fuel pressure question
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From: Grand Bay, Al
fuel pressure question
i have a 01 f150 with the 5.4. when it starts it takes a minute or two to do so.. after i shut it off and try to restart it wont unless i turn the key on and wait. i checked the pressure and when ideling its running about 28 soon as the key is turned of it goes to 0. is my pump bad??
Lead Driver
Joined: Sep 2006
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Here is what you should have.
The pump has a check valve to hold the pressure as long as 20 min or more as long and there is no leak down from the regulator or the injectors.
The running pressure should be over 35 psi and hold after shutting down the motor.
Either one or both could be the problem.
First thing to check is to see if the pressure you do have changes when the regulator hose is removed.
If yes assume the regulator does it's idle pressure control but you still don't know if it's the cause of the low pressure.
Best I can tell you so you have to work it out to see.
Good luck.
The pump has a check valve to hold the pressure as long as 20 min or more as long and there is no leak down from the regulator or the injectors.
The running pressure should be over 35 psi and hold after shutting down the motor.
Either one or both could be the problem.
First thing to check is to see if the pressure you do have changes when the regulator hose is removed.
If yes assume the regulator does it's idle pressure control but you still don't know if it's the cause of the low pressure.
Best I can tell you so you have to work it out to see.
Good luck.
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Posting Guru
Joined: Sep 2010
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From: Grand Bay, Al
ok. just checked pressure again. ideling it sits on 28psi. take the vac line off the regulator it goes up to 33psi..
also noticed my scanner will not work with the truck..... strange because it works on everything else
also noticed my scanner will not work with the truck..... strange because it works on everything else
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
The regulator 'control' is acting correctly.
The lower pressure could be gage accuracy error or the regulator's base pressure regulation is on the low side.
Honestly the pressure your seeing should not cause a cranking problem because the fuel requirements at that time are not very high.
At cranking, the vacuum is lower to the regulator so the pressure is higher than running idle.
I would check pressure with a second gage.
If no major difference is seen the cranking/start issue is something else.
Hot restarts are even less demanding of fuel than cold starts.
Check the fuel pressure under as many different conditions as you can to see a possible pattern before doing any work.
It could still very well be the fuel pump or it's control circuit.
Good luck.
The lower pressure could be gage accuracy error or the regulator's base pressure regulation is on the low side.
Honestly the pressure your seeing should not cause a cranking problem because the fuel requirements at that time are not very high.
At cranking, the vacuum is lower to the regulator so the pressure is higher than running idle.
I would check pressure with a second gage.
If no major difference is seen the cranking/start issue is something else.
Hot restarts are even less demanding of fuel than cold starts.
Check the fuel pressure under as many different conditions as you can to see a possible pattern before doing any work.
It could still very well be the fuel pump or it's control circuit.
Good luck.
Thread Starter
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Posting Guru
Joined: Sep 2010
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From: Grand Bay, Al
is the check valve in the tank with the pump? as soon as i turn the key off the pressure hits 0.
no SES light. just has a rough idle
no SES light. just has a rough idle
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Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
The check valve is inside the fuel pump.
If the pressure falls to 0 instantly, the pump needs replacement and will lose prime taking time to build pressure in the whole line and fuel rails before the injectors will meter enough fuel at rated pressure.
Remember that 8 injectors opening one at a time during cranking slows the pressure build up in the system even longer.
Good luck.
If the pressure falls to 0 instantly, the pump needs replacement and will lose prime taking time to build pressure in the whole line and fuel rails before the injectors will meter enough fuel at rated pressure.
Remember that 8 injectors opening one at a time during cranking slows the pressure build up in the system even longer.
Good luck.
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