Here's a combination I have not been able to find an answer for.
2003 F150 Supercrew 4x4. 121,000 Miles, After market catback, unknown make or age.

Exiting after about 1.5 hours of freeway driving coming to a stop, windows up ac on. The cabin is deluged with an intense rotten egg smell everyone can hardly breathe. Stop the truck, cannot detect the smell on the outside, under the truck, in the engine compartment, tail pipe. Nothing but hot engine driving smell.

Also noticed I was getting a HORRIBLE gas mileage on the trip (about 50% from normal). But, the truck didnt seem to be running rough, and no noticeable loss of power, no check engine light, all gauges register normal (except the plummiting fuel gauge).

Thought it might have been the car in front of me.
But No. Today, lingering smell, not as intense. Only smell it with the windows up and I come to a stop. Again run around, smell nothing from the outside, engine, or tail pipe.

The only other variable here is that I noticed I was running in 4wd mode for about 30 minutes at the beginning of the trip at 65 mph and switched it back to 2wd (yes at 65 mph). I later read that you shouldn't change the 4wd to 2wd above 45, but everything seemed ok, light went out, major clunk or anything).

Ive read read possible Cat Back - but I dont smell anything from the exhaust,
Read possible O2 sensor, but no check engine light.
Read possible rat in the cat, but no smoke and no smell

Anything else before I take it to the dealer and have them charge me $$ to say I dont smell anything!

Thanks
Ray

 

Might be your cats I think they usually smell and can contribute to bad gas mileage. or a exhaust leak lol I have no idea. sounds like an interesting problem though someone will chime in soon

 

If it's rotten eggs, your cats are bad or WILL be bad soon from running excessively rich.

 

Cheap gas with a high sulfur content will give you a rotten egg smell. Couple that with an exhaust leak where the aroma can be sucked in the vent might be possible.

How is your charging system, if you are over charging the battery it can give a similar but more pungent odor. That one is just a shot in the dark as a possible-maybe.

Or get a different air freshener, I personally haven't seen the rotten egg aroma, but it might exist.

 

Everything points to an exhaust issue. You say aftermarket. I'd suspect that might have something to do with it.

 

You using Costco or Arco gas?

 

Typicaly use Costco Gas, but have for years and its not in any of my other cars

 

Im pretty sure its the cat, but do they just go out (?) within an hour and would it smell out the exhaust?

 

Costco and Arco in California are both crap gas.

 

If the honey comb guts of the cat broke up it would clog the exhaust and give you crap for performance. I big bump could of been the last straw. smack the cats and see if they sound like BBs in it. if it does they are toast. If your truck is from California you have 4 cats. crap from the first cat can end up in the second sometimes. Check readings on your O2 sensors they may give you a better Idea of whats going on.

 

I believe all 50 states have 4 cats.

 

Crazy. talk about holding it back...lol