Exhaust System Backpressure
WARNING: This page does not describe the selected car, but rather 13 other vehicles, including the 1998 BMW Z3, 1998 BMW M3, 1998 BMW M Roadster, 1998 BMW 740iL, and 1998 BMW 740i. However, it is still accessible from the selected car via links, so may be relevant.
The exhaust system can be tested with a vacuum or pressure gauge. If using a pressure gauge, remove HO2S or air injection check valve (if equipped). Connect a 0-5 psi pressure gauge and run engine at 2500 RPM. If exhaust system backpressure is greater than 2 psi, exhaust system or catalytic converter is plugged.
If using a vacuum gauge, connect vacuum gauge hose to intake manifold vacuum port. Start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge indication slowly drops after stabilizing, inspect exhaust system for restriction.