I have a problem with my truck. Its a 78 F150 with a 351M. I bought the truck in Las Vegas and it seems to have been sitting for some time as the tanks (both) were bone dry. The previous owner did some basic maint. to get the truck running, new carb plugs, wires, clear fuel filter just before the carb. (more on this in a min.). So I get the truck and drive it to the gas station and fill both tanks. The truck runs great for about 20 mins and then dies. I get the truck out of traffic pop the hood and look at the fuel filter sight glass and see its empty. So I switch tanks. The truck starts up again no problem. I get on the road again and get maybe a mile up the road and die again. So I open the gas caps and let some air in as the caps look to be the non-vented type. Once again the truck starts and now I get maybe another mile. Same thing. So I have it trailed to a mechanic. I have the fuel pump swapped, again same thing get a couple of miles and same thing dies with no fuel in the fuel filter before the carb.. Next is we bypass the solenoid for selecting the tanks and run the rear tank to the rear fuel line. Same thing happens. next we try the front tank and same thing happens again. Oh I didn't mention blowing out the line, replacing the rubber fuel lines as they looked original. Next the rear tank was dropped and we looked inside the gas was all cloudy, but this shouldn't cause this problem. The only thing we didn't swap was the carb???? As it is after the fuel filter. Long story but I need help, I can't trust the truck and I just got it....I spend a ton of money on the truck as there is 0 rust and the truck runs great when it runs, interior is immaculate. I am really screwed as my wife wants me to sell it ASAP.

 

I'd rig a gas can up under the hood and run it off of that and see what happens.

 

I know the filter appears to be dry but are you SURE its a fuel problem?
You might have an ignition problem and not realize it.
What you describe is a classic symptom of the DSII ignition module failure.
I would replace the box and the pickup coil in the distributor (they both can display the same symptoms when they fail). A new coil wouldn't hurt either.
I wouldn't bother getting them tested. Heat is the enemy, if they're cool they will check good regardless.

 

Mike,
Thanks for the help. When there is fuel in the filter it is a medium yellowish color. I just went out and looked. I spoke to an old timer over at O'Rielly's and he was saying it might be vapor lock. He was showing me the difference between Fuel line and fuel vapor line. It looks like I have fuel vapor line on which is much thinner rubber. I had forgot to mention that the fuel pump was leaking oil around the crimp. The guy was telling me that that could have saturated the rubber hose lines coming out of the metal line coming up from the fuel pump and could be contributing even more to my problem.

How would I isolate the DSII problem?

 

Great suggestion, I would but the mechanic had run it 30 mins after each of the above with no problems just sitting there....

 

OK so I go outside and run the truck idleing for about 15 mins. during this time I'm looking at the site glass in the fuel filter. As its idling I'm watching the fuel surging, bubbling etc. so now I know its fuel, and it looks like air getting in to the lin somehow. Next is to do the input line from the fuel pump into a gas can and run it to see what happens. This will eliminate a DOA fuel pump.

 

If you can, you need to run a fuel pressure test to see whats really going on instead of guessing. If there is a Harbor Freight near you they might have the setup you need.
In my previous post (#3) about the DSII, for some reason I was thinking you had already replaced the fuel pump. Sorry.
If you run the pressure test and it checks okay, I would still strongly suspect a DSII problem.
As for how to isolate, with DSII the only sure way is to replace the parts and see what happens. The parts aren't that expensive and with the DSII, carrying a spare is not a bad idea. The only caveat with the DSII parts, buy QUALITY PARTS, ie Motorcraft, etc.
avoid the cheapie off-shore junk. They're just about guaranteed to fail in a short period of time if they're not bad right out of the box.

 

I had a problem very similar to this a couple of years back. It turned out to be a problem with the floats in the carb being stuck in the full up position not letting fuel into the bowls. This was on a one year old Edelbrock, but the truck sat alot and the fuels today will "gum" things up pretty quickly.

 

That's a great suggestion to try. What I don't understand is how can this be causing the fuel in the filter to be acting this way, with the fuel level in the filter to empty out and have just a little to 1/2 full or surging and bubbling from the fuel pump side?

 

OK, so here is the latest. I put on a fuel pressure gauge today in front of the fuel filter. Coming from a gas can under the truck into the fuel pump I started out with 7 lbs cold and increased to 91/2 in 10 mins. next I put the line on from the front tank and the pressure went to 10-101/2 lbs. I ran this way for 20+ mins and then the fuel pressure dropped to 31/2-4 lbs and then it surged (looking through the site glass of the fuel filter) and went back to 9 1/2 lbs fluctuating between 7 and 9 1/2. Now that I was seeing the problem I put the gas can on and started the process again. The fuel pressure was a steady 7 lbs for 10-12 mins. and then it also dropped to 3 1/2 lbs. so now the olny things in the path in common are the new fuel pump and the carb. Since the old fuel pump was doing the same thing its looking like the carb. What I'm not understanding is how does the carb affect the fuel pressure coming into it?

I have a intake, carb and air cleaner on the way but was hoping the resolve the problem before I possibility add to it.

 

i would drop the tanks and pull the senders out an check the filter on the end to see if there the problem

 

2X on what tinaheels says. Your having similair symptoms to what my truck was doing.

 

In the first post he said that they had dropped the rear tank and looked inside. If the nylon sock was plugged bad enough to cause problems it should have been readily apparent when they pulled the fuel tank sender out plus he said that they blew out the fuel line. But it never hurts to double check.
Fuel pressure should be 4.5-6.5 PSI, fuel pump volume should be 1 pint in 30 seconds @ 500RPM.
I know I'm beating a dead horse, but I still suspect a DS II problem.

 

I ran a line straight from a gas can into the fuel pump. Same problem

 

When we looked in both tanks the nylon socks were gone.
Mike,
How can the DS II cause the fuel issue? That is what has me confused. I can get the DSII from a parts truck I'm buying Friday. Thanks for the help.
Mark