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Fuel Pressure

11K views 91 replies 13 participants last post by  69mach1377 
#1 ·
My fuel pressure has been between 40psi and 60psi for the last year, the weather has cooled off about 20 degrees where i live and my pressure is now more constant at 60psi a runs great. Any explanation? My pressure should stay at 60 psi right ?
 
#32 · (Edited)
Your right but it just seems like good practice and the pump box says no warranty w/o pre-filter. Not sure what the warranty is anyway. I've always used filters in front of fuel pumps on my old mustang and don't want to risk ruining a brand new one, that's all.
Maybe I'll test my old pump first like in that video, but I can't simulate driving under load, so what will it prove?
Also, the pump box has a part # of 69136 but on the pump body is stamped the following;
Bosch 0580464074
12v (506) 75503 made in CzechRepublic
F81U-9350-AA
 
#36 ·
Also, the pump box has a part # of 69136 but on the pump body is stamped the following;
Bosch 0580464074
12v (506) 75503 made in Czech Republic
F81U-9350-AA
My spare F/P is is made in the Czech Republic too, so it's the same pump.

Best thing to do is get those gauges fitted and the F/P and test drive, so it eliminates those parts.
 
#33 ·
Maybe a bench pressure test of the old pump vs. the new pump w/pre filter? Just to prove the old pump is worn.
Followed by a road test with the new pump/filter. If the new pressure is higher and/or does not drop much a WOT then that would confirm the pump was worn, right?
 
#34 ·
is the filter designed for suction?

If you use diesel clean ever fill up I doubt the in tank screens are cloggedz
 
#35 ·
Good question, haven't got one yet. Any suggestions appreciated.
Fingers crossed about clean screens...:thumbsup:
 
#37 · (Edited)
So I got the old one off and setup a bench test but lack some 5/16" hose, filter and maybe the clamps. Will got those tomorrow and test both, dead head style. I used the new one as an overnight plug ;)
I plan to apply 12v to each and see what the pressures are before doing final install and test drive. Being retired does have the benefit of having some extra time to do things like this...besides, working sucks after several decades, right?

The warranty is lifetime but only if there is a filter in front of it. Maybe a metal one is better for suction than a clear plastic one?
 
#38 · (Edited)
#40 ·
Thanks for the good advice above. I found a Motorcraft FG986B stainless steel one with 5/16" ends that I'll give a shot for now. Should be able to splice it in pretty easy (nothing comes easy for me the first time I do something).

So I just applied 12v to my old pump and it pegged the 100 psi gage, so I guess there is nothing to learn from a bench test?

On to installing the new one and filter and go for ride to check the pressure...after lunch.
 
#41 ·
A deadhead test won't tell you much, other than what pressure the pump will produce with no flow. It takes flow for pressure loss to show up, so a better test would be to have normal flow thru the system and tap pressure/vacuum gauges in at various points to measure the resistance across each element of the system. It's kind of analogous to measuring voltage drop on an electrical circuit to isolate a high resistance connection.
 
#42 ·
Well I was hoping it would tell a difference in the pumps and condemn the old one. I'll just put the new one in and repeat the first pressure test at wot.
 
#43 ·
I decided on installing a Dahl Baldwin Model 150 Diesel Fuel Filter which includes a water separator and filter element. You can google them and find that they are very common as pre-filters on diesel engines and several diesel vendors recommend them. Although an engineer I do not claim to be an expert on filtration but I know there is a significant difference between the filtering done by a screen and that of a multiple pass paper or cellulose element. If you have ever rebuilt an engine you have probably seen that the gears in the oil pump can become highly scored or worse from wear. The fine abrasives in the oil pan are carried right through the screen where they chew-up the gears over time. The oil filter is on the output of the oil pump and keep these parts out of the bearings and other critical engine parts. The same idea is true with Ford's design of the screen in the tank, followed by the fuel pump and then the fuel filter. The pre-filter saves the pump from being exposed to such abrasive particles.
 
#44 ·
Well I finally got to read the pressures of the new pump...55 idle and dropped about 8 on wot. So my old pump wasn't so bad after all. But diesel scent all over me ain't so bad, it really pulls the chicks in ;)
 
#45 ·
eau de gazole [French]
 
#46 ·
Misspelled it...should be eew, ;)
Anyway, it looks like I may doing the tank mods.
So the guys who have 60 psi and idle and wot, where did you measure it and what mods were done?
 
#47 ·
Don't everybody reply at once now...sensing the silence, I set up a clear hose to 2 gal 'can' of new diesel bypassing the stock fuel tank. And guess what???


The pressure drops the same as when the fuel pump was connected to fuel tank. Only made one run up my street before it started sucking air, so back into my driveway I went.

Now what?
 
#54 · (Edited)
@ Googs
I set up a clear hose to 2 gal 'can' of new diesel bypassing the stock fuel tank. And guess what???
The pressure drops the same as when the fuel pump was connected to fuel tank.
Based on this I'm inclined to work on the regulator now but that doesn't mean tank mods are off the table in the future. I have all winter to improve things.
 
#49 · (Edited)
Thanks Brandon, I had not seen that yet but that was where I was thinking next after replacing the 1/4" ID hose between the tank and the pump. I can't believe how small it is actually measured, it's supposed to be 5/16", which seems small also.
Funny thing, I had a weird dream last night that my temporary fuel pressure hose was spewing wildly as I drove around...need to find a cap for it today I guess.
Oh, and it looks like that post shows a mechanical fuel pressure gage...in the cab, not a good thing.
 
#50 · (Edited)
#51 ·
That's a whole replacement for the stock part. Billet aluminum instead of the stock steel piece.

Looks like there is a serious "oh sh*t" warning for when you're taking the old stuff off, similar to how a simple thermostat swap turns into a whole new water pump for a lot of guys when the rusted housing bolts break off at the heads.
 
#56 ·
That's a whole replacement for the stock part. Billet aluminum instead of the stock steel piece.
The stock FPR housing isn't steel, it's cast aluminum or pot metal. I've seen a few of those break off with the fitting stub stuck inside the return line fitting. Isn't a lot of fun to get that out so you can install the line on a new FPR housing.

69mach1377, dropping the tank isn't a terrible ordeal. I've done it a bunch of times. Probably the hardest part is getting the filler and vent hoses off the inlet. Use a couple of ratchet straps to hold it in place till you get a jack underneath. And be sure to do it with a nearly empty tank.
 
#52 · (Edited)
So let's see...more adjustable, costs less, looks better, which one should I get? I think I have to use some of the old stock guts, right?

Yeah, I seem to get the OS a lot. Like today, I wanted to tilt my bed up to get access to the top of the tank but only 4 of 8 bed bolts were visible. The other 4 have 17 year old bed lining on them, which is real hard to get off. Got one going but the second one broke my T50 pos bit. That's why I only replaced 10" of 1/4" hose with 3/8" instead of several feet. But it seemed to yield 2 more psi at idle :)
It still sags several psi on wot though :(
 
#53 ·
So you've done the tank screen filters correct and you still have seven or so psi drop at WOT?
Did you use the billet FPR rebuild kit? if so did you use the silver spring?
 
#57 ·
I should get my billet FPR today, but who knows...truck is dead until then since the stock one broke as I tried to shim it.

For those who have done the hutch mod, I look to Guzzles instructions but can't believe the need for 3 feet of of 5/16" tubing. Maybe 2 feet will do, or some number of inches?
I'm looking to source the parts locally vs. buying a kit ($).
 
#58 ·
You want to run the feed to the bottom of the tank

Then move the return line farther away
 
#59 ·
Right...the feed uses a few inches of 3/8" tubing.
YouTube U shows a zee bend on the 5/16 tubing going out past the float, but not 3 feet worth, surely.
 
#60 ·
So I installed the new FPR with the gold spring and got 61 psi at idle.
Getting the fuel tube into the new housing was a bear, but no leaks :)
Went for a romp and it dropped into the low 50s...
 
#61 ·
That’s not terrible. It’s well within the acceptable range.

I would fit the stiffest spring in.

Did you rebuild the wheel regulator or just swap the billet piece with springs?
 
#62 ·
Just the swap, the guts looked good and clean.
I guess what still bugs me is the pressure drop on wot.
I'm probably going into the tank this weekend anyway and do the hutch and harpoon. I've been wanting to fill the tank completely forever now.
Do I really need $5/ft, 3/8" fuel line between the tank and pump? How many feet do I really need, surely not 10'???
 
#64 ·
Just the swap, the guts looked good and clean.
I guess what still bugs me is the pressure drop on wot.
I'm with blue on this one , your pressure sounds okay, at 55-61, try the black spring before you drop the tank .
I have similar pressures when towing heavy so i have fitted these High Flow Fuel CVD Fitting Set for Ford 7.3L 99-03 to combat the drop in pressure , hope it lifts it.
I don't thing your going to see exact pressures at idle and at WOT , you'll alway get some variant.
 
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