I'm going to reply to this the old-school way, i.e. by quoting the sections one by one.

Our Arty B. indicates in prior posts that the -5xx has and -9xx is without the cold start valve injector logic.

You are in Netherlands?

Have you verified the state of exhaust gases using a quality digital multimeter as the Volvo 700 / 900 Series FAQ, Bentley, or Haynes indicates? +VDC at the oxygen sensor electrode terminal disconnect from the wiring harness and the –VDC multimeter electrode secure at a good ground connection. Between 0.1 to 0.9 VDC and swings wildly between. There is a method using a Dwell meter you can search for here.

Do not use the multimeter to touch the single wire oxygen sensor wire harness connect with heavy green wire in a usually round white plastic connector with terminal. Wrap that terminal so it does not touch ground so I have been told to do.

If the digital electric multimeter shows a slow action or that trens not far away from .5 VDC, replace the oxygen sensor.

Below .5 is lean. The 02 sensor may not be faulty. Running condition is the issue.

Above 0.5 VDC is rich. The 02 sensor may not be faulty. Running condition is the issue.

2-2-1 suggest lean at idle. Netherland marklet 240s use EGR with PulsAIR fitted? Pulsair can become faulty at the two vacuum control valves at the engine exhaust side at the engine front. The EGR vaccum valve can get stuck open and drive-ability is almost impossible and not good for the engine.

I guess 9xx is newer, made by a different company, and maybe more robust (that I've read). Better performance and economy. No cold start. Has the immobilizer logic. Or some 9xx ECU's have the immobilizer logic? Unsure.

If no EGR / PulsAIR, and indeed lean at idle and when you take up drive (load) you have an air induction intake leak.

If you have the preheat silver accordion hose connected between the exhaust manifold heat shield and the bottom of the air intake filter box where the silver hose route is obscured by the radiator fan shroud, you must maintain the air filter box preheater thermostat (Wahler brand 70411). These all fail to all hot air all the time ruining an AMM and on the way to ruin the fuel to air ratio is lean until the AMM fails and the LH-Jet ECU goes into limp home mode.

The fuel pressure regulator directly affect fuel pressure at the injector. With low air intake manifold vacuum, as you are coasting downhill, and so the FPR open allow fuel to return lowering the injector fuel pressure. When you floor it, the air intake manifold pressure become closer to ambient, the FPR closes, raising the fuel injector pressure to inject more fuel. The FPR third failure mode is gas leaking through the vacuum line whether the FPR is failing else wise or not.

Remove the hose or replace the air filter box preheater flap valve with a new Wahler PN 70411.

So exhaust leaks can lean the air to fuel ratio, and a weak or failed oxygen sensor, when encountering the lean and high oxygen exhaust, cannot signal extra exhaust oxygen and so engine control responds by richening the exhaust by injector duration.

You can inspect the plastic and rubber hosts at the connections, whether the large accordion style hose, between the air intake downstream of the AMM / MAF and the throttle body, has holes. That hose can twist and distort, more so with bad mortor / transmission mounts, and contact the inner fender forming holes. Use strong light to find hole at the inside, degrease, use fine sandpaper, and a UV resist adhesive. I've used Eclectic products UV resist Goop to seal the holes.

You can use dish detergent on a cold engine, soaking the exhaust unions from top to down, asking an assiatnt to start, and seeing an bubbles at unions. All unions down to the union at the header pipe output and catalytic convert input.

Also, verify the exhaust hanger union that supports the end of the header pipe weight that secures to two bellhouing to engine bolts. The union fails at the weld of the plate with the threaded fastener. Volvo used weak hardware at the header pipe output to catalytic converter input union. I had the three forward-facing at the catalytic converter front pressed out and used quality hard of a larger gauge.

I guess that does it. Questions?

Thanks for your response. That may explain a thing or two.
If there is a vacuum leak, then it's not bad enough to cause the idle to hunt.

Today I pulled the tiny hose from the flametrap off the throttle body while the engine was idling high, i.e. 950 rpm. It dropped to 750 until I put the hose back again.
The strange thing was that just before I did that, the engine had been idling at 750 until I dabbed the throttle. The idle then went to 950 until I pulled that hose.

Is there some sort of valve in the PCV that is responding to inlet manifold pressure?