Hello! I just bought my first one of these cars and the thing is great I've just had trouble fixing this problem. Im new to these cars and dont know a ton about them hopefully someone can help.
1990 volvo 240DL B230F M47 LH3.1

As of right now it's throwing a code 232 and 231. Studders a bit off idle when i try to accelerate but she does eventually figures it out and revs out fine. Does the same thing when driving and it sits at one rpm for around 30 seconds. Also does afterfire a bit on decel. Definitely is running rich but can't figure out why. Here's what I have done so far:
-New IAC Valve
-New MAF sensor (Delphi from ipd)
-Cleaned throttle body
-Checked FPR doesn't seem to leak and looks quite new
-Checked for vacuum leaks (70% sure there's none)
Theres newer looking injectors that the previous owner changed along with the o2 and cat.

 

maybe try resetting the ECU? sometimes that helps with weird issues on these old volvos.

 

Thanks for the help! I tried that this morning before work and it actually helped a ton , but the idle is still a little rough. And it just threw another 232. Still has some afterfire on decel and at idle. But still a huge improvement.

 

Having a car that runs rich at speed and a bad idle may not have a common root cause short of a weak spark. for idle issues, normally I'd look for intake air leaks/vacuum lines etc, check the air intake tubing (via a smoke test?) and then check spark. Note on older cars, you can get vacuum leaks from some unexpected places like the injector seals so you may want to spray carb cleaner there to check. Since you are also having issues at speed, you may want to start by checking spark. Have you pulled the plugs to check for color/check the gap? how does the rotor/cap look? What's the date code on the wires? have they been replaced say in the last 5 years? How's the intake look? air filter etc ok? From there you may want to check fuel pressure. As to the running rich at speed, that's where the O2 sensor would be involved. Not sure what it would take for an older car to throw a check engine light on a pre OBD2 car, but if your car has been running rich or burning oil, its quite possible the sensor is fouled.