To diagnose a plugged catalytic converter, you can check intake vacuum or exhaust back-pressure. 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.
CONVERTER OPERATION
Under normal operating conditions, the converter should not have to work very hard to accomplish its job. If an engine has good compression, is not sucking oil down the valve guides, and the fuel, ignition and engine management system are all working properly, there should be relatively little HC and CO in the exhaust for the converter to burn (a few tenths of a percent CO and less than 150 ppm of HC when the engine is warm). In many late-model engines with multipoint fuel injection, combustion is so clean that the converter has little to do and the difference between the inlet and outlet temperature may only be 30 degrees F at 2,500 rpm - which is a lot less than the old rule of thumb that says a good converter should show at least a 100-degree F difference fore and aft at cruise. At idle, the converter in many late-model vehicles may cool off so much that there's almost no measurable difference in fore and aft temperatures. So checking exhaust temperatures fore and aft of the converter at idle and 2,500 rpm is NOT an accurate way to determine if the converter is working properly or not.
One thing temperature measurements will tell you, however, is if the converter is working too hard. An infrared non contact hygrometer or a temperature probe will tell you if the converter is running unusually or dangerously hot. If the converter outlet temperature is 200 or more degrees higher than the inlet temperature, it means the engine is running rich and there's a lot of CO in the exhaust that needs to be burned. A rich fuel mixture will often produce a "rotten egg" odor in the exhaust (the smell is hydrogen sulfide). Underlying problems may include an engine management system that is not going into closed loop (check the coolant and oxygen sensors, or for a thermostat stick in the open position), plugged PCV valve, or excessive fuel pressure (bad fuel regulator). High CO levels in the exhaust can also be caused by an inoperative air pump system.
If the outlet temperature is a lot hotter (more than 500 degrees F) than the inlet temperature, it indicates unburden fuel in the exhaust. The most likely cause would be ignition misfire (fouled spark plug, shorted or open plug wire, cracked distributor cap, arcing rotor or weak coil), or a compression leak (burned exhaust valve). But other causes may include lean misfire (check for vacuum leaks, leaky EGR valve, low fuel pressure or dirty injectors). A single misfiring spark plug can cause an increase in HC emissions of 2,500 or more parts per million, which can push the converter's operating temperature well above its normal range.
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