The factory vacume-route diagram was missing off my ('89) truck when I bought it. Some of the line-colors have become faded over the years and I have since changed all the rubber hoses, so none of mine have the original color stripes.
Having said all that, I THINK that the plastic line you are describing goes to the plastic valve which sits next to the passenger-side (RH) exhaust manifold, which is part of the thermactor pump hose routing.

If that don't work, and if no one else pops in with a better answer, try to post a photo.

Hope this helps.

DGW

 

On my 1990 ex-5.8L, the Yellow Vacuum Line goes from the TAD Solenoid (driver's side forward solenoid) to the TAD (Diverter) Valve located at the rear of the Intake Manifold. You have to climb up onto the engine to feel the Diverter Valve or remove the Intake Manifold.

The Pink Vacuum Line goes from the TAB Solenoid (driver's side rear solenoid) to the passenger side (adjacent to the Vacuum Reservoir Can) and down to the bottom of the TAB (Bypass) Valve. Note that my Pink Vacuum line appears Yellowish in this pic and is not yet connected to the base of the TAB Valve as I was working up a solution at the time.

Here is a pic of my VECI (Vehicle Emissions Control Information) Label for a 1990 5.8L Bronco. Be advised that the ACV (Air Control Valve) is the TAD (Thermactor Air Diverter) Valve and the AIR BPV is the TAB (Thermactor Air Bypass) Valve. The VRESER is the Vacuum Reservoir Can where the red & black vacuum lines plug into and the EGR is actually the EVP (EGR Valve Position Sensor).

I hope that this helps! :thumbup

 

Thanks man good info ! this helped me out a lot.:rockon