Help Tracking Down Rotten Egg Smell

[My Mighty White 95 Z28]

Hi everybody, for a while I have been trying to track down this rotten egg smell I notice sometimes when I drive my car. When my car was a A4 I only notice the smell when I floored the car and when I got to a stop light, the smell would araise. When I converted my car over to a M6 I notice it when I cruise at high rpms and when I get to a stop the smell araises. So today I decided to get my cat replaced with a new one that I bought. The converter I bought was a carsound cat. I though it would fix the problem but apparently the smell is still there. Also last week I replaced my spark plugs. The plugs look good, no signs of the car running rich. So now I am out of ideas. I had a buddy scan my car, and he was not able to see anything wrong. My car use to be a dual cat but I converted over to a single cat. Would programming be required for this? Also my friend loaded up a M6 file after the T56 swap. The O2s have been replaced. Would a bad EGR cause this or a bad fuel injector? Or would I have to change out my opti? Any help would be apprecaited. Thank you in advance for your time and help.

[N20LT4Bird]

The car is running rich. What are your oxygen sensor readings? The most common cause for this is a bad O2 sensor. It could be caused by a bad injector, one that is stuck open. The PCM will compensate for an overly rich mixture unless: a) it has erroneous info from the O2's b) the output (fuel inj) are not doing what they are supposed to c) the PCM is faulty d) the problem is so severe that the fuel trim is not capable of compensating.

I would take a hard look at the O2 sensors again......

[Performin Norman]

Check the battery also. Overcharging can cause it to smell like that.

[Joe 1320]

Possibly the charcoal canister used to trap fuel vapors may be kaput.

[Injuneer]

Try a different brand of fuel. Excess sulfur levels can cause the sulfur to be converted by the cat to sulfur dioxide = rotten egg smell (actually, its nowhere near "rotten eggs".... my science teacher kept a rotten egg in a jar, just in case anyone wnated to debate the issue )

[Bond Hawk]

Shell has one of the highest sulfer content of major gas brands.

[and11200]

I had the same problem on my Cutlass. It was caused by a bad cat. conv.
Try all the other suggestions too and your car will be back in perfect,.....smelling condidiotn.

[My Mighty White 95 Z28]

N2OLT4Bird, you mention about checking the readings of the o2s. I was wondering what should I look for when I am scanning my car. The scan tool I am using is Diacom. Also what else should I look for? Should I do the test at wot or just regular driving conditions? Injuneer, I have tried different brands of fuel but I have stayed with the highest octane and I am still having the same problem. I just replaced my cat yesterday with a brand new carsound cat. Thanks for the help guys I really appreciate it.

[N20LT4Bird]

N2OLT4Bird, you mention about checking the readings of the o2s. I was wondering what should I look for when I am scanning my car. The scan tool I am using is Diacom. Also what else should I look for? Should I do the test at wot or just regular driving conditions? Injuneer, I have tried different brands of fuel but I have stayed with the highest octane and I am still having the same problem. I just replaced my cat yesterday with a brand new carsound cat. Thanks for the help guys I really appreciate it.

You want to make sure both O2's are bouncing around between .1 and .9 volts or 100 and 900 millivolts, depending on how your scanner displays it. They should be switching back and forth constantly, especially under varying throttle positions. If the O2's are stuck up in the .8-.9 range, then it is running too rich. If thats the case, you need to determine if you have a problem such as the stuck fuel injector you mentioned earlier. If the sensors are stuck at .1 or .2, it may falsely be driving the system rich, and in this case you need a new sensor(s).