I have a 2006, 5.4l. Went to change the oil the other day and there was a milky/yellowish sludge on the underside of the oil filler cap, along with some rust-like residue, and the rust-like residue was also on the inside of the oil filler neck. There also appeared to be steam coming from the filler neck.

When I drained the oil, it didn't seem to have water in it.

Think it's a head gasket?

 

Might have been a headgasket in the past and it was fixed. Are you loosing coolant?

And if the oil looked pure I bet its fine.

I have seen goo on filler caps before could just be a result of bad oil or too far between changes.

Hope for the best cause it sounds like its fine.

 

It doesn't appear to be loosing coolant. I've had a couple of times in the past year and half where I went too far between changes, including this last time.

 

Kinda sounds like normal condensation maybe..Hopefully.

 

This is a common issue on all vehicles with plastic oil-fill tubes. The plastic ends up gathering condensate and unless you operate your vehicle to full operating temperature everytime this condensate forms to flash off the water, you will inevitably have a yellow sludge on your cap.

In other words, if you are not losing coolant and your oil is clean, then you have nothing to worry about!

This is common in this generation F150s and mine will consistently have a small yellow sludge buildup when I do not operate it to full temp. However, I notice when I always operate it to full temp prior to shutting down I never get buildup. My truck has 179k and no issues.