[Baffler]

Pumping the primer resulted in no noticeable effect.

So no air getting into the system. That's good.

Jacked the car up to take a look at the loom. It's in hard(ish) corrugated tube and looks just fine. Any closer inspection is definetely not a job to do outside, swimming backstroke under the car, in these temperatures. It's been awfully warm lately and only about -9 C today, though.

That's the problem with the loom, it looks fine on first inspection, but inside that conduit the loom is wrapped in insulating tape which traps moisture close to the wiring so try to imagine what effect years of heating/cooling/freezing/heating has on it. Just be mindful of this if you start to see some strange electrical issues.

Found this under the rubber cap. injectionpumps.co.uk says it's an advance solenoid. Does it act as a stop solenoid, too?

My mistake, I'm not a trained diesel expert, I thought that was the stop solenoid. The only other electrical connection I can think of is the Throttle Position Sensor on top of the pump so if you follow the harness it should take you to the stop solenoid.

You may have stumbled onto something...have you tried to actuate that advance solenoid when you had it out on the bench?

I don't know what to call the other part in the last photo, but the o-ring under it was hardened. The other o-ring was in better condition, but they both seemed quite thin to me. Anyway, I picked two similar from a local parts store. They said, they're not really meant to be in touch with diesel, so I'll have to replace those with something decent.

Made a really quick test run and at least it's not any worse. We'll see.

Solenoid has some weird marks on it, as seen in the second photo above the o-ring. What could that be? Corrosion?

That will be a return banjo pipe. The marks are probably just tar deposits left behind from the returning fuel.

Oh, and I somehow forgot to mention, that there's been occasionally a significant amount of white/blue smoke from the exhaust. It's not burning too much oil. I guess air in the fuel system and/or something electrical could cause that?

When you say occasionally...on starting from cold or when up to normal operating temp? If from a cold start it would point to bad glow plugs and if from warm engine it might be slightly advanced/over fuelling.

These don't typically burn oil but being a Renault unit they do tend to push out oil everywhere else