fuel system problems
#1
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???
Last edited by luke1; 11-09-2015 at 11:17 AM.
#2
Senior Member
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.
Vapor lock is indeed worth investigating further and that's probably where I would start if those other procedures have all been done recently.
#3
Senior
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.
Just some Ideas. Hope they help.
#4
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
#5
Senior
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)
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luke1 (11-14-2015)
#6
Senior Member
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.
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.
#7
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.
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.
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#8
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)
Last edited by luke1; 11-14-2015 at 03:36 PM.
#9
Senior
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
#10