luke1

Ok I have an 85 f150 with dual tanks. I am having an intermittent fuel loss issue that I cannot figure out. I have about a half tank of fuel in each tank. I installed a clear filter and fuel pressure gauge by the carb for diagnosis. The truck will run great then out of nowhere start bucking and kicking but usually stay running just rough when I check that filter it will be empty and fuel just spitting through and only like 4psi then I switch tanks it will take a little fill the filter and have like 8psi run great for a like a mile then do the same thing sometimes I can drive around for 30 mins. No problems sometimes I can't get to the end of my block. I have replaced the fuel pump cleaned and verified operation of rollover valves, vent lines, gas caps, and filler neck vents all appear to work properly I put some heat shield tape around the fuel line where it comes around the frame rail near the header thinking maybe vapor lock. Also Replaced the selector valve cleaned both fuel tanks and the pickup filters. Any other ideas???

burnthelight88

Sounds like the pump or the pickup but you claim to have replaced both. I'm not sure if that means that you're done them both recently or not, but that's what comes to mind.

Vapor lock is indeed worth investigating further and that's probably where I would start if those other procedures have all been done recently.

GhostriderI

Is there another filter inline? Do you have vented or non-vented fuel caps? I think yours might use vented caps? When you had the tanks down did you check the vapor valve on top of the tank?

Just some Ideas. Hope they help.

luke1

Thanks for the fast responses. all of the diagnosis and parts replaced have been donewithin the last 2 months the filter I installed is the only one in the system I believe the vapor valves Are what I referred to as rollover valves they are functional the gas caps are vented type and seem to work this system seems way too simple to be this hard to diagnose...ha

GhostriderI

It is not hard, just have to be logical. When you had the system apart did you blow the fuel lines clear? Might have some junk blocking the lines. Also double check the fuel connections on the inlet and outlet sides of the selector valve, inlet side of pump, and fuel lines at the tanks, might be loose and letting the pump suck air instead of fuel. I hate to say it, but you might have gotten a bad pump also (pressure relief valve in pump bad)

burnthelight88

What work had you done on the truck prior to the issue starting? Are your headers aftermarket? Is your fuel line the stock line or has it been replaced and routed differently?

I'm thinking vapor lock if its only happening when the truck is getting warmed up. Usually doesn't start happening out of nowhere though.

If you haven't already, swap out all of the old rubber for new hose that the fuel passes through. If the lines are stock, I believe that means 3 short lengths. If the hoses are older, they could be sucking air and causing a problem for you.

luke1

The headers are aftermarket but were on the truck when I got it I put an aluminum intake on cause I had it abd the factory (cast iron 4bbl ho intake) was leaking both oil and coolant at the back so it had to come off regardless but I drove it like that for almost 3 months with no issues until it started this now it seems the more I replace the more pronounced the problem is. When I had the tanks down I blew through every line insuring that there was No blockage and verified that the selector valve was in fact switching tanks ( that was what I first believed the problem to be cause I ran the rear tank pretty low then switched to the front which showed it had fuel but died anyhow so I thought the gauge was switching but it wasn't actually switching tanks)When I put the New fuel pump on I replaced the all the rubber lines all the lines appeared to be the factory lines

luke1

What are the symptoms of the fuel pressure relief valve being bad I noticed (since you mentioned this)when I first start the truck and watch the fuel pressur gauge I installed it will flicker from like 10 -14 psi then once warm (like 5 -10 mins. Later)will be way more steady at like 8.5 is this a sign that isn't working correctly?? When I used to rebuild and dyno engines if we didn't set the regulator properly usually12 psi it started pushing fuel out the vent tubes of the carb which is not happening so I don't believe it's getting that high

GhostriderI

The pressure relief valve is inside the pump sounds like it is working correctly with the readings you gave here. Do you have the 351 HO?

luke1

Yes