Hard warm start + low long-term fuel trim (mixture too rich) DTC. How to diagnose?

Just wondering what my next steps are here. Vehicle is ’05 Volvo V50 2.4i.

Initially had two misfiring cylinders, which was tentatively diagnosed as a blown head gasket by the shop. I changed it myself, but the old gasket appeared in serviceable condition, no splits or cracks – go figure.

Anyway on the way in there I cleaned out the fuel rail because it looked to have a bit of crud in there, sort of black flakes, which worried me a bit.

Once Humpty was back together again, she fired up hitch-free other than a stuck throttle that was fixed after cleaning. Performance while driving seems fine, cooling system is tight as a drum with no loss of coolant in months.

However, ever since then I’ve had a hard warm start. Cold or hot (i.e. shortly after turning off) start is 1-2 cranks and go. But leave the car for half an hour and it’ll take maybe 5 seconds to fire, sometimes with a rough idle for a moment before idling normally.

So I lived with this condition for several months, waiting for any more clues to arise, which I now have. CEL with "long term fuel trim – lower limit", which I understand to indicate a rich mixture detected by the O2 sensor.

I had long suspected leaky injectors and this appeared to confirm my theory. I have since:

• cleaned throttle (again)
• cleaned fuel injectors using a power supply to open the pintle valve
• replaced the fuel injector O-rings, filter baskets, and pintle caps

All to no avail.

I have crudely tested the injectors for internal leaks by taking the fuel rail off the manifold and turning the ignition on to pressurise the rail, but there doesn’t appear to be any fuel dripping out.

My list of suspects is longer than I would care to buy all parts for. Possible culprits include:

• Vacuum leak after the MAF sensor
• Fuel pressure sensor
• MAF sensor
• Manifold air pressure sensor
• Injector operation compromised somehow
• Something else fuel related

How can I reliably test for these issues? Any other ideas?

Thanks.