I'll chime in here with another case of the bypass port cap failing on a Dorman replacement intake manifold. I have a 2002 Limited 4.6L; I replaced the intake manifold in September 2013 and have put less than 10,000 miles on the truck since then. I was driving it on Tuesday and floored the throttle briefly; when I backed off, I immediately smelled coolant and noticed a copious amount of steam behind me. I pulled over and discovered coolant pouring out of the bypass port in the right rear; the cap had split open most of the way around, just above the spring clamp. When I removed it, I noticed the whole cap was pretty deteriorated, both inside and outside - similar in appearance to dry rot on a tire. It was enough to make me wonder if the cap was only intended for vacuum or something like that, rather than exposure to coolant.
The auto parts store was closed at the time, so I went to the farm and home and bought a short piece of 3/4" heater hose, some hose clamps, and a 3/4" bolt to use as a plug on the other end of the hose. So far it is doing the job; I have ordered a package of bypass caps of assorted sizes, but I don't know that I'll bother replacing my homemade cap unless it starts to leak.
While I was typing this, I just remembered that I installed a Dorman manifold on a friend's van with the 5.4L a few months after I did mine. I don't recall if it has a cap or hose in that location, but this makes me think I should probably check!
