The same carbon build-up in the intake manifold that Phil pointed out is also within your plenum. You can spend time with various chemicals trying to clean it up and who knows??...you may have some success.

If you go this route; on first start-up you'll likely see tons of white smoke that some would identify as a blown head gasket. Not to fear. It's just all of the intake cleaner that's exiting the engine. It will take awhile for it all to burn off.

Be sure to remove the IAC(idle air control) valve - Haynes manual - page 4B-12 photo 9.48 from the throttle body after the TB has been reinstalled and cleaned. Lots of the same stuff you're seeing in the intake manifold. This effort may make for a smoother idling motor.

The o-ring gasket parts numbers are:

1-09623-130-0 & 9-09561338-0

The big water pipe has 2 connections at the back of the motor - 2 gaskets needed here. The part number is:

8-94364-756-0

These part numbers are from a 95.0. Suspect yours will be the same.