Do I have to vent the cooling system by removing the rubber cap located by the firewall? Last week I was at junkyard and I tried taking off that black rubber cap which was very hard to remove, but when I got it off, I found the tube it was covering was clogged and I had to use a skinny screwdriver to unclog it. I'm afraid my vent tube will also be clogged and if I try to clear it, the junk will fall down the tube and get into the system. If I do the radiator flush and don't vent it, will I get air trapped in there? I've done this once in another car, and didn't have to vent anything. Thx.

You do need to vent the air relief plugs when you refill your radiator. Otherwise, you will get air trapped at the top end of the motor. Preventing air from getting trapped is what the relief plugs are for. I don't know about the QX4 but my Pathfinder has two of them. One at the rear, like you mentioned, and one in front. You don't have to remove the plugs, just loosen them so that air will leak out while you are refilling the radiator. When fluid starts to flow out of the loosely capped relief holes, tighten the caps and then continue to fill the system. Having the caps loose, with a rag in place to catch the fluid as it starts to leak out, causes less of a mess than with the caps off and allowing the fluid to split out uncontrolled. I have never seen one clogged up before but if it is and you poke at the clog and it falls into the system, it should get flushed out when drain and flush the system.

In this coolant replacement thread, I detailed how I cleared the vent pipe plug.