So when I bought my car whenever I got on the throttle hard, the cabin would get an burning oil/gas/exhaust type smell. For some reason as I slowly replaced/upgraded parts that had to do with making the car run better the smell has lessened over the years but still faintly exists. The two things I decided to check are exhaust leaks (not done yet), and checking out the oil filler neck and cap. I removed the cap and noticed only plastic threads were there. Am I missing a rubber seal or ring?

 

I do believe there should be an o-ring there

is that sloped section a flattened o-ring? Kinda looks like it

 

Mine's a 5.0, but it does have an o-ring.

 

My 2006 4.2 NA has the same as in the photo posted. THe cap body is hard plastic and the sloped "O" ring is of firm rubber and the slope matches the camfer on the oil filer.

 

Turns out the sloped part in pic is the rubber gasket mashed into that shape. Got a new cap and tube combo on ebay. Couple people are selling the genuine parts for very reasonable price. The filler neck gasket was in the same shape along with a crack in the plastic. I thought my new mufflers were the cause of my mushy throttle response and was disappointed my tuner couldn’t do more but it was a vacuum leak. Throttle response is best I’ve had it so far.

 

Thanks for posting the outcome. Many people don't and the thread slowly dies.