2002 XC70 misfire on cylinder 1--ECU issue?

[DavidE7]

... I like to wire brush and put dielectric grease on the contact surfaces for the two ground bolts. ...

Oops, bad practice, don't do this! TL/DR: Never use dielectric grease inside an electrical path. Use it outside or around the path.

Dielectric grease is an insulator.

Its basic purpose is to keep things, like moisture or corrosive chemicals, out of an electrical connection. By putting it on the mating surfaces you effectively interrupt the path to ground.

Obviously your engine grounds are working well enough so presumably you're squishing out enough dielectric grease that there is still a sufficient conductive path through there, but why sabotage the grounds!

I disagree for environments with high humidity and salt or chloride used on roads for snow and ice melting. Keeping the junction sealed from salt and corrosion is critical for maintaining electrical connections and grounding. The surface tension of dielectric grease isn't high enough to create the problems you are describing. There is more than enough pressure with a ground bolt the squish the dielectric grease out from the connection. The same applies to electrical plugs. I have significantly more problems with corrosion inside electrical connectors than I do with dielectric grease that will prevent future corrosion. The DEM (Differential Electronic Module) was non-functional in my 2004 V70R because a little salt water got splashed into the back seat area next to the door and ran down the harness into the unsealed connector and corroded the CAN high and CAN low plugs. I cleaned them with several applications of DeOxIt and then smeared in dielectric grease to prevent future problems. The problem was eliminated and prevented from recurrence.

None of this applies if you live in Southern California, Nevada, Arizona or New Mexico without humidity and salt. You don't need to coat every metal fastener in anti-seize either.

The main reason I like P2 Volvos is the rust protection. These cars aren't perfect after 20+ years but they resist rust better than any other old car I see in Michigan.

[dikidera]

There is also electric grease! A conducive grease to improve electrical contact.

[volvo245fan]

Grease wars aside (I side with the conductive grease comment here), I'll provide a brief update on this:
I compression tested the engine this morning:
Cylinder 1: 180
Cylinder 2: 175
Cylinder 3: 175
Cylinder 4: 180
Cylinder 5: 180
So, that looks like a dead-end (happily).
I installed the new injector on cylinder 1, reassembled the fuel rail and ignition system, and gave her a good long test drive, no problems (yet), but this is of course the challenge with intermittent problems and I'm not waving the flag of victory just yet.
I'll hope that new injector fixed the issue and will report back for more advice if the misfire returns and the saga continues. I'm still confused why all my codes would be related to cylinder 1 ignition if it was just a bad injector, but will cross my fingers and wait.

[vtl]

The rest injectors will follow. I replaced them all when the first one started to give occasional misfires. Nothing I could notice, but the counter was ticking from time to time.

[erikv11]

Thanks in advance for keeping us posted OP, hopefully you've found it.

Grease wars!

Yes as mentioned, I can see that squishing dielectric grease out of the connection is one approach to having it ultimately seal and surround the electrical path, I see your point there. Obviously this approach works for you overall, so thanks for elaborating.

I guess I'm mainly trying to help prevent unsuspecting people from extending the "just grease it up" suggestion to doing something more likely detrimental like "so I squirted dielectric grease onto the spark plug connector in the coil pack, to make the connection real good."

DeOxIt is good stuff, nobody will argue that!

[volvo245fan]

Solved!
Car has been running great, no more CEL or misfire.