I have a 2003 Dakota with a 4.7L V8 engine, and there is a vacuum line going to the back driver side of the intake, when this line is hooked up the idle drops to below 500rpm and stales, but when not hooked up the port sucks air and allows the engine to idle at 600-700rpm. I traced the line and it runs to the Vacuum Purge Solenoid and I replaced it with a new one but the issue persists.

I have included a picture to hopefully clarify my description. The red circle is the port on the intake where the line should be connected, and the blue circle is the purge solenoid that I replaced. There is another line coming out of the purge solenoid that looks to go under the cab/firewall of the truck but I haven't traced it further than this.

I am wondering if anyone can help diagnose the issue. Thanks!

 

I don't think the purge solenoid is related. You've created a vacuum leak, allowing more air basically bypassing the IAC valve. It's like opening the throttle a little bit.
Have you ever cleaned the throttle body? I've had to several times in over 250k miles. Your IAC valve may also not be functioning properly.

 

IAC valve issue.