Assuming the issue is an air leak, the sources to consider are:
- The vacuum nipples located in the middle of the plenum; one on each side.
- The plenum gasket
- The IH gasket
- Injector "O" rings
- TB gasket
- TB Idle Air Control (IAC) malfunction
- TB main throttle plates ajar
- Power brake booster
- Fuel pressure regulator
- PVC evacuation line
- Evap canister evacuation line
With exception of the gaskets and "O" rings little disassembly, this should help:
1990 FSM, but later models are substantially similar:
