Numidian

So when I get just below a half tank on my truck, if I come to an abrupt stop (especially if I'm headed downhill) my fuel pressure will drop to 0 for a second or two, then jump back to normal.

I figure there are two possible causes -
1. Poor tank design and the fuel is sloshing away from the pick-up.
2. My sending unit is screwy and not reading an accurate level.

I never noticed this happening over the summer, so I do wonder some if the cold/thicker fuel has something to do with it as well?

Thoughts, experiences, fixes?

BarryG

There are no baffles within the tank. Just a canister type unit that the pickup sits in and draws fuel from and the return fuel is dumped into this canister as well to avoid this very situation. The link shows a picture however it is not a stock one. The stock one's canister section is taller but this is pretty similar.
http://www.vulcanperformance.com/Mod...ule-p/fmds.htm

It is entirely possible that your sending unit is reading incorrectly as that seems to be a common failing with the 2nd gen trucks fuel level sending unit.

Does your truck have a draw straw installed? If so one of the common issues at least early on was that at lower fuel levels it may have been cut too short and hence can't pickup fuel under lower fuel levels upon hard acceleration, quick deceleration or steep hills. Typically referred to as the 1/4 tank issue.

Perhaps your pickup line is screwed up within the module or even at the top of it where it exits the tank which with the steel lines that some of the early ones had rusted out. However you would think that would be an issue all the time not just at lower fuel levels.

Only way to get the canister out is to drop the tank or raise the bed. Not enough room under the bed to lift out as it sits.

Numidian

I do have a DrawStraw II (at least according to my signature that I stole from the previous owner LOL)

I haven't actually done any work to the truck myself. I bought it already built, so I'm kind of learning as I go.

So can the straw being too sort be checked without dropping the tank?

Lary Ellis (Top)

Common issue when installing the draw straws is for a person to set the tank on the ground and then cut it to fit. Problem with that is the tank when hanging from both ends in the truck drops down and inch or two in the center so the straw ends up too short.

The other issue is the sending unit gets damaged by the pressure put out by a big fueling nozzle at truck stops because they pump a LOT of fuel in a hurry.

As stated earlier both these issues are very common but you will have to drop the tank or lift the bed off to fix them. If you end up putting a new draw straw in be sure to support the tank on both ends so the center can hang down as it does in the truck

Numidian

I'll just keep filling up when I get to a half tank or 350 miles, whichever comes first LOL

SIXSLUG

His original post stated "fuel pressure" dropping not fuel level.

An accurate fuel level sender would show a change in fuel level in that instance, but mine is slow to read and I believe this is because the electronics sample the reading intermittently so the gauge doesn't do this...

If it is indeed fuel pressure, he should get a stutter after sucking air, shouldn't he??

Numidian

Correct. My fuel pressure gauge goes to 0 for 2-5seconds, then bounces back to 18.

I do not get any sort of stutter or miss from the engine, but I'm also always at idle when this happens, because I'm stopping, so my foot isn't on the throttle.

I only brought up the sending unit, because I was thinking maybe it thinks I have half a tank and in reality, my fuel light should be on or something along those lines.

SIXSLUG

I can't imagine the fuel pulling entirely away from the pick up unless you have very little fuel in the tank, even at a half tank you would have almost 18 gallons in there, if a longbed quadcab, and if it was moving around that much I would think you would feel it in the brakes.

I would address the FP gauge and its wiring and the snubber for starters, even then I would think there wouldn't be an instant notification of fuel pressure loss, I would think it would be delayed a bit...

Just thinking out loud.

Numidian

It happens when I get below half a tank... Probably 3/8th or so. I know in every vehicle I've ever owned the 2nd half of the tank really only represents about a 1/3 to 1/4 of the actual tank capacity. Thus why I'm wondering if it isn't some "tolerance stacking", where my fuel gauge reads a little higher than actual level, my draw straw is a hair shorter than it should be and fuel sloshing with the other two to combine for my situation.

BarryG

As six slug pointed out an accurate gauge would show the sloshing however after delving deeper than I ever wanted to into my fuel sender(having gone through 3 of them) I found, again as six slug pointed out, there is a delay in the gauge reading. My aftermarket fuel level sending unit also has a delay built into it so I have effectively doubled my delay (the aftermarket sending unit plus the stock ecm's delay). Which in normal operation is nothing to worry about. Anyone needing to watch their fuel level to the second has other problems. The shape of our tanks long and narrow does promote plenty of fuel moving around fortunately just fore and aft and the pickup is in the approx. center. For example in my driveway which is fairly steep when I start my truck it will read approx. a solid 1/8 tank lower compared to level ground. Within a few minutes of driving it has stabilized up to the correct level level. Now prior to my aftermarket sender it would only take a minute or less. On my stock sender when my low fuel light would come on which is supposed to be 1/8th tank(according to the gauge anyway) it would take 24 gallons to fill it up (35 gal tank). So in reality it still had almost a third of a tank left. I am sure Dodge had their reasons for doing this since when it runs out of fuel it is not as simple as putting in some fuel and starting it backup you have to bleed the heck out of it, which for the average person means a tow as they don't know why it wont start up so it appears Dodge put in a healthy cushion to avoid this situation. Now back to the fuel level my after market sender is much more accurate and linear. 1/8 tank when the fuel light comes on means I have approx 5 gallons of fuel left. The stock gauge I could travel approx. 80-90 miles before it left the full level now it is more like 40-45. So this just to say that the stock fuel level gauge is not very linear or perhaps accurate is a better term. When you look at the tank if the draw straw was say 2 inches short(based on Larry's post) how much fuel is that that you cant use when level much less on a hill, several gallons at least and that would be exacerbated stopping or accelerating or being on a hill due to the fuel moving within the tank. At least during the initial accel/deccel event it would start to stabilize very quickly though. However in that little bit of time it may allow your fp to drop simply because for that second or whatever amount of time wasn't able to draw fuel but does recover quick enough to keep the truck from stumbling. Anyway that was my thought process based on your original post.

While I certainly don't know this is the answer it is just postulation on my part, six slug does bring up some other valid points to consider. Hard to tell for sure unless you lift the bed and see how short the draw straw is with the tank in place. However as you said keep it above 1/2 tank and let it go is also a viable option at least until or if it progresses.

Numidian

Sounds like we're in agreement, your just more of a wordsmith than myself

BarryG

Oh great! I'm a long winded wordy EDIT
My wife (whom I love dearly) would certainly disagree with the amount of communication I convey to her. Must admit try to explain my thoughts in as much detail as possible without going too overboard when on the net and then usually go off on a tangent making it even longer, kind of just like this post apparently couldn't let it go with the first line and the smiley.

patdaly

Simple enough, the next fill up, fill when it hits 1/2 tank, see if it takes roughly 17.5 Gallons to full.

That takes the fuel sender out of the equation. I have run for years on the odo on Greenie, but on Brownie, the fuel light would come on barely above E and I still had about 7 Gallons, so if your gauge reads no less than 3/8, your light will never come on.

SIXSLUG

Same here...I've got about 5 gallons left when on E and overhead showing a range of about 20 miles...haven't done this more than a few times and usually in the winter so not too worried about hot fuel cooking my vp.

BarryG took the words right off my keyboard!!