S40 Misfire on 1 and 4
 

Hello! I’m the new owner of a 2003 Volvo S40 1.9T with 84k miles from Georgia (United States).

While accelerating from 40 up to 65 mph, I simultaneously had both Cylinder 1 and 4 misfire while accelerating but resumed normal operation once at a constant speed. The car drove fine for 2 hours with a stop in between, until I got home and tried restarting the car. The car hardly started and was accompanied with audible misfires. Once again I had both Cylinder 1 and 4 misfire. So I changed the spark plugs and performed the In-line diode spark plug check. The only effect I now have is that both ignition coils now have codes.

Additionally while i can get the car started amid all the misfiring, if i turn the air conditioner on or put the car into gear it will often stall out.

P0301 Cylinder 1 Misfire
P0304 Cylinder 4 Misfire
PO351 Ignition coil A Primary/Secondary Circuit
PO352 Ignition coil B Primary/Secondary Circuit

Just below is a list of the troubleshooting I’ve performed since the codes appeared;

-In-line diode spark plug check (looked fine to the naked eye)
-New spark plugs gapped to .028
-Compression test (went great, all cylinders were within 1% of each other)
-Fuel injectors swapped between good and bad Cylinders. All injectors checked great with the Ohm resistance check and audibly with mechanics stethoscope
-Swapped Ignition coils with new Ignition coil (had no effect)

I’ve ran out of ideas at this point. I look forward to hearing your thoughts!

Did you check the wiring and connectors into the coils? they can crack and you may try cleaning the contacts.

Replace the wires and coils both. Both can get cracks in the hard plastic that fits down over the plugs which allow arcing to the head that you can't see because of the location. To be sure you can inspect the plastic that fits down into the spark plug well and look for small fine cracks. I have heard of someone using RTV to coat the plastic outside in an attempt to block the arcing. Don't know if it works.

Fixed
 

Bingo! It was the connector going to the coil. I'm going to try RTV on the connectors for a permanent solution

The coil pack on #4 fires for both 1 and 4. I just went through this and found both coils (at 3 and 4) had cracks on the long portion that extends down to the spark plug. Bought two used replacements at Pick-N-Pull and the problem returned on just #2, turned out one of my used ones had the crack as well (and I didn't know where to look for the crack when I harvested the used parts).

I used some thick self-adhesive sailboat rigging tape and wrapped the crack to get me by until a new one arrived (electrical tape would work too, I just grabbed the first roll of tape I could find in the garage). Worked great! The spark "leaks" out the crack to ground and thus starves both plugs of good spark. The cracked plastic is just failed insulation. Not a good long term fix, but good to know if this crops up while on the road.

I'm wondering if a good two part epoxy like JB Weld would serve? Experiment time....