Low fuel press. at full throttle

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Low fuel press. at full throttle

Postby mitchk » Sat Oct 06, 2018 3:52 pm

Hi. Haven't been on here in quite a while. Been doing some work to the daily recently and have been noticing my fuel pressure dropping around 3/4 to full throttle. Just pull the pickup from the tank tonight to check it. Cleaned all the screens and blew out the feed line. Everything seemed fine but cleaned it up just cause it was out. Has 3/8 line from the tank to lift pump. Has a hungry diesel 15psi lift pump. Cleaned the screen on it a little while ago. Has fresh fuel filter. Can't find any leaks anywhere. If I roll into it it'll hold about 10-12 psi till about 3/4-full throttle then drops quick down to about 5 psi. Not sure what it could be at this point so just trying to get some ideas of what else to check or if I'm overlooking some thing. Thanks Mitch.
Daily: 91 d350 727 gear vendors over drive, 3/8 feed line, THD lplp, maxed pump, Denny t 2, timing to head, #366 spring, 60# valve springs, arp studs, 60 horse injectors?(po), THD bahf, s362sxe

Project: 93 w250 5 speed, ground stock pin, #366 spring, 60# valve springs, THD lplp, intercooled compressor non intercooled turbine, diamond eye 4 inch, custom intake, south bend hd single
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Re: Low fuel press. at full throttle

Postby PToombs » Sun Oct 07, 2018 1:26 pm

There is a screen in the inlet of the piston lift pump, you might want to check that. If it starves for duel it can scratch the piston and you'll lose more pressure. BTDT.
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Re: Low fuel press. at full throttle

Postby mitchk » Tue Oct 09, 2018 3:48 pm

That was actually the first thing I checked. Prob couldn't hurt to check it again though. Had a thought the other day and want to see if my theory is correct. I have the fitting from thd to run my fuel pressure guage off the outlet side of the fuel filter. With the way that's set up would the guage read a loss in pressure between that and the injection pump say if I'm losing pressure closer to the pump or would it really only read from the tank to where the guage is. Was just thinking to narrow down where I could have a leak or air getting in somehow. Have a lot of time to think about this seeing as I drive trucks cross country but won't be able to actually do anything till I get home in a couple weeks. Need to do an oil change and am going to throw a new fuel filter on just to see if that makes a difference too.
Daily: 91 d350 727 gear vendors over drive, 3/8 feed line, THD lplp, maxed pump, Denny t 2, timing to head, #366 spring, 60# valve springs, arp studs, 60 horse injectors?(po), THD bahf, s362sxe

Project: 93 w250 5 speed, ground stock pin, #366 spring, 60# valve springs, THD lplp, intercooled compressor non intercooled turbine, diamond eye 4 inch, custom intake, south bend hd single
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Re: Low fuel press. at full throttle

Postby PToombs » Tue Oct 09, 2018 4:55 pm

I just reread your original post, sorry, I missed you said that. :oops:
That gauge will read a loss of pressure feeding the IP, whether it's a plugged filter, sucking air, bad LP. You get the idea. If you move the fitting to the inlet of the filter head and the pressure comes up it means the filter is plugged, if not, the problem is between the tank and the filter.
What are you using for the 3/8 line? Rubber, steel, plastic?
You could also gently pressurize the tank, 5 psi or so, and then climb under and look for any wet spots which will indicate someplace it can suck air.
pete

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Re: Low fuel press. at full throttle

Postby matthewh » Tue Oct 09, 2018 9:18 pm

Couldn't he be drawing enough fuel to cause it to loose pressure under 3/4 to full throttle? I thought these setups when tuned can drop the pressure right down to 0, even with the higher psi lift pump
'93 W350 5spd Getrag, 4.10 gears, 83k miles and counting, Mods: LPPP AKA Ma Dually
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Re: Low fuel press. at full throttle

Postby DMan1198 » Wed Oct 10, 2018 3:24 pm

That higher volume pump if plumbed right pushes enough fuel for around 1000hp (theoretically based on fuel density). Not plumbed as effectively it’s still enough fuel for more than your average ve truck.
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Re: Low fuel press. at full throttle

Postby mitchk » Wed Oct 10, 2018 5:26 pm

No worries. And that's what I was figuring was that the guage would only read up to wherever it was plumbed in.
And the previous owner put the line on. It's just a clear braided fuel line. I've followed the line from the tank to the pump and can't notice any obvious leaks or scuffs where it might be pulling air.
How would you go about pressurizing the tank? An air line on the breather regulated down to about 5psi? The previous owner was also to lazy to drop the tank(or smart enough) when he did the line and just cut a flap in the bed over the pick up. Looks like s&$t but makes all this much easier than having to drop the tank. Haha.
And like I said once I get home the first thing I'm gonna do is put a fresh filter on and go from there.
Thanks for the replies. Have to say it's good to know this sight is still going strong. Learned a lot from here when I got my first truck. Now I'm on my third and still learning.
Daily: 91 d350 727 gear vendors over drive, 3/8 feed line, THD lplp, maxed pump, Denny t 2, timing to head, #366 spring, 60# valve springs, arp studs, 60 horse injectors?(po), THD bahf, s362sxe

Project: 93 w250 5 speed, ground stock pin, #366 spring, 60# valve springs, THD lplp, intercooled compressor non intercooled turbine, diamond eye 4 inch, custom intake, south bend hd single
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Re: Low fuel press. at full throttle

Postby Donkey Kong » Thu Oct 11, 2018 1:40 pm

A flap in the bed? Thats awesome!! Why didn’t I think of that? Its almost as good as the guy who cut a hole in the top of the fender to drive the kingpin out of the front axle. When asked who was going to fix it, he said “body shop will take care of that”...genius, pure genius.
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Re: Low fuel press. at full throttle

Postby PToombs » Thu Oct 11, 2018 4:51 pm

I use a blow gun with a rag in the filler neck, but Ise a perfessional so don't try this at home! :lol:
Did he put the 3/8 line on the original suction tube? That could be a restriction right there in the pickup! Or if he put the 3/8 right on the 5/16 plastic on the tank it could be sucking air right there. Maybe have somebody ride in the bed and watch for bubbles? :mrgreen:
When I went from the original 5/16 line to the 1/2 inch with the draw straw my pressure went from 11 up to 14 or so. Then when I went to the sump I gained a couple more, even though I was pulling it thru a 30 micron prefilter.

I know some of those "flap in the bed" types! ;)
pete

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Re: Low fuel press. at full throttle

Postby mitchk » Thu Oct 11, 2018 7:47 pm

Ye. I'm still scared to go through some of the the wiring he did seeing as that's already not a strong suit of mine. Pulled out at least a pound or two of wires that weren't hooked to anything when I ran all my new guages. Still need to completely rewire the overdrive unit e put in cause that hasn't worked since I got the truck and I'm turning about 2k at 55-60 with 3.01s. Buts that's a whole nother problem.
And he has the line hose clamped to the original 90 deg. Out of the top of the pickup. that makes sense cause even with the larger line I'm only pulling whatever can squeeze through that factory pickup. Didn't even think of that.
How did you go about putting a draw straw in. I would like to run a sump eventually but just can't afford to have the truck down for any amount of time right now as it's my daily driver and it's the busy season at work.
Daily: 91 d350 727 gear vendors over drive, 3/8 feed line, THD lplp, maxed pump, Denny t 2, timing to head, #366 spring, 60# valve springs, arp studs, 60 horse injectors?(po), THD bahf, s362sxe

Project: 93 w250 5 speed, ground stock pin, #366 spring, 60# valve springs, THD lplp, intercooled compressor non intercooled turbine, diamond eye 4 inch, custom intake, south bend hd single
mitchk
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Re: Low fuel press. at full throttle

Postby Remps » Thu Oct 11, 2018 7:53 pm

I don't recall if it was the 2nd or 3rd time the getrag had to come out of my 1st 93, but I just cut the shifter hole big enough to get the top cover out. Added a hinge and a latch to the piece I cut out lol. Meh, it was worth it.
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Re: Low fuel press. at full throttle

Postby PToombs » Fri Oct 12, 2018 5:20 pm

If he just clamped the new line on the old fitting I bet it's sucking air there.
Don't bother with a draw straw. Save your money, run the tank low on fuel and put in a sump. The straw will suck air under a 1/4 tank, everybody says so, and they are right! I could watch my fuel pressure drop from 14 to 2 or 3 sometimes when I got on it, it was scary to watch. :lol:
I filled a 5 gallon can with fuel, ran the tank down to an 1/8 or so, then siphoned out all I could. I got about 8 gallons from it. I then jacked up the back of the truck as high as I could and put it on stands. (safety 1st! ;) )I located a spot behind the sender module where it was mostly flat, drilled the hole and filed the edges. I got a Sinister Diesel sump, and it came with a gasket. Throw the gasket away, tell you why later. I figured which direction I wanted the line to go, the drilled for the mounting bolts. I drilled 2 across from each other, then put the bolts in, and drilled the rest while holding the sump in place so all the holes would line up when I was done. I then installed the sump with the gasket.
Now here is where it went bad...... :/
A year or 2 later, I knew the gasket had been swelling because I could see it pushing out around the sump. The truck had been sitting out back for a month because I was driving the truck with AC. I jumped in, started it up, and the gauge showed empty! I pulled up by the house, pulled the line and drained the tank. (still got about 5 gallons from it) I jacked it up and pulled the sump, cleaned off the gasket goo, which must have been made of rubber. I went and bought a tube of Permatex Black RV, closest to fuel resistant I could find. I then reinstalled the sump with a nice film of RTV. (notice I didn't say fat bead!)I left the line off over night and close to 24 hours before I put fuel back in, and it's been fine since. I check it regularly too.
Some sumps have an o-ring, I think I would still put a film of RTV on it.
Sorry it was so long, but I wanted you to know what can and does happen. ;)
pete

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Re: Low fuel press. at full throttle

Postby mitchk » Fri Oct 12, 2018 5:55 pm

Thanks for that. I actually ended up contacting Eric at the hungry diesel today and he said the same thing to just go with a sump. So I decided to shell out the money and went with the trp sump he has and the braided lines. Don't want to deal with this anymore. Have had this new turbo setup on for a couple months and am tired of only being able to touch full throttle and then letting off. Wanna see what it can really do.(until my trans goes at least lol). Hopefully it will get there by the time I get home and I can take the weekend off and throw it in. Thanks for all the help. Will let you know how it works when I get it installed. Thanks again.
Daily: 91 d350 727 gear vendors over drive, 3/8 feed line, THD lplp, maxed pump, Denny t 2, timing to head, #366 spring, 60# valve springs, arp studs, 60 horse injectors?(po), THD bahf, s362sxe

Project: 93 w250 5 speed, ground stock pin, #366 spring, 60# valve springs, THD lplp, intercooled compressor non intercooled turbine, diamond eye 4 inch, custom intake, south bend hd single
mitchk
fuel screw!!!!
 
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Re: Low fuel press. at full throttle

Postby PToombs » Sat Oct 13, 2018 5:30 pm

No problem man, BTDT so just trying to save you the hassle and some money. ;)
pete

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