98 S70 throwing p0304 code
 

I bought a 98 S70 base model (non-turbo) automatic with 156k on it about 2 weeks ago. It seems to run fine. Since the service records end at 77k I took it to Meineke for an oil change and they looked it over, said everything looked good but they recommended a fuel injector cleaning since the mileage is so high, and it looks like they could be dirty, from the outside anyway...I looked under the injector cover and yeah its kinda dirty in there, but for a car with this many miles it should be a lot dirtier. I passed on the cleaning.

About a week ago the CEL came on, no flashing, just solid. I took it to Autozone and the codes were P0303 and P0304. I thought maybe it was the fuel injectors. I put a bottle of Liqui Moly in it and the light went off 2 days later. The light came back on today, again no flashing, just solid. I took it back to Autozone and the code read P0304 and P0300 (random misfire). So i'm thinking the problem is cylinder 4 and that is causing the other codes.

The car doesn't spit or sputter or show any symptoms, though I have noticed when I'm stopped at a light or sign there is a slight bump/jump every few seconds, but nothing major. It's barely noticeable.

I got under the hood today. The spark plugs and wires look fine, probably replaced in the last few years, as well as the ignition coil. I took out the #1 and 4 spark plugs and compared them to a used spark plug chart and they looked what can be considered good. I can hear the injectors ticking. Overall it sounds and looks smooth and quiet. No smells either, and no loss of fluids or leaks of any kind, though there were a few spots of oil under the spark plug cover, which I read can be normal on a higher miles volvo. I sopped them up and I'll check back on them in a few hundred miles.

I'm not really sure what could be causing the misfire. I'm not the best mechanic. I can handle tune ups, replacing alternators, that sort of thing. But the inner workings of the drivetrain are over my head.

Anyone know what the issue could be? Maybe I should have had the injector cleaning done?

I usually recommend staring with plugs since they are cheap, but if you checked them (did you re-gap them too?), I'd take a look at the cap and rotor next.

We had the same issue on our 98 S70. I replaced the plugs, plug wires, cap and rotor (about $100 for the parts) and viola, no more issue. Volvo's , especially 850s and S70s for some reason are finicky about the plugs they like, so go with either Volvo plugs, or Bosch, and it should work just fine. Also, buy yourself a code reader (about $60-$100) so you can read any codes and reset the light yourself in the future.

+1 on the OEM plugs - My 850T preferred simple copper core plugs. Just make sure the gap is correct before installing. I've also had wires weaken/fail at the distributor end due to corrosion under the boots. a simple mist test in a dark garage may give some clues.

Misfire almost always has to do with PLUGS and since yours is a 98 like mine,replace the Cap and Rotor,to see if that solve the problem,if not The wires should be next.