[comptiger5000]

True, if it's built with lower temps in mind, then it's ok (although ring wear is still a little faster, as cylinder wall lube isn't quite as good). However, the 4.0 wasn't built for that, and isn't meant to run that cold. They're toasty engines by nature.

It's tough to vapor lock an injection system, but given enough heat, certainly possible. There have actually been some issues with vapor locking the 4.0 in hot weather. Newer ones had a heat shield over the exhaust manifold runners where they connected to the head, as well as some shielding for the injectors to avoid this.