1999 explorer start problem
 

Yes, another thread on an explorer that occasionally has trouble starting. One cold morning the fuel pump would not operate. When turning the key to run the volts to the pump (with plug disconnected) was 8.6 for a second, then dropped to the 6v range. I took the inertia switch from the firewall and back probed the wire going to the fuel pump, getting 12 volts. I went back to the fuel tank and measured the voltage with the key in run position, and read battery voltage. The car started, and has not failed in 3 days. I know it will quit again. I was thinking of replacing the inertia switch and fuel pump relay. Does anyone have more suggestions?

Based on the low voltage readings you originally measured I'd be looking more for a corroded connection or bad ground.

You mention that you "back probed the wire going to the fuel pump, getting 12 volts." Are you saying that all you did was unscrew the switch from the truck body (while still electrically connected) and then had battery voltage to the pump? If that's the case, I would focus my efforts on the connection to the inertia switch, the nearby ground connection, and any splices for that circuit in the area versus replacing the inertia switch or the fuel pump relay. There's probably a ground screwed to the body in that area that may be prone to corrosion if a drain for the sunroof gets unplugged, if the windshield leaks, or if a heater core leaks.

-Rod

After trying many times to get it to repeat the no fuel pressure condition to happen, it failed again. This time here was no pressure at the inertia switch to the pump. I then cycled the fuse in the distribution box that supplies the pcm and fuel pump relays. After several times connecting and disconnecting the fuse, the fuel pressure went normal. I replaced both relays, hoping this works and will remember what you advised if the problem is still happening. Thanks for the advice.

Fuel pump relay and Pcm relay are now replaced. Worked good for the last few days but started to act up again today when i started it and ran for couple seconds and shut off. I primed it up by turning key part way and it fired right up...

Sounds like you have either a sketchy ground or a corroded wire that needs cleaning and dielectric grease. Every time you turn on the switch, you are essentially forcing a 12v charge to the pump and it will try to jump the gap like a spark plug, but it's always going to look for the path of least resistance, which is why you were only seeing 6v. Look for a bad ground or just clean up the grounds near the tank.

The fuel pump ground behind the left kick panel shows good continuity. Should I check he fuel pump for specifications like amp here draw, thinking that he fuel pump could be going bad?