i have a 2001 s40. blown head gasket a month ago,did an identical engine swap with 112,000mi on it. Everything went fairly well! timing belt, thermastat, oil change, tranny dump and fill, new spark plug wires and plugs, new radiator all at the same time. she started right up. but slowly gained a studer, running really rough when in drive but stopped. slowly got worse, CEL came on. when it started to rain (might be coincidense) it started running real rough, studering while driving not just when stopped. pulled codes p0303 and p0304, which is misfire cylider 3 and 4. bought new coils put them in and runs even worse! no clue whats going on! help!

 

I've seen new wires and new plugs bad right out of the box. Don't assume anything is working just because it's new. Some of the stuff being manufactured near to where I'm sitting now is junk from the start.

I'd start by pulling each spark plug and reading it. Is it still white? Is it discolored? Check the gap, maybe tighten them a bit to .025-.030". Then I'd ohm the wires. Should be no more than 5k resistance, good quality wires even less. Take a close look at the plastic part that covers the spark plug. When they go bad the spark jumps to the head instead of the plug. Sometimes you can hear the "pop, pop, pop" if it's really bad. Then I'd check the coils, both ohms and for damage. They can also jump spark to the head causing a misfire. Last, make sure the wiring harness is fully seated on each coil. The plastic connector holding the female wires tends to break down over time allowing the wires to back out of the coil. You can usually push the female wires onto the male coil prongs with some wire pliers. Once fully seated they should stay put.

 

also pulled another code today P0128 (coolant temp below thermostat regulating temp)

 

I believe that means a bad t-stat.

 

Check the feeds going to the coils.I have find that the connector does fall apart and doesnt make good contact.I ended up taking the plugs apart and connecting the two wire to the coils after cleaned the slip-on ends up naked.

 

Were you able to fix it?
Rough idle, sometimes several tries to start. Runs fine while moving or at higher speeds.
I also have only those two codes p0303, p0304. Replaced coil (Cracked) in #3, wires (discolored) this week, and replaced the plugs several months ago.
Car still has the same codes after this. (p0303 p0304) Running out of ideas...

 

Did you clear the codes and they came back? On mine I ended up replacing both coils and wires before it stopped missing.

 

One coil was cracked. So I replace it, new wires too. One old coil in #4.
Erased my codes, and it still had codes for misfiring in #3 and #4. New coil was in #3. I changed it to #4 no difference in rough idle.
This morning I switched the fuel injectors 3-4 with 1-2 to see if that is the possible problem.
Did both of your coils fail?

I'll attempt to post a picture.

 

In my case I did what you did replacing only one coil, then new plugs, then new wires, and finally replaced the other coil before it stopped acting up. Both coils ohmmed good so I can only guess it was a hairline crack I did not find. I hate being a parts changer but I was fairly new to the S40 back then. The car had just hit 100k when it acted up. Lately I've read of someone using epoxy to fix the hairline cracks and save the coil. Never tried it myself.

 

I'm at about 110k. I will try that with getting another coil. This cars driving me crazy haha.
I purchased the first coil from O Reilly and they claimed it was OEM, but it was not labeled just had a black top. Do you recommend where I should get the coil?

 

The dealer or a parts outlet that sells OEM Volvo parts.

 

I bought my first replacement from Autozone. The box was labeled Duralast, just like their other parts so I'm thinking it's just another aftermarket part when, holy smokes, a genuine Volvo coil is in there. Same for the 2nd coil. Several years and 50k miles later and I replace both coils again but with Advance Auto parts el cheapos. Those are still going today with about 180k on the engine.