Ford explorer won't start

I have a 1997 Ford Explorer with the 4.0 Liter SOHC engine. Sometimes it does not want to start and the only way the car starts is by spraying starting fluid into the air intake while the engine is being cranked. What could be wrong?

The next time you go to start it, turn the ignition switch on so the dash lights come on for two seconds and then turn the ignition switch off. Repeat this a half dozen times and then try starting the engine. If the engine starts right up there’s a problem with the anti drain-back valve in the fuel pump assembly.

Tester