Catalytic Converter: Testing and Inspection

Restricted Exhaust System Diagnosis

A blocked or restricted exhaust system usually results in lack of power or popping through the carburetor. Check that the problem is not caused by ignition or timing conitions. Perform a visual inspection of the exhaust system. If the visual cannot locate the restriction, proceed with the following program:

1. Attach a vacuum gauge to intake manifold. Connect tachometer. Start engine and observe vacuum gauge. Gauge should indicated 16 to 21 inches of vacuum.

2. Increase engine speed to 2,000 rpm. Vacuum will drop when speed is increased rapidly, but should settle at 16 to 21 inches and remian steady. If vacuum drops below 16 inches, exhaust system is restricted. Stop engine.

3. Disconnect exhaust pipe at manifold. Start engine and increase speed to 2,000 rpm.
a. If vacuum settles at 16 to 21 inches, restriction is in exhaust pipe, catalytic converter or muffler. If vehicle is equipped with a catalytic converter, reconnect exhaust pipe at manifold, remove muffler and check vacuum gauge.
b. If vacuum drops below 16 inches, restriction is in the catalytic converter. If vacuum is normal, muffler is restricted. In the event of a converter failure, always check muffler to be sure converter debris has not entered muffler.

4. If the vacuum drops below 16 inches with exhaust pipe disconnected, exhaust manifold is restricted.
a. On six-cylinder engines, remove exhaust manifold. On eight-cylinder engines, remove both exhaust manifolds.
b. Inspect ports of exhaust manifold for casting flash by dropping a length of chain into each port. Don't use a wire or light to check ports. The restricted opening may be large enough for them to pass through but small enough to cause excessive back pressure at high engine rpm.
c. Remove casting flash. If flash is at lower end of port, it can usually be chipped out. If flash cannot be removed, replace manifold.

5. Reassembly exhaust system and remove test equipment.