Fuel pressure low when warm and under load - Ford Truck Enthusiasts Forums



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Fuel pressure low when warm and under load

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Old 01-27-2011, 07:58 AM
doctrdan doctrdan is offline
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Fuel pressure low when warm and under load

Hi Ford truck enthusiasts . My name is Danny and I'm a retired smog tech and shop owner ,so I have some diagnostic skills and equipment(though not as much as I had when I had my shop.I bought a 1986 Ford f150 with efi 5.0 auto and dual tanks about a month ago and noticed after I drive it about 2 or 3 miles and shut it off for a few minutes it would run rough under a load and sometimes die.After a few seconds it would pick up again and so far has always got me home.
Not having any history on the truck other than knowing it had set for a few years off and on I suspected either fuel or cat convertor so I changed the canister fuel filter(dosen't appear to have an inline filter)and replaced the air filter and checked the housing for rat nests and such.Had no noticable effect.
So I hooked up my fuel pressure guage and had 40lbs key on engine off ,27-35 engine running at idle and 40 with fuel regulator hose off cold .After a warm up and heat soak pressures stayed the same until I brake torqued the vehicle in my drive way for a few minutes then the pressure would drop off to 10-15 lbs and truck would start bogging down as you would exspect. I changed the high pressure pump thinking it could be breaking down when warm under pressure but didn't help.Have the same pressures cold except engine running stays from 30-35 still loses pressure under load when warm.
Then I came on here reading up on the reservoir and low pressure pumps(thanks for all the quality info I am impressed with the knowlegable folks on this sight.)
Now I think it might be a supply problem although it doesn't seem to make any difference which tank it is on.So thinking it must be a problem with the dual reservoir.
My question is if a person were to bypass the reservoir by installing a filter on the inlet side and a straight tube on the return side for one tank and disable the other tank as a test would there be enough supply from the low pressure pump.If that won't work I guess I will probably go to the wrecking yard and get a single reservoir and try that since I saw that suggested on this site.
Any suggestions or advice would be appreciated.

PS I thought I should mention that the front fuel gage work intermitently and the rear tank doesn't seem to work at all.
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Old 01-27-2011, 08:06 AM
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nstueve nstueve is offline
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Hey Dan...

Not to be mean but you'll have better luck with in the next forum down 80-86 trucks; there are more carb 5.0 guys there. 1980 - 1986 F100, F150 & Larger F-Series Trucks - Ford Truck Enthusiasts Forums
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Old 01-27-2011, 09:30 AM
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You are in the right forum for your FI truck.
It sounds to me that both of your in-tank pumps may be bad and not running at all.
You need to find out if they are running when selected first by pull in the supply line from each tank off and sticking it in a bucket and turning the key on and off. It should pump fuel @ 8-11 PSI for one second each time the key is turned from the selected tank.

If you bypass the selector valve on the pressure side for a test you still have the return line from the FPR going back to the tank so you need to leave this one on the valve.
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Old 01-27-2011, 09:33 AM
doctrdan doctrdan is offline
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Thanks for the reply,actually it is fi and I figured there would be more fi traffic in this forum because of the predominant amount of carb trucks from 80-86.I did post in that forum also though.
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Old 01-27-2011, 09:42 AM
doctrdan doctrdan is offline
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Thanks subford I apreciate the response I will have to test those in tank pumps today and see what I have.
If I am understanding you correctly to test the reservoir you would not bypass on both pressure and return side of the reservoir you would leave the return side hooked up. Thanks again Danny
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Old 01-27-2011, 09:47 AM
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I am just saying do not leave the return side from the FPR hanging while doing the test.
Yes you could send it back to the tank outside of the valve.
As you might of read the reservoir is needed for WOT, hills & and to stop engine surging.
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Old 01-27-2011, 10:27 AM
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Quote:
Originally Posted by doctrdan View Post
Thanks for the reply,actually it is fi and I figured there would be more fi traffic in this forum because of the predominant amount of carb trucks from 80-86.I did post in that forum also though.
my fault dan... I should have read closer and seen the EFI.
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