Fuel rail leak

Hello, that port should not be leaking fuel even with the cap missing, there is a Schrader valve in there that opens when depressed by a fuel pressure gauge, the Schrader valve is much like a valve in one of your tires. Its almost exactly the same except the seal inside is made to withstand fuel. So you can check to see if its possible loose, but most likely the seal is leaking. Considering the look of that fuel rail with all the rust. But I will see if I can find a picture of a Schrader valve tool to try screwing it back in if thats what has happened.
You may want to look around at some local used automotive parts yards and see if you can get a fuel rail cheap, if the price for a new one is an issue.

The first picture are some different Schrader valve tools, you may have seen these before, but you may be able to take the valve out and clean it, then tighten it back up. Some of these thread in backwards so dont force it. The rest of the diagrams here are for the fuel rail and fuel lines if you do end up needing to pull the rail. In that case I would also replace the fuel injector seals, because the old ones probably wont seat correctly with that much rust around them. Was this vehicle in a flood? Just wondering how it ended up with so much rust on everything there. If you have any further questions at all, dont hesitate to ask.

Make sure to put a rag around the valve before attempting to remove it, there will be some fuel pressure there, most likely not much since its leaking, but you dont want to have gas ending up everywhere.

Yeah the plastic cap that goes on the test port is only there to keep dirt out. I suspect the seal on the Schrader valve is probably dry rotted and causing the leak. I would also be concerned about the fuel injector upper and lower seals. If the lower seals are not fully sealing the injector into the intake manifold, it can actually cause a vacuum leak because of intake manifold vacuum pulling air right around the seals. This will cause a lean running condition, and the leaking test port can also cause fuel pressure to be lower that it should depending on how much is leaking when the engine is running, this will also cause a lean condition with lower fuel rail pressure/volume to the injectors. This is something that can be checked by looking at live engine data with a scan tool. You may see higher 'Positive' Long Term and Short Term Fuel Trim data PIDs (parameter IDs/ sensor data readings).

I will look around today, and get back with you this evening. See what we can come up with, there are quite a few used auto parts lots in my area.

So I do see quite a few fuel rails for sale on ebay, within a bunch of different price ranges. Any new ones are going to cost a lot more. Unless you were able to take the Schrader valve out of the test port and check it, with no cap on there I think youll find its probably full of dirt and the spring on the valve is not returning it to its closed position, or its loose.