FWIW - here is GM's procedure to check for an exhaust restriction:
EXHAUST SYSTEM RESTRICTION CHECK
MODELS W/AIR INJECTION
Run engine until it reaches normal operating temperature, then stop engine.
Disconnect and remove check valve from air injection tube or manifold.
Connect suitable pressure gauge to nipple from propane enrichment tool J-26911 or equivalent, then insert nipple into injection tube opening, Fig. 32.
Start engine and run at 2500 RPM while observing gauge.
If gauge reading exceeds 2-3/4 psi, inspect entire exhaust system for collapsed pipes, heat distress or possible internal muffler failure.
If all other components are satisfactory, replace catalytic converter.
MODELS WITHOUT AIR INJECTION
Run engine until it reaches normal operating temperature, then stop engine.
Connect suitable vacuum gauge to any convenient port to direct intake manifold vacuum.
Disconnect EGR solenoid electrical connector, if equipped, or reroute hoses as needed to connect EGR valve directly to vacuum source, bypassing any switches or solenoids.
Run engine at 1000 RPM and record vacuum reading.
Slowly increase engine speed to 2500 RPM and record vacuum reading at a steady 2500 RPM.
If vacuum reading at 2500 RPM decreases more than 3 inches Hg from reading at 1000 RPM, check for exhaust restrictions as outlined for models with air injection.
If no restrictions are found, repeat check with exhaust pipe(s) disconnected from manifold(s).
If vacuum readings still differ by more than 3 inches Hg, check valve timing. On models without air injection or EGR, connect suitable vacuum gauge to intake manifold vacuum, and observe gauge with engine idling at normal operating temperature. If gauge reading regularly drops to zero, check exhaust system for restrictions and repair as needed.
