My 16 year old daughter bought her 1st car 3 weeks ago, a silver 2001 S60 NA automatic with 233,000. This is our families 4th Volvo covering 4 consecutive decades ( 71 1800, 89 240, 98 xc70), so I was pretty excited. I loved all of those previous cars.
Anyway- a week into driving the car, she asked me to drive it. She said something weird was happening with the suspension at freeway speed. Took her on a somewhat spirited drive up the freeway, and when we came to our stop, car was running very, very rough. No cel, but it died frequently.
Quick interweb search suggested sticking/dirty throttle body.
Did that and installed new gasket. Little to no improvement. Seemed like it was missing. Got it home, googled the problem. Suggested troubleshooting was to pull each coil with engine running and listen for click, click, click.
Cylinder 1 good, cylinder 2 no click, cyl 3-5 all had clicks.
Cool, we found the problem!
Order cheap spectra (or some similar sounding brand) coil from Rock Auto.
Install and there is a slight improvement.
Talk to a couple of mechanics. They always replace plugs when that happens.
Go to the local parts house. Only have Autolite 3923 in stock. Buy and install. There is a slight improvement that dissipates over time. Eventually running as poorly as initially.
Do some more focused research. Paraphrasing, but basically Autolites suck.
Shop for better plugs. very discouraging as there is no universal agreement as to which plug is good for this application. Plugs in the car were 7955. But multiple sources say those aren't right.
Went with Bosch 7401. Gaps seem crazy huge, but I've never seen a 4 electrode plug before. What do I know? Install them and get a slight improvement.
More research leads to buying all 5 new Bosch coils. Install and it is almost right! Idle is rough, lights dimming and brightening. Only throwing the cel every 3 or 4 starts and stops. Almost right, but not quite.
My daughter is tearing up, wife thinking about the same. What am I missing?
The only code that I have seen is p0300.
Any help will be appreciated. My daughter is so distraught and I am tired from scratching my head so much.

 

0300 is a non-specific misfire code - as you noted its typically coils or plugs. For some reason Volvos tend to like traditional copper core or OEM plugs. If you've done coils and plugs, the next thing is to inspect connectors to the coils. After that you need triage possible causes for mixture issues - fuel pressure ok? EGR valve ok? The rough idle could indicate a fuel pressure sensor (not sure if your model has a pressure regulator). Vacuum lines/intake tubes checked for air leaks?

 

Status update. Coils replaced with Bosch. Bosch 7955 (that's what was in the running car when we got it) plugs gapped at .028. Replaced the fuel injector O-rings. Replaced all vacuum lines but the oil trap related ones. Tested fuel system pressure at the fuel rail. Key on engine off pressure would bump up to about 57, then drop to 54 ish when key is turned off. Idle pressure around 58 until idle gets rough, then bounces rapidly (really shaky needle) between 52 and 59. Nice smooth 58ish when idle is smooth, ie not misfiring.
From my research, idle fuel pressure is high. This kind of leads me to think it is fuel pump or pressure regulator or pump control module.
Thoughts?

 

Did you change the throttle body, or replace it? If you replaced it, you will need to have your firmware updated, as I believe the 2001's used the Magnetti Marelli Throttle body.

 

I just cleaned and reinstalled it.