This is a repost of an issue I was having last week, but I've been testing everything and I really need some expertise here.

I finished rebuilding my 2007 XC70, I got her with a broken valve, tore her down and put her back together. She ran like a rock star for the first day but I started losing voltage, I replaced the
battery, the voltage regulator, and finally bought a new alternator altogether.

That issue is resolved but now my #2 cylinder won't fire. I've switched coil packs, the plug, nothing. If switch the connectors between #1 and #2 both plugs out, she'll keep running on 3 cylinders while I can test for spark, I get spark on whatever coil pack the #1 connector is connected to and no spark to whichever the #2 connector is connected to. This test rules out the coil pack and the plug. I replaced the connector, no difference. I tested the terminals at the connector, the ground, is grounded, the green stripe is at +12, the brown wire I assume is a ground wire or a low voltage signal that the computer sends to the coil pack to tell it when to fire. I don't know for sure, I can't figure out what it should read but it's all over the place on the voltmeter so I think I've got it understood. I traced this wire all the way back to the computer and cut it and spliced in a new wire, no dice.

So now I think it's the computer itself. All the wires on this car are brittle and easily will fault. Once I get her running perfectly, I will replace all the coil pack connectors.

But my thinking now is that the issues is either with the ECU or could be resolved if I just reseat (not reset, but take it out, clean the connectors and replace it) the ECU; I could easily see that connection rusted. Today I pulled off the cover and the sides off the ECU housing and pushed all 4 of the little purple rails all the way forward, as far as they would go, anyway. But the ECU's won't budge, I'm either doing it wrong or they are rusted in place.

Am I chasing a red herring? I redid the timing in case that was it, and it was off a little (less than a tooth), but I redid it anyway and now it's perfect. In fact, the car runs remarkably well on just the 4 cylinders. But I've got to get to the bottom of this.

Any suggestions, tips, debugging ideas, anything? It's a sweet car, clean and when she was running, she ran like a champ with acceleration and power I've never experienced with a Volvo so I cant' give up. I can't see how it could be the timing since a) there are no errors thrown and b) she runs pretty much fine on 4 cylinders, just low power and not a smooth ride.

Finally, I tested the compression for #2 and she's sucking 140 PSI, I didn't check the rest but that seems fine to me. But that's actually irrelevant. no spark is no spark.

Please please help, any suggestions at all, will be appreciated,