The Effects of Ethanol on a Catalytic Converter

A catalytic converter is a kind of self-sustaining blast furnace that converts unburned fuel, nitrogen dioxides and carbon monoxide into carbon dioxide, nitrogen gas, water and heat. The converter contains two ceramic blocks that look like a honeycomb when viewed from the end. The walls of the honeycomb passages -- called "microducts" -- are coated with a mixture of precious metals like rhodium, platinum and palladium, among others. When chemicals in the exhaust gases come into contact with this "washcoat," they react to swap atoms around in order to produce less harmful gases.

The popular misunderstanding is that ethanol's 30 percent energy deficit means a 30 percent richer air-fuel ratio, which means more unburned fuel flowing through the cat; and that supposedly produces more heat, which kills the cat. While the first part is true, the richer air-fuel ratio won't affect converter temperature any more than it will combustion chamber temperature -- which is to say very little. Energy content is energy content, regardless of whether it's spent in the combustion chamber or catalytic converter. So that myth is busted.

Engine oil isn't just refined dinosaur juice: It contains dozens of different additives designed to keep your engine clean, keep the oil stable at high temperatures and to reduce wear in the valve train. Wear-reducing additives usually take the form of soft metals and elements that impregnate into the metal to form a slippery surface. Phosphorous and zinc additives, known collectively as zinc dithiophosphate, or ZDDP, have long been the go-to additive for most oil manufacturers.

In 1996, the federal government dropped the allowable ZDDP content in standard engine oil from 1,600 parts per million to 800, and then dropped it again to 400 in 2004. If you know your car history, then the year 1996 may ring a bell, since it was the watershed year that the EPA made OBD-II and catalytic converters mandatory for all vehicles sold in the US. The Fed reduced ZDDP levels because the same properties that allow ZDDP to penetrate the metal in valve train components also make it deadly to catalytic converters. If these metals absorb into your converter washcoat, they'll coat the precious metals and render your converter useless.

Your engine's cylinders never achieve an airtight seal; at best, they'll always allow at least 1 percent of the combustion gases to escape past the rings and into the crankcase, where the oil is. The problem with ethanol is that it's highly reactive with phosphorous, combining with it to produce any one of several free-floating phosphor compounds. These compounds get sucked up through your positive crankcase ventilation system, sent through the motor and then make it to the catalytic converter. Once that happens, you're looking at converter poisoning not directly as a result of the ethanol, but the way that it interacts with the phosphorus in your oil.

Note the strategic use of the word "standard oil" two sections above. While the government did indeed cut allowable ZDDP content in standard oils to about a quarter of what it was 20 years ago, manufacturers can still get away with using huge quantities of the stuff in "specialty" oils. Racing oils often contain large amounts of ZDDP to cope with high valvespring pressures and steep cam lobes, which would conspire to induce premature cam and lifter wear without some sort of additive. But you're more likely to find high ZDDP levels in any one of the many "extended life" oils on the shelves today -- they're excepted from the government's ZDDP limitation by virtue of being specialty compounds. So standard oil plus ethanol: good. "Extended life" or racing oil plus ethanol: bad.