Check for intake air leaks       

• Visually inspect intake ducts and duct joints
  • tight clamps
  • no splits/cracks/holes

   

• Visually inspect all vacuum lines and connection points.  Here's a picture (for a 91, non-TCS)
  • no loose fits
  • no disconnected hoses
  • no broken plastic nipples
  • no splits/cracks/breaks
  • check all vacuum connections
    • turbo bypass line
    • turbo boost gauge line
    • feed line for both of those, from manifold
    • PCV lines
    • fuel pressure regulator line
    • evaporative canister line
    • vacuum take-off points at the manifold 
      • check the rubber grommets for pliability and a good seal
        • a bit of lubricant helps

       

• Check the turbo bypass valve
  • Disconnect the vacuum line from the tee
  • suck on the line
    • no resistance = bad valve

NOTE: These are my notes from a generic fuel injection book.  I have not done all of these procedures, but they sound reasonable to me.  Corrections and updates would be appreciated.

• Perform propane-injection leak test (if nothing found visually)
  • First, disable the Idle Speed Stabilizer by clamping the line or disconnecting the electrical connector
    •  (this is true for '91 and later; not sure about earlier)
• Fit a piece of tubing to a propane torch
  • Preferably, use several concentric sizes to step the diameter down to about 1/8" I.D.
• Perform the test with the engine cold so the cooling fan does not come on and disturb the air flow
• Start the engine
• Crack the valve on the propane torch to obtain a small flow of propane
  • Direct the propane toward any suspected leaks
    • An increase in RPM indicates a leak (Why?)
  • Don't forget to check the intake manifold joints and fuel injector seals