[Non Ford] Intermittent Rough / Sluggish Running

[Citroënbender]

There are two cats from the collectors, each with their own sensor before the cat. Sensor 1 is cylinders 1&4, Sensor 2 is cylinders 2&3.

There is only one catalyst efficiency sensor, this the after-cat Sensor 1. Hence a couple of inaccurate data lines.

Cylinder 2 plugs on the left, cylinder 3 on the right. Probably less than 3000km of mostly urban driving.

This is not a great photo, I tried to show the fairly visible clean(ish) piston top with oil film.

When I changed the belts in January 2022, I checked the cam lobes for wear by visual inspection. All looked normal. I did not remove any lifters, hence not check the valve seals or guides. The engine does enjoy a sip of oil, but I’d assumed it was the usual reason - weak scraper rings.

Is it possible, that a valve guide or stem seal have become faulty (ie; split/worn guide or deconstructed seal) and at light throttle (high vacuum) it’s sucking oil which is causing the stumble and whacked lambda reading? I’ve no personal experience to comment about the real world effect of oil burning on oxygen sensors.

[oldel]

The piston looks too clean, the spark plug has white discolouration.
I'd suspect head gasket.

[Citroënbender]

There’s no excess or rapid pressurising of the coolant, or coolant loss beyond the known weeps (since 22 year old o-rings don’t take kindly to being disturbed). The whiter plug is from the adjacent cylinder to the one whose piston top is shown. I’d more think the ECU is leaning out the mix for that cylinder pair until it pings, as a response to the over-rich oxygen sensor readings.

These cars are a huge hassle to get the cylinder head off in-situ, so that’s another reason I really hope it’s not a gasket issue.