My 86 efi is only getting fuel from the back tank...
Ok so on my 86 5.0 EFI f150 4x4 im only getting fuel from the back tank. No matter what the switch in the cab is in front or back, the back tank is feeding the motor with fuel. I have gas in both tanks. The front tank reads full and it is full. Whats my issuse. I thought it might be fuel tank selector vale but i went to all my auto part stores and all they have are 6 port universal selector vales. I also look at LMC and NPD and they all show 3 port but they dont know if the vale is for an EFI motor. Is there a way to check it without having to shoot from the hip? Is the vale my problem?
There is a manual fuel valve in the system that changes where the fuel comes from. If the low pressure pump in the tank is not working, then it will never stop using that tank, no matter what the dash switch says.
When you have the switch on the front tank, can you hear the pump inside the tank run when you turn the key to on? You will probably have to crawl under the truck to hear it over the noise the external pump makes.
If you don't hear it run, it could be wiring, a bad dash switch, or the pump itself could be bad. I would try to narrow the problem down before buying parts, although the switch doesn't care if it's EFI or not.
IIRC when the valve goes out it defaults to the rear tank (at least it did on my 83). to find out if its the switch or not, you can check it with a multimeter, or just splice the feed lines manually and see if it feeds off of the front tank. thats how i diagnosed mine. you'll get some fuel on yourself, but if you're quick, its not much. if yours has the in tank pump, you shouldn't get much on you at all. let us know how it goes
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Hi ya, Papa! As luck would have it I am working through a similar but slightly different issue on my '85 F250 4X4 with the 460cid engine and what they call the "hot fuel" option. My issue is that the truck will ONLY run with the rear tank selected. If I switch to the front tank using the dash mounted switch with the engine running it will continue to run for maybe a minute or two and then die. When I get under the truck and put my hand on the valve/motor assy. on the drivers side frame rail about where the cab and bed split is, and have someone flip the switch from inside, I can hear and feel the valve motor run for approx 1 second. This tells me that the switch and valve are probably working as they should. From your post it sounds as if your truck will run regardless of where you have the switch. Is that the case? If so then you must be getting fuel through the valve regardless of the switch position (perhaps the valve has failed somewhere in the middle of its travel, leaving both ports open). One other thing to think about: does the fuel gauge show a different fuel quantity when you switch between tanks? In the Haynes manual they seem to say that the fuel level signal is also switched through the valve circuitry. I hope that this helps but as I say, I have a carbureted engine and that may be quite different from your EFI setup. I'll keep watching this thread and maybe we will become experts in the dual fuel tank world! Let me know what you find out!
From your post it sounds as if your truck will run regardless of where you have the switch. Is that the case? If so then you must be getting fuel through the valve regardless of the switch position (perhaps the valve has failed somewhere in the middle of its travel, leaving both ports open). One other thing to think about: does the fuel gauge show a different fuel quantity when you switch between tanks? !
Yes my truck will run no matter where the switch is and when the front tank is selected it will read full (just filled up and then noticed I had the problem) and when the rear tank is selected it will read the fuel level in that tank (about 1/2 a tank right now). Oh I just remembered something, I had the cab switch in the front tank selection and I drove for about 6 miles (15 minutes) and when I got to work me smelled gas really bad. I looked around and I noticed that fuel was leaking out of the front tank filler spout. (Now just so we are all clear no matter what tank is selected the rear tank feed the motor for fuel). When I took the fuel cap off about 1/2 a gallon or so spilled on to the ground. The tank was pressure. Oh I have a question, could I swap the fuel line on the selector valve? If I did swap the fuel line on the valve and my truck run fine that would tell me the valve is good and maybe it’s the switch in the cab? Someone said that I should find the problem first before I start buying parts and your right. Ill glad that the auto part stores didn’t have what I was looking for.
if you switch the lines and the truck starts drawing from the front tank, it means that the valve is bad. your tank being pressurized would seem to suggest this is the case. the in cab switch is just a simple switch, you should be able to check that with a multimeter by taking off the plate. IDK if the selector switch also controls the in tank pumps or not.
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