I have a 93 E-250 with a 4.9L six.
Yesterday afternoon, my wife was driving and it ran like complete crap, no power. It is very hot here lately. I changed the fuel filter and it did the same thing. Barely running and not driveable, with a couple stalls, fuel pump made an awful sound at one point. Went back about 2hrs later and I was able to drive it home, although it seemed sluggish.
Today it drove me 30 miles to work without a problem. I bought a $15 fuel pressure tester at Harbor Freight and attached it. 1st turn of the key the gauge got to 25psi, so I turned it again and it got to about 30-35psi and I started it. The gauge got to around 45-50psi and I drove it up and down the 45mph street. By the time I parked, the gauge read 38psi, and dropped to 25-30psi after idling for around 10 minutes.
I am thinking this is the pump going and getting weak with heat? Yesterday maybe it got so bad that the Van would barely run?
Do I need to drop the tank and replace the pump, or could this be a regulator?
I also got the computer scanned and all that came back was knock sensor. (i will have to look into that too, 'cause this thing feels like a dog sometimes)

 

I would vote for the pump being bad or the sock in the tank getting plugged. Regulators do not act that way.

 

I dropped the tank and put a new pump in Saturday. The tank was clean. The sock had a little dirt on it, but not bad. I'm still not certain the pump was the problem, but the new one is quieter and pressurizes the system quicker. Van has 130K on it.