Throttle body icing up, what to do?

Throttle body icing up, what to do?

I have a 1998 Ford Expedition 4.6L with 241K miles.

This winter I've started having a throttle ice up issue. After driving for 15-20 minutes (usually freeway driving) ice builds up in front of the throttle butterfly and holds it open or freezes it in position at 1500 to 2000 RPM. I've noticed it happening 4 or 5 times, I pulled the air intake off and found a bunch of ice build-up in front of the the butterfly on the bottom of the throttle body. Once I shut the engine down and wait a few minutes the ice melts and the problem is alleviated.

I often get moisture inside the oil cap for as long as I can remember but I'm not losing any significant antifreeze. I wipe out the cap every time I open it.

Last week I tried a can of sea foam throttle spray in the throttle body, this didn't appear to help. The outer cylinder and the front of the butterfly are clean, the back side of the butterfly and deeper into the throttle appear to have carbon build up (but this is not where the ice is forming).

This weekend I pulled the PCV after the car had sit overnight, it was all ice'd up. I replaced the PCV and also put some cardboard in the grill to see if I could keep the engine a little warmer, after 150 miles the problem hasn't happened again yet.

I think the moisture is coming up the small tube connecting the driver's side of the engine block up to the front of the throttle body because that tube was full of the white gunk.

I've found a Ford TSB 01-21-5 that calls out a "PCV service kit" for this problem, but I don't know what this kit is and I can't find a picture of it. Sellers want anywhere from $218 to $2K for this kit.

Do you have any suggestions for remedies I may try? Can you tell me what the TSB 01-21-5 service kit is and if it's worth giving it a shot for this problem?