Greetings. Have a 2010 Impala 40,000 miles.The smell from the exhaust has been there for about a year. GM has replaced the cat,checked fuel trims and cannot figure outwhy the smell is so bad. Tried different fuels and different octanes. Can not park car in garage because of the terrible smell when engine is at operating temp. Any clues? Could a fuel injector be leaking.? Not getting any codes.. thanks.

 

https://www.google.com/search?q=exha...tten+egg+smell

 

The rotting egg smell is an indication of excessive NOx and Sulfur emissions indicating that you are running lean in one or more cylinders. The catalytic converter should remove some of this but it does definitely indicate a hot combustion chamber due either to a lean condition or a bad EGR valve if the 2010 have them or not...not sure. Gasoline will produce greater NOx emissions as cylinder temperature increases.

A leaking or clogged injector could cause a lean condition as well that the O2 sensor would not pick up on necessarily. If the fuel trim is high, then this could indicate that the ECU is attempting to make up the difference of a clogged or leaking injector by putting more fuel to the other injectors. It would definitely have more symptoms than just a rotten egg smell though. I would pull your plugs to see if they all look the same. The hot cylinder would have an ashy look while the other cylinders would either be golden brown or even black and or wet if they are running rich from an increased fuel trim. On the other hand a rich condition would increase the output of sulfur that was not oxidized in the process of combustion and that could definitely cause the problem as well. I would start with pulling the plugs, which the dealership should have done but you would think they know what they are doing...

 

If it is not throwing any codes, you may want to start a different approach. I had a Chevy pickup that was not throwing any codes but was running (barely) so lean the exhaust would make your eyes water. Like Jaybird1189 said the fuel trim could be trying to compensate for the problem. With the pickup I had, (it used throttle body injection) I brought it to the dealer and they told me since it wasn't throwing any codes that the computer must be bad. So they replaced it at a cost of over $ 300. It somewhat worked ok for about a month then would refuse to keep running as before but with no codes being thrown. I started with basic troubleshooting. I put a gauge on the fuel line to check the pressure. It was almost non-existant, under 5 lbs. I replaced the in-tank fuel pump and the engine started running normally, although it was on the rich side at first until the fuel trim returned to normal. Also check the fuel pressure regulator while checking the fuel pressures, they have been known to cause some strange problems.