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Discussion Starter · #1 · (Edited)
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.

Another suggestion is to remove the O2 sensor in front of the Cat (which will allow the exhaust gases an avenue of escape) and see if the engine runs better. If it does, the Cat is plugged.

Plugged Cat test with pressure guage...courtesy of Poppy
10 Posts
alot of times we drill a small hole in the pipe just before the converter

symptoms of bad conerter include: ongoing rotten egg smell for some time periodm weeks or months,

overheating/glowing/red hot converter

stalling dying hard starting after warmed up/being driven, etc,

but starts/runs fine when engine is cold.(bad converter swells up internally and thus blocks exhaust flow horribly)
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