Are you sure it's not coming from the exhaust pipe. The rotten egg smell (hydrogen sulfide) is a by product of the catalytic converter working on the exhaust gases. you may have a leak behind the cat if it is coming out from under the car. Have you installed a cat-back system, if so check for proper fit at the connection to the cat, and at the axle back section.
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Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming ...."WOW! What a ride!!!!!"
LUNCH with THEOLDMAN...On a break for now...