Originally posted by LarryBible
The easiest way is the old fashioned vacuum gauge method. With vacuum gauge attached, rev the engine to high RPM while watching the gauge. A clogged exhaust system will show the vacuum go way up then fall back very low in just a few seconds.
This is incorrect. What LB descibes above is NORMAL vacuum gauge action for a well balanced engine
ASE A8, Engine Performance manual, page 60, reference states:
"To check for a restriction in the exhaust, attach a vacuum gauge to the intake manifold and run the engine at approx. 2000RPM, observe the vacuum gauge as the engine is allowed to quickly decelerate.
If the exhaust is FREE of restriction, the vacuum gauge reading will increase immediately upon closing of the throttle, then settle down to a normal reading. If there IS a restriction, the needle of the gauge WILL NOT show an increase."
This is logical if one sees the engine as a air pump that happens to have gasoline pouring into it, the action of the pistons creates a movement of air at the intake ( vacuum) and pushes it out of the exhaust ( pressure) if the exhaust is plugged or restricted, then the vacuum will not increase.
Another way to check for same is to attach a pressure gauge at the after cat O2 sensor port ( if so equipt), a normal reading will be around 2psi at idle and 3 psi at 2K RPM.