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1991 F-superduty 7.3 ZF5
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Discussion Starter · #1 ·
I need help. I got a 1991 F-super duty that keeps losing prime within seconds. I just got the truck and was told the fuel selector valve was removed because it went out, and the previous owner tried to prime it just using the starter and fried the starter. I got a new starter today and have been trying to get it to run all day. It will run with a rag with gas on it or when the schrader valve on the fuel filter housing is removed, but when the valves removed it will only run about 10 seconds before it dies again and it’s a fight to get it to reprime. I’m stuck and need major help, I need the truck running by Monday.
 

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Remove the inlet for the lift pump. hook a hose up to it, and put some fuel in a clean can, see if it will run like that. Then look at the selector valve and make sure it is hooked up right.
 

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1991 F-superduty 7.3 ZF5
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Discussion Starter · #3 ·
Remove the inlet for the lift pump. hook a hose up to it, and put some fuel in a clean can, see if it will run like that. Then look at the selector valve and make sure it is hooked up right.
It has the fuel selector valve deleted by the previous owner, I put a mr gasket micro fuel pump to it and now it will run for a minute but it just stated pissing fuel all over the front of the motor and I can’t find the leak. It’s wettest around the lift pump, but fuel isn’t getting in the crank case
 

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If you put an electric fuel pump on it, did you by-pass the mechanical one? That could be your leak. The diaphragm if it has a hole in it will leak out the weep hole. Check downstream of the electric pump, a leak that big should be trying to slap you up side the head. Check while you're running the pump. you should be able to see the fuel running out. If you plan to leave the electric pump there, first make sure it doesn't put up more than 7 PSI. Then by-pass the mechanical pump.
 

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1991 F-superduty 7.3 ZF5
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Discussion Starter · #5 ·
It has the fuel selector valve deleted by the previous owner, I put a mr gasket micro fuel pump to it and now it will run for a minute but it just stated pissing fuel all over the front of the motor and I can’t find the leak. It’s wettest around the lift pump, but fuel isn’t getting in the crank case
If you put an electric fuel pump on it, did you by-pass the mechanical one? That could be your leak. The diaphragm if it has a hole in it will leak out the weep hole. Check downstream of the electric pump, a leak that big should be trying to slap you up side the head. Check while you're running the pump. you should be able to see the fuel running out. If you plan to leave the electric pump there, first make sure it doesn't put up more than 7 PSI. Then by-pass the mechanical pump.
it’s still got the mechanical pump on it but putting that electric pump on got it to run for about a minute or so instead of 10 seconds without it. I know it’s leaking on a return line but that’s right by the front tank not up by the motor. I’ll check out it out
 

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That would lead me to believe that the mechanical pump isn't working as desired. If you're still running fuel through it, then by-pass it and try it again. My bet is that your leak is the mechanical pump. If you aren't still running fuel through it, then start the electric pump and with it running find the leak.
 
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