ok so i have thrown a bunch of money at this problem and still cant figure it out.. both my father and I are mechanics (he has been a truck mechanic for 30+ years).. here is what we have so far.. a 1985 ford f150 with a 5.0l v8 fuel injected (dual tanks), it is a 4spd 4x4.. The problem is as follows. the truck will start and run fine but after about 20 minutes of continuous driving (takes less the hotter it is outside).. it will stumble and act as if it is running out of fuel (inside the truck it feels as if someone is popping the clutch over and over again).. i hooked up a fuel pressure regulator to look at the fuel pressure when this is going on and the needle will show anywhere from 35-40 psi when it is running fine and <20-25 when it is stumbling and jerking the car around (running out of fuel).. now when this happens i usually pull over, as i do not want to cause a accident, and if i try to restart it it will idle very rough (barely idle) or wont run at all.. so far i have changed both in tank pumps, cleaned tanks, blew out the lines with air, changed boost pump on the fuel rail, changed both fuel filters, changed the fuel presure regulator, the line from the boost pump to the fuel rail, changed the ignition control module, spark plugs, spark plug wires, cap, alternator (for other reasons), exhaust (its same as stock, but all new).. i honestly dont know what else it can be.. it is obviously running out of fuel.. of course there is gas in the tank etc..any help would be appreciated

UPDATE 8/22/09
ok so i changed the tank switch valve (which i learned had a check valve in it) and cleared out the return lines.. and the problem is the same.. but now it appears to be a fuel pressure regulator problem (i already changed the fuel pressure regulator).. it barely idles, pours black smoke out of the exhaust but has no problem running up to 6k rpm. (still pours black smoke).. i swear to god if i dont figure this out soon im going to fill this thing with a couple thousand pounds of dynamite and push it off a bridge and blow it up at the same time

so far i have changed
-in tank pumps
-boost pump
-map sensor
-fuel pressure regulator
-spark plugs
-spark plug wires
-spark plug cap
-air filter
-fuel filters
-some parts of fuel lines
-ignition control module

i have done / tested
-blew out fuel lines
-ran codes (nothing)
-checked injector pulse (good)
- checked vacuum (good)

what the syptoms used to be
- would run, drive and idle fine especially on the freeway. if i was on the freeway for more than 10 minutes and got off, the second i would hit a red light it would sputter and jerk the truck back and forth
-the hotter it got the more likely the problem would be to happen

symptoms it shows now (might post a video later)
- will start but barely idle (200-250 rpm)
- puke black smoke at any rpm

 

I don't see where you changed the injector(s)...possibly an injector problem? Any codes? I had a similar problem on the wifes car after checking all that stuff & it is a bad intake valve...bummer for me.

 

Suggest cleaning MAF sensor if equiped or replacing the MAP sensor. They control the amount of fuel to the injectors according to air flow or density. You have replaced the pressure regulator already, but pull off the vacuum line and see if there is presence of fuel on that side of regulator. (leaking internally)

 

Did not see that you checked the items that I mentioned in your previous thread.

 

replaced the MAF sensor, replaced the regulator, pulled off the line and there is vacuum

 

"replaced the MAF sensor, replaced the regulator, pulled off the line and there is vacuum"

Was actually looking to see if there was fuel present in the vacuum chamber of the regulator. Should be no evidence of fuel.

 

I'd suggest going to library & ck ALLDATA for symptoms
print out flow chart & save money on pts

 

How about a coil pack