR confirming the high IAT temps:

FLIR confirming the high IAT temps

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

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  • I'm not sure I'd use the FLIR image as "confirmation" of the temperature at the IAC. This is because your FLIR image is showing the temperature at the outside of the of the housing, which is outside of the rubber hose. Temperature is going to be very stable in this area with air inside not providing enough cooling of the parts to change it as much or as quickly as what's going on inside of the tube. That said, the amount of fluctuation from the sensor is really strange. Not sure if there is an EGR which might be feeding warm air at given times which might be affecting the intake temps. Jun 13, 2023 at 15:30

2 Answers 2

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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

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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:

enter image description here

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.

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