Catalytic converters:- are one of the greatest emission add-ons ever to be installed on vehicles. By cleaning up the pollutants left over from combustion, they reduce tailpipe emissions of hydrocarbons (HC) and carbon monoxide (CO) to extremely low levels, when everything is operating normally, that is. But sometimes things do not operate normally, and when that happens engine performance may suffer or the vehicle may fail an emissions test.
Driveability symptoms such as a drop in fuel economy, lack of high speed power, rough idle or stalling are classic symptoms of excessive backpressure due to a plugged converter. Checking exhaust backpressure and/or intake vacuum will tell you if there's a blockage (more on this subject in a minute).
Elevated HC and CO tailpipe emissions, on the other hand, are often symptoms of a fouled converter or a faulty air supply (bad or leaky air pump, diverter valve or pulse air system). A fouled converter may not cause any increase in backpressure, so other methods of checking the converter are required for this type of problem (which we'll also get to shortly).
The important point to remember here is that converters don't just plug up or die for no good reason. There is usually an underlying cause which must also be diagnosed and corrected before the problem can be eliminated. Diagnosing a plugged or fouled catalytic converter is only half the fix. Replacing a bad catalytic converter will only temporarily restore things to normal because unless the underlying problem that caused the original converter to fail is identified and fixed, the replacement converter will likely suffer the same fate.
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 s**king 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.
CAUSES OF CATALYTIC CONVERTER PLUGGING
Prolonged overheating or short term severe overheating are the leading causes of catalytic converter plugging. The underlying cause here is often fouled or misfiring spark plugs, or a burned exhaust valve that leaks compression and allows unburned fuel to pass through the combustion chamber into the exhaust.