Low fuel pressure

[SuperAwesomeGuy]

Getting 10 psi on the rail though what's weird is that the pressure quickly decreases

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[adsm08]

If it doesn't come up after a few priming cycles, first check fuel level, then filter, then make sure the external pump is working, in that order.

You can't test pressure bleed of accurately when it is that low. Get it right, then worry about bleed off. On return style systems it can drop up to 10 PSI from the idle pressure almost immediately and that is acceptable. Then it may bleed another 5 PSI off slowly over a few hours.

If pressure loss is great or faster than that first check for a ruptured FPR, and if that is OK look at replacing the little fuel module near the transfer case. Unlike newer single-pump trucks your pressure check valve is in that unit, not in the tank pump. It's an artifact of the way the system was designed for use with a single or dual tank setup.

 

[SuperAwesomeGuy]

If it doesn't come up after a few priming cycles, first check fuel level, then filter, then make sure the external pump is working, in that order.

You can't test pressure bleed of accurately when it is that low. Get it right, then worry about bleed off. On return style systems it can drop up to 10 PSI from the idle pressure almost immediately and that is acceptable. Then it may bleed another 5 PSI off slowly over a few hours.

If pressure loss is great or faster than that first check for a ruptured FPR, and if that is OK look at replacing the little fuel module near the transfer case. Unlike newer single-pump trucks your pressure check valve is in that unit, not in the tank pump. It's an artifact of the way the system was designed for use with a single or dual tank setup.

4 cycles gives me roughly 9 psi of fuel pressure. Filter is new. Pressure bleeds out within 5 seconds after priming

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[adsm08]

4 cycles gives me roughly 9 psi of fuel pressure. Filter is new. Pressure bleeds out within 5 seconds after priming

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Ok. Verify that the fuel pump on the frame is running. I doubt it is, since the tank pump is supposed to produce about 10 PSI, and the frame pump does the rest.

Once you are able to get a proper 35 to 40 PSI you can look at the bleed down rate.

 

[SuperAwesomeGuy]

Ok. Verify that the fuel pump on the frame is running. I doubt it is, since the tank pump is supposed to produce about 10 PSI, and the frame pump does the rest.

Once you are able to get a proper 35 to 40 PSI you can look at the bleed down rate.

U sure there should be a valve on fuel pump that would cause bleed off so would might answer for both problems

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[adsm08]

U sure there should be a valve on fuel pump that would cause bleed off so would might answer for both problems

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Can we please try that question again, but with more punctuation?

I am very sure of the things that I did say though. I have been building and re-building these 86-88 trucks for almost 15 years. Your frame pump is responsible for any fuel pressure above 10 PSI, you only have 10 PSI, so that pump clearly isn't working.

You need to fix that and get your fuel pressure back to where it should be before you can evaluate the fuel pressure drop off. Since 10 PSI is an acceptable amount to drop right away when the pump is shut off, and that is all you have, you can't look at that 10 PSI dropping to 0 right away and say "this is a problem". You need to fix the low pressure and THEN and ONLY THEN can you properly evaluate the pressure drop.

Doing it any other way is like taking a guy who needs glasses, taking his glasses away, and asking him to tell you if there is any smudged ink on a letter you just wrote. He can't tell if anything is wrong, and right now neither can you.

 

[SuperAwesomeGuy]

Just looked. No external pump, am in process of changing in tank pump.

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