I have a puzzling P0304 code (misfire on # 4 cylinder). I swapped the plugs and coils between #3 and #4 and the code stayed on #4.

The circumstances of the check engine light coming on are suspicious, so here is the entire background...
Some of you will recall that I was wrestling with foamy oil. I took off the intake manifold & changed the PCV parts. Then I dropped the oil pan to replace the o-rings, which fixed the foamy oil problem. At that point, the car developed a nasty engine shake because the 20 year old motor mounts which were mostly perished, did not like the twisting involved in loosening them up to raise the engine.

While waiting for the mounts to come in, I drove the car about 45 miles, until the code reader showed that all was well and it was ready for the emissions inspection. Then I replaced the mounts, which is a very fiddly job on a turbo with the electronic throttle body. Started the car, and it was much smoother. I don't think I got out of the driveway before..BOOM... Check Engine light.

Since swapping the coil didn't help, and I could hear the injector clicking with a stethoscope, I figure a wire had gotten damaged in the course of changing the motor mounts. The parts of the wiring that are visible (i.e. before it goes into that massive plastic sheath that runs above the subframe) did not show any damage. (I assume that the sheath, which is not smashed, would protect any wires inside from damage), and there were no wires pinched between the motor mount and its bracket.

So, I verified 12V and ground on their respective wires on the #4 coil and pronounced myself stumped. I don't have a scope and I don't know a safe and effective way to check whether or not the coil trigger signal is reaching the coil. (Since it comes directly from the computer, I don't want to risk damaging the computer [like by exposing a 5V circuit to 12V]. I'm told that the computers are flashed to be VIN specific, are discontinued (and cost as much as the car is worth), and thus irreplaceable.)

Anybody got any thoughts?

I thought I had solved it, but I guess it is intermittent. The code came back after 50 miles. The next step is to disassemble the connector on #4 coil and see if the pins are too loose.

Similar scenarios I've experienced. Replaced/swapped plugs & coils all around, problem solved for up to 500 miles or as short as one day, then problem would reappear on #3: P0303. I think that's why the Indy shops were stumped.

Good luck!

I examined the coil connector plug. It looked fine. Since I had the fan & the intercooler pipe out(the big curvy plastic pipe that runs from the top of the radiator to the bottom of the electronic throttle body), I temporarily reconnected the pipe and ran the car without the fan.

I pulled off the cover over the fuel injector connectors, started the car and pulled the connector off of the # 4 injector. The engine slowed down briefly & sped up. I tried this a couple of times with the same result. Then, as a control, I pulled the injector off of # 3. Exactly the same response. This told me that #4 cylinder was firing, ruling out a wiring problem to the coil or fuel injector.

I am a big believer that, when you do a job and suddenly have a new problem, the problem was caused by something you did in the job you just finished. Since the P0304 code appeared after replacing the motor mounts, and there is nothing around the mounts that could cause it, the only other part of the motor mount job that did not directly apply to the mounts was removing the fan & intercooler pipe to gain access to the upper bolt on the front mount. I then sprayed carburetor cleaner around and under the intake manifold. The engine appeared to speed up a bit when I sprayed under the manifold. I then removed and carefully examined the intercooler pipe. There was a small amount of debris around the end of the pipe that connects to the throttle body.

I made sure the pipe was clean, not cracked, and carefully seated, and then took the car for a 4 mile drive. No codes. I then reassembled the car drove it through the Inspection readiness drive cycle (about 20 miles), until my code reader showed green (meaning ready for inspection), and drove straight the a Smog Inspection station, where it passed with 3 days to go before my registration is due.

So, was the P0304 code caused by a leak at the throttle body, either due to debris or the an incomplete installation due to the fact that you cannot see that point and have to install it entirely by feel? I do not know. But, for now it is code-free.

Had a similar problem on 2000 V70. Intermittent misfires and CEL; always on cylinder #3 (P0303 code) even after swapping plugs and coils around. Had the car into two Indy shops who work on Volvos for a living; neither could diagnose this correctly and their only advice was to "monitor," which I did until the #3 misfire occurred again, and again, and again.

Turns out through my own troubleshooting that it was the #3 coil receptacle housing that needed to be replaced and repinned. After so many times unplugging and plugging the wires into a coil, the receptacle housing clamp weakens or breaks and the pins no longer make good contract with the coil terminal blades. From what I've observed, most troubleshooting about this problem is limited to swapping coils and plugs around, and not examining this connector. (Same as the Indy shops, apparently).

Here's a writeup about replacing and repinning that coil receptacle housing, which is a part available from Volvo. The wires must be removed from the old housing and repinned into the new one:

https://www.brickboard.com/AWD/volvo/1663046/V70-XC70/coil_receptacle_extraction_tool.html

Definitely worth considering. Thank you.

Is there any way to visibly inspect the connector for burning?

Once unplugged, conduct a visual inspection of the coil terminal connecters; i.e., male/blades in the coil port and female/wire terminal ends in the receptacle housing. From my experience, the blades in the coil were visibly degraded from arcing that had occurred, which is a clue that the receptacle housing is not staying fully plugged in. The coil then fails to fire. The suspect coil was a throwaway at that point.

Would recommend acquiring a new receptacle housing from Volvo, repinning it, and finish the job in #4 cylinder with a new coil. Then, you're confident all contacts are good.

Some with more patience than I can use small screwdrivers to release the barbs that hold the wire terminals inside the receptacle housing, and then pull out the wires. The referenced terminal extractor tool makes a much easier task of it IMHO.

Peter, the nasty engine shake was because of the 304. The old worn mounts just made it feel like they were at fault. The third time you started the car, the pending 304 became live and turned on the CEL.

So, you have to think about what happened to the wires/intake manifold when you fixed the PCV. Check the plug on the wire ends, make sure the pins haven't been pushed back and they make good contact.
--
Keeping it running is better than buying new

What I find puzzling is that I was able to drive it 45 miles with no codes. Then the P0304 happened almost immediately after changing the mounts. I swapped in another coil, cleared the codes, and the P0304 happened again within a couple of minutes. Then I swapped the plugs and moved the #3 coil to #4 and put the original #4 coil in #3, cleared the codes, and the P0304 happened a third time within a couple of minutes.

There is another weird symptom, that has existed since last year. After clearing codes, the car won't start (like a bad immobilizer...runs for a second, dies and no start). After doing the clear code process a second time, the car starts. This happen once last summer, after a NJ gas station attendant didn't put the cap on properly. Then, no problems with the car for 5,000 miles until now, and after clearing codes, I see the same weird need to clear it twice.

Sounds like an intermittent fault somewhere, a bad connection that came about during your recent PCV and engine work. I second Klaus's suggestion of checking over your electrical connections as there may be something right on the edge of connected/disconnected; that would explain why it was broken, worked for a bit, then broken again after moving when the mounts were replaced. Check everything over, electrical and vacuum connections, not just connections related to cyl. #3. You'll figure it out sooner or later.

I've been caretaker of a couple of cars that were finicky about OBD readers. The Mazda 6 would die as soon as the code reader was connected; the Honda will read codes at any time but won't clear codes unless the car is off and connected after being in key pos. II for a few seconds. So your Volvo may just be picky about being probed.