850 running lean.

Hello,

I am working on a 1997 Volvo 850R 2.3l turbo, automatic, FWD, with 94k miles. It has seen a few minor upgrades, detailed below. About 3 weeks ago the CEL came on reporting P0171, long term fuel trim lean. The car has been running “fine” starts fine and runs smooth, however acceleration and boost pressure are way below normal. These components were replaced or upgraded prior to the CEL. Everything seemed to be fine after they were upgraded:

Spark plugs (went from platinum to iridium)
Cap & rotor
Plug wires (8MM Performance)
Fuel injectors (higher capacity)


I did some research on the code and the following components were replaced:
Mass airflow sensor
Throttle position sensor
Front O2 Sensor
I also upgraded the coil, it had no effect on the CEL.
I have extensively checked for vacuum leaks, found 2 small ones and repaired them, no effect on CEL.

I failed to change the fuel filter when I put the new injectors in and recently did that. The old one was the original factory filter, fuel flowed out of it with no problem. This had no effect on CEL or performance.

I have also noticed that the temp gauge has gone up slightly over what it was for the last year. It was up before the CEL and the poor accel issue presented itself, so I also have flushed the cooling system, replaced the thermostat and replaced the coolant temp sensor. The gauge is still at the same elevated spot, however it is well within the normal range

Yesterday I checked / changed the spark plugs. They were white. Referring to the generic plug color chart, they look like the vacuum leak, lean fuel mixture plug dead on. So I put a set of older ones in. I also pulled the fuel rail and checked the injectors to make sure there were no obvious block and nothing was leaking, all looked good. I did drain the fuel in the rail and “charged it up” by turning the key on and off a few times, (I know the regulator should keep it constant and I had a shop test the fuel pressure and they said it was good). For 1 trip, of all of 6 miles it ran perfect. Then after an hour it was back to the way it was.

I am planning on putting the old injectors back in to see if that makes a difference. Beyond that what’s left? Fuel pressure regulator? Fuel pump? Air temp sensor?
Why would it run fine the one time? It has never stalled out or failed to start normally...

Sorry for the long question however I am pretty much at a loss..... Any ideas?


Thank you!