A question that seems to arise quite often in the shop and in discussions with other techs is, "How much CO (carbon monoxide) and HC (hydrocarbons) can a catalytic converter handle without damage?" Also, how do we effectively test a converter? We decided to test some of these questions on my 1984 Ford van, with a 4.9 liter engine with a feedback carb. We made a test hole in the exhaust pipe just ahead of the converter to get the before-converter gasses. I don't recommend drilling holes in exhaust pipes anymore because many pipes have special anti-corrosion coating and/or pipe-in-pipe configurations.

One of the most important gas analysis tests to run is the cold start test. This test gives a good indication of fuel control and engine/tune-up condition. This test can be run from full cold start or rerun at any engine temperature after the converter has fully cooled down (usually 20 minutes). Note that the before and after converter readings in this chart are the same, because the converter hasn't reached light-off temperature. So, what do we see from the first gas readings? The HC readings are pretty reasonable, so no major misfire exists. The O2 readings are high, and without misfire we can assume that the O2 is from a good air injection system, an exhaust leak upstream or a leak in our sniffer hose. It is difficult to be sure about all the gas levels because they are being diluted by the introduction of O2 into the exhaust.

Table #2 shows the same data with the air pump disconnected and the lines plugged. I prefer to disconnect at the air pump and plug the lines rather than disable the pump using diverter valves, as they often have small internal leaks. Also, the disconnect and plug technique requires less effort to understand the air injection system operation. So, what do we now know? Well, the O2 dropped to a reasonable level, so we have no O2 leaks and we know the O2 was from the air injection system, so it's working well. We also see that the CO percentage is fairly reasonable for a cold engine and HC levels are good for a cold engine. CO2 is reasonable for a cold engine. From these readings, we can assume that there is no outrageous fuel delivery problem, the engine is in reasonable tune with no major misfires and the engine mechanical condition is probably OK. My brother's 1988 5.7 L TBI Chevy truck runs 0.5 percent CO, 75 HC, 13.5 percent CO2, and 1.0 percent O2, 30 seconds after starting. His converter never shows a higher outlet temperature under no load in the shop; no wonder there is almost nothing for it to catalyze.

The next step is to define a baseline standard with the engine at operating temperature. It's a good idea to watch the gasses during warm-up to detect any abnormalities that may occur. The before-converter gasses in table #3 are all pretty reasonable. The after-converter changes in the gasses show the converter has lit off. Note the change in front to rear converter temperature of 30 degrees Fahrenheit - not a huge change, but enough to show the converter is working.

In table #4, we drove the fuel trim rich by disconnecting the MAP sensor - you can see the decrease in the CO2 percentage and the reduction in O2. The converter handled the increase of CO without a great deal of thermal stress. Keep in mind that these readings are no load; with greater load, these reading would surely increase substantially. By pumping the accelerator, we managed to reach a maximum of 10 percent CO - the converter still had no problem cleaning the tailpipe to almost zeros on HC and CO.

Table #5 shows the gas results with one cylinder misfiring. The before HC rose substantially as expected; also, the CO went up as a result of the O2 sensor interpreting the higher O2 (from the misfire) as a lean mixture. We aborted the test at 2,500 RPM when the converter shell temperature went over 800 F. The converter internal temperature was probably well over 1200 F at this point. The converter was doing a good job of cleaning the gasses as evidenced by the before and after; however, it was starting to suffer some major thermal stress.

The O2 increase before the converter responded as expected. A four-cylinder engine with one cylinder misfiring will emit 25 percent of the O2 taken in, a six cylinder about 19 percent and an eight cylinder 12.5 percent. So, if the ambient air has 20 percent O2 and this engine emits 19 percent of that, then 20 percent multiplied by 0.19 equals 4 percent. The O2 baseline was about 0.7 percent before the converter; add the 4 percent from the misfire and the O2 should equal 4.7 percent, as it does.

It appears the converter was capable of converting a great deal of CO without too much stress. This makes sense because there is only one carbon atom per molecule to oxidize. The HC molecule is made up of many carbon and hydrogen atoms requiring greater catalytic action and therefore much greater thermal output. This demonstrates the damaging effect of misfire on a converter. We'll do some loaded dyno testing later to see where CO really starts to stress the converter thermally.

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