Catalytic converter emitting foul odor

Question: I have a 2003 Toyota 4Runner V-8 with the standard automatic transmission, air-conditioning, and four-wheel drive. It has only 49,000 miles, mostly in the city. I purchased it new, have regularly changed the oil, have had the timing belt and serpentine belt replaced, as well as the water pump and whatever else was required at the time the timing belt was replaced.

The vehicle runs perfectly. It was slow-rear-ended about six years ago, with the rear bumper, trailer hitch, and rear muffler then replaced.

Recently, I have smelled a hydrogen sulfide odor (rotten eggs) after the engine has been running about 10 minutes and the converter is obviously warming up. I went to my local emissions-testing station about two months ago as required for license renewal, but the technician said new regulations do not require measurement of tailpipe emissions, and he checked only the gas cap and under the hood (no changes have ever been made to the engine) and said I passed. I subsequently received the 2016 sticker.

The service person at the Toyota dealer told me that the odor indicated that the converter was failing and that I would have to replace it sometime in the near future. He had zero advice on how to extend my catalytic converter's life.

I looked on the Internet and noted that numerous companies market various products that can be added to gasoline that promise to extend a catalytic converter's life or even restore failed catalytic converters.

I don't know if their claims are possible, much less true, but want to extend the converter life as long as I can. I like the 4Runner and hope to keep it five more years.

Answer: Taking action may save your catalytic converter from premature failure. The odor you've noticed is caused by sulfur found in motor fuel being acted upon by the catalytic converter.

I disagree the converter is faulty, but it sounds as if the 4Runner may be running rich (excessive fuel delivery) or misfiring somewhat during warm-up. Either can shorten converter life or may seriously overheat and damage the converter - if it's reached operating temperature while the fault is present. Since you haven't mentioned an illuminated check-engine light, it appears the fault may be slipping beneath the on-board diagnostic system's radar.

I'd recommend having a sharp engine performance/emissions-savvy technician take a look at fuel pressure, fuel trims, and the possibility of a stuck open thermostat or contaminated/sluggish engine-coolant sensor causing lingering cold enrichment and delayed closed-loop operation.

My son's car did this some years back due to a clogged coolant passage. It started and ran flawlessly when cold and when at engine operating temperature, but in between ran rough and rich/stinky due to the lagging temperature-sensor reading. Checking the 4Runner's diagnostic system misfire records might also lead to a smoking gun.

I'm not sure if a tailpipe emissions inspection would have made much difference since the 4Runner would/should be at full operating temperature prior to testing.