http://www.asashop.org/autoinc/july97/techtotech.htm

Quote:

My other converter test requires extreme caution. On a questionable converter, I will cause one cylinder to misfire by disconnecting and grounding a spark plug wire. While running the engine at 2,000 rpm, a helper monitors the converter shell temperature. A good converter will heat up very quickly, usually 800 degrees Fahrenheit on the outside in less than two minutes. The 800-degree outside temperature usually indicates the core has reached 1200 degrees, and that's plenty. Too hot for too long is going to melt the converter substrate. This test has two functions - first, it confirms the converter is working, and secondly, it "cleans" the converter of contaminating coatings. The converter can become coated with sulfur and/or carbon, usually from older drivers that drive short trips at low speeds. We have some elderly customers who never achieve closed loop. Every year we have to change their oil to remove the fuel in the oil and light up their converter to pass emissions. Many techs advise the highway burn out for the converter, but we have found the misfire trial by fire a better solution. Be careful!