stumped newbie

[1stscrewcab]

Recently purchased my first F150 supercrew and absolutely love it. I had a GMC previously-yeah I am not sure where my head was. Anyway 2004 Supercrew Lariat 4x4. I recently noticed once the temp comes to half and I am sitting idleing I am getting a sour propane/natural gas type smell coming into the cab through the vents. Once I start moving again it is gone until the next slow down or stop. It does not occur until the temp gauge reads half. The truck seems to be running fine with no loss of power. I am thinking cat? The smell seems to change with differant additives like fuel injector cleaner. Local dealer is stumped or is not interested in solving. I love the truck too much to quit trying. Any ideas? Thanks for any input.
Rob

[FX4Matt-06]

Welcome to the site?

Would that be coolant smell,
Do you have any visual leaks?????

[1stscrewcab]

No, coolant is fine, no visible leaks. When I use the recycle button the smell goes away. Thanks for the response.

[Keeks]

Sometimes new exhuast parts can "stink" for awhile until everything is burned in. If nothing has been replaced, I would suspect the cats.

[1stscrewcab]

Thanks. Thats where my thoughts are pointing to

[loganskinner]

did you ever suspect your heater core? my buddy has an 04 and at 80K miles his HC went out. smelled like something was burning so at first we didnt have any idea what it was... never suspected the HC going out at only 80k. then slowly it started to smell like antifreeze so then we knew what it was.

[hillbilly4u]

My truck has a propane like smell to the exhaust but not in the cab. Sure you dont have an exhaust leak?? and its pulling it in the cab while you're sittin still??