1992 E350 460 RV with 45,000 miles vapor locked 3 times in last 40,000 mi. First in death valley at 117, next going down the pass @ silverton CO (10,000 ft and 95 heat), then in Ohio @ 104. Don’t know if there is a fix or if should just not drive in extream conditions?
There are thermal insulation wraps to cover the fuel lines with. Look to http://www.jcwhitney.com/autoparts/Search?catalogId=10101&storeId=10101&sku=vapor+lock&searchbtn.x=11&searchbtn.y=13 for one.
I doubt the problem is vapor locking. It’s more than likely related to the ignition module.
Most Fords of this era used the TFI-IV module and these were prone to failures. Heat is what does them in and summer temps make it much worse.
The symptoms can vary, but often resemble running out of gas or vapor locking. Sputtering, stalling, or just flat quitting; it varies.
Look at the distributor and if you see a small rectangular gray module on the side then that is the TFI-IV module. They’re relatively inexpensive and easy to replace.
If you do this yourself make SURE you use the special electrolytic grease that comes with the new module.
Do a net search for TFI-IV igniton module and you can read the details behind them.
To really solve the problem permanently, the best thing to do is mount the module to a piece of aluminum heatsink and mount it to the fender. This would require extending the wiring pigtail to reach.
I mounted the module on a heatsink and placed it inside the airbox on the last Ford I owned that used this module. Whenever the engine was running the incoming air kept it cooled and never had a problem after that.
You would really be surprised at just how hot that little metal plate on the back of the module gets. An egg (a small one) could be fried on it.
Hope that helps.