How to do a Cat Check

[adanac]

Found this, not sure if the process is sound. Think we need other opinions.

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 may not be the most accurate way to determine if the converter is working or not.


One thing temperature measurements will tell you, however, is if the converter is working too hard. An infrared noncontact pyrometer 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 then 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 unburned 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.
A common external clue of overheating to look for is a badly discolored or warped converter shell.

Catalytic Converter

[jwtimme]

I have seen exaust guys drill a hole in front of the cat and behind the cat. They hold their hand over the hole if the flow is about the same then the cat is good if the flow is diminished in the hole after the cat the cat is bad. Then just weld up the hole if everything checks out.

[Bear River]

That does not tell you if you have a bad cat, that is just one test of many. The real test involves using a pressure gauge to check actual backpressure. That also only detects cats that are plugged. Most modern OBD-II converters do not plug, but fail in other ways.

Thermal testing of the cat can be a quick and easy way to verify some things, it can teel you if a cat is overheating, etc. But a lot of applications will have little or no temperature difference before and aft. That is what can make the test inconclusive. To use the temperature test to its fullest advantage, you need to know how much HC, CO, O2, and NOX is going into the cat and how much is going out, combined with the temperatures. This requires equipment that most exhaust shop do not have.

The process you show is sound, but not conclusive. It mostly pertains to cats that are overheating, but cannot make a full determination of a cat that is weak.