From aa1car.com.....
EXHAUST BACKPRESSURE CHECKS
To diagnose a plugged catalytic converter, you can check intake vacuum or exhaust backpressure. To check intake vacuum, connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading at idle. Then increase engine speed to about 2,500 rpm and hold steady. Normal vacuum at idle for most engines should be 18 to 22 inches Hg. When the engine speed is increased there should be a momentary drop in vacuum before it returns to within a couple of inches of the idle reading. If the vacuum reading is lower than normal and/or continues to drop as the engine runs, it probably indicates a buildup of backpressure in the exhaust. Remember, though, that intake vacuum can also be affected by retarded ignition timing and valve timing. What's more, some engines are much more sensitive to small changes in intake vacuum than others, so checking backpressure rather than intake vacuum may give you a better indication of what's going on.
Checking backpressure requires connecting a pressure gauge to the exhaust system. Use a gauge that reads up to 8 to 10 psi and is calibrated in 1/2 inch increments. Or, use a metric pressure gauge calibrated in kilo-Pascals (kPa). One psi equals 6.895 kPa.
A backpressure gauge can be connected to the exhaust system one of several ways: by removing the oxygen sensor and connecting the gauge to the hole in the exhaust manifold; by removing the air check valve in the air pump or pulse air system and connecting the gauge here; or by drilling a small hole into the head pipe just ahead of the converter to attach the gauge (never drill a hole into the converter itself!). One drawback of drilling a hole is that the hole will have to be plugged by a self-tapping screw, plug or welded shut after you've taken your measurements. Also, drilling is not recommended if the head pipe has a double-wall construction.
Once you've made your connection, start the engine and note the backpressure reading. Depending on the application, the amount of backpressure that's considered "normal" will vary. On some vehicles, backpressure should read near zero at idle, and should not exceed 1.25 psi at 2,500 rpm. Others can handle 0.5 to 1.25 psi at idle, but should have more than 4 psi during a snap acceleration test.
If you find a relatively high backpressure reading (say 8 to 10 or more psi), there's obviously an exhaust restriction that will require further diagnosis. Don't jump to conclusions and assume the converter is plugged because it might be a collapsed pipe or muffler.
One way to rule out the pipes and muffler is to visually inspect the exhaust system for damaged components. Another way is to drill a small hole in the pipe aft of the converter and check backpressure here. If the reading is lower (or is less than about 1 psi), the rest of the system is OK and the converter is what's causing the restriction. Or, disconnect the exhaust pipe aft of the converter. No change in backpressure would indicate a blockage at or ahead of the converter. If backpressure drops back to normal, the problem is not the converter but a collapsed pipe or muffler.
If you suspect the converter is plugged, you can disconnect and remove it. Then hold a shop light by one end of the converter and look in the other end. If you can't see the light shining through the honeycomb, the converter is plugged and needs to be replaced.
You can also recheck backpressure readings with the converter removed. If readings are at or near zero, you've found the problem. But if backpressure is still high, there's an obstruction in the head pipe or manifold. Sometimes a collapsed inner tube inside a double-wall head pipe will create an obstruction that acts just like a plugged converter. Another cause can be a heat riser valve on an older V6 or V8 exhaust manifold stuck in the closed position.
