Test Method (Vehicle Parked):

Remove the oxygen sensor.
Insert Gauge Bar Set SA9127E into the exhaust manifold. The thread designation is M18 x 1.50.
NOTE: Do not use the 0-60 in. H² O gage when performing this test, as it will be permanently damaged due to the excessive back pressure during accelerations. Use the 0-690 kPa (0-100 psi) gauge and appropriate hoses in service gauge bar kit SA9127E.
Connect the gauge bar set, 0-15 kPa gage (0-60 in. H² O to the back pressure adaptor.

Fig 1: Testing Exhaust Back Pressure
Courtesy of GENERAL MOTORS CORP.
With the engine at normal operating temperature, observe the exhaust back pressure at 4000 RPM.
NOTE: If the exhaust pressure exceeds the specification, inspect for a dented or collapsed exhaust pipe. It may be necessary to disconnect the exhaust pipe before or behind the converter and repeat Step 4 . If removing the muffler does not lower the back pressure, disconnect the three way catalytic converter from the front exhaust pipe and repeat the test.
The exhaust back pressure must be less than 5.0 kPa (20.13 in. H² O).