gnatnoop

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
- sounds like it is running on less that 8 cylinders (im going to clean plugs, to see if that changes)

01f1502wd

hows your battery? you might have a bad alt. i dont know much on the earlyier models..but bare with me

gnatnoop

front battery is perfect (brand new).. the alternator is BRAND new, this problem was happening before i replaced the alternator. it is a 200 amp powermaster alt. btw.. resting voltage on battery is 12.7 load tested good.. voltage when truck is running = 14.4..

01f1502wd

not sure how both of the tanks are hooked up. but could you cut one of them off (disconnect the wires/hoses etc) and run it off just one? your computer might be trying to switch between the two therefore starving it? (although im sure you have a switch in the cab, but feel free to kill this idea)
does it die or just chug when it does this?

Mike

overheating? my truck did that once when i overheated it

gnatnoop

that was going to be my next step, bypassing the tank selector valve. i do have a switch in the truck.
well after about 20 minutes of driving, it will chug and if i keep trying to drive it (which is near in possible as it jerks the truck pretty good) it will eventually die.

gnatnoop

i doubt it is overheating it has a large aftermarket radiator with a 160 degree thermostat (stock is 190) and the gauge on the dash reads the temp is low. not to mention that 20 minutes of driving would not to be enough to overheat even a air cooled engine.. but thanks for the suggestion, if nothing else you guys are making me think..

BLiZZNECK

valve stems on the flux capacitor need to be roatated, cleaned, and lubed. Sorry for the jokes. I loved your PAYPAY title and had to have a look at the thread. Way over my head. Hope all works out for you brother!!!!!!!!!!!!

01f1502wd

well....just to throw shi.t at ya, could you somehow do something to make it run at 190? i doubt this has anyhting to do with it but maybe....

and your using good gas im guessing. ya I would mess with the valve and see what happens.

gnatnoop

i could put a 190 back in it, although i dont think this is it (but im willing to try anything at this point.) tomorrow morning, i will bypass that valve with some rubber fuel line and see if it helps any..