lift pump?

[DaveNY]

Well another puzzlement, I guess the truck is running well for a long time now and I must be looking for stuff, but it seems that the truck runs noticeably better after I top off the fuel tank. I have checked the lift pump and it is definatly working. Did not put a gauge on it. So im thinking maybe weak pump? I can definatly feel better response and power with full tank. Maybe I need a hobby?
Dave

[Hubert]

I dunno if its the feeling of fresh fuel but I get that also and have it with most vehicles too. Maybe its the same as just washed and/or fresh oil change feeling just feels good???

I have often thought maybe its just good fuel where all the bad stuff has settled out and or has something to do with the temperature of the fuel. Where are you and what is air temp usually? I'm in SC and would think fresh tank filled fuel is cooler and denser than the stuff in my tank.

[DA BIG ONE]

Perhaps a little off subject; Lately, I had some issues thinking the worst as in needing an IP, but I found that changing out the IP driver/new fuel filter and lots of additives to fuel and turning down boost was all I needed. I found too much boost can super heat the aircharge to the point of cutting back fuel this is why I had been going crazy in the past w/lower boost egt's would climb, crank up the boost and egt's would become steady @wot only because the fuel was cut back. I had never considered the pressure drop through the IC about 2.5 lbs and this amount put the turbo beyond its efficiency range thus superheating aircharge......w/o a IAC before turbo/after turbo gauge I was clueless!

I can't tell you how many times I had put the fuel gauge on only to find the lift pump was not the problem. Yep, maybe I'm just slower than most w/6.5td know how!

I've decided to go w/gauge install in cabin using fuel isolater to gauge which is around $100. +- USD, gauge is another $60.+- will be a big help for any situation to rule out lift pump as issue.

[DaveNY]

I am in NY on Long Island, I can feel the difference all year and I have tried to rationalize it away with the clean vehicle ride and cool fuel ect. but it just has more throttle response and runs smoother after I top it off. I buy my fuel in bulk 275gal at atime and top off at 1/2 tank, the fuel all has the same shelf life.
I have to thank you for you response cause I was really doubting what I was feeling in the truck. I know I cant tell you when the perfomance slightly degrades because it is so subtle but it sure feels better upon topping off.
PS Does the same thing at Trade Mart in NC.
Dave ??????

[Robyn]

Now this is just a slim chance but its worth thinking about.
Now just suppose there is a small (itsy bitsy teeensy weeensy) air leak in the suction pipe that becomes covered with a full tank?? HMMMMMMMMMM
A small air leak won't kill the thing but it will run crappy.
Take note of how far down the tank has to get before it starts running poorly. Fill it back just at that point and see if the issue resolves.
If this happens I would drop the fue tank/s and see. Or if it has dual tanks maybe an issue with the switching valve.
Put a clear plastic tube in the return line off the IP and when its running sour look for bubbles. You could be getting a tiny stream that will affect power but not bring on the classic stalling and failure to run.
Beings its ok on a full tank I am suspect of the fuel tank pickup. **** You mentioned runs smoother***** Think about it :0)

Robyn

[DaveNY]

Well Robyn the thing is it doesnt run crappy just not as snappy as with a full tank. I cant even tell you when it runs bad because it dosent. It just runs crisper with a top off. Im thinking lift pump cause the gravity push from the full tank just maybe fills the gap for a slightly weak pump? Other than that its really nothing I would go to any length to correct. Ive noticed this for as long as I have had the truck.(63,000 at purchase and now 125,000) more a curiosity than a problem, just wondering now if others have noticed the same?
Dave

[Hubert]

I've noticed it too. Seems almost any vehicle runs better on fresh tank fill. I wonder why. I can see it as a warm fuzzy feeling like freshly cleaned vehicle but could it be something else???

I have heard advice to fill up at a station a day or 2 after the station has recieved its tank fill and do it early in the morning when the station has not seen much traffic. But it does not really matter when I fill my tank it just feels fresh. I have kind of wondered about the temperature of the fuel but it doesn't seem too sensitive to season. A recent pondering is could it be the foaming action of the nozzle oxygenates the fuel and it burns better for a little while????? Its not that I can feel the freshness deteriorate so maybe it is just a warm and fuzzy feeling don't know just curious what others thought.

Are there any bulk additives that possibly evaporate or other reason????

Since the install of fuel pressure gauge I haven't noticed any significant change in fuel pressure compared to tank fuel level but wasn't specifically looking for it.

[DaveNY]

Hubert, thanks at least I know its not my imagination. I guess I will never know what causes it but at least I know that it is really happening cause I'm starting to forget where I put things!
Dave

[JohnC]

Sounds a lot like what pilots call "auto rough". Aircraft piston engines switch to "auto rough" whenever flying over large bodies of water, or in pitch black conditions. No one has ever been able to pinpoint the cause...

[Hubert]

OK I am bored and still curious if there is anything to it. Yeah I know its silly to wonder about these things but .... could it have something to do with stuff evaporating off.

Bulk tanks sit still and are generally sealed or have minimal air exposure. Once you put the fuel in the vehicle tank it sloshes almost constantly exposing different fuel surface area to air. So maybe the fuel does degrade ever so slightly sloshing in the vehicle tank. I know gas evaporates and degrades if left in an open container and if you shake it it will evaporate/degrade faster. Vehicle tanks are vented somewhat and with sloshing / parking and temp changes a little air is exchanged in the vehicle tank. That would be a subtle difference between fuel in a vehicle vs in a still bulk tank. I doubt you could really measure any of this performance wise in diesel over a few gallons but might be something to consider. Hey its a theory anybody have anything better???? Anyway thats what I am going to believe if anything until a smarter person says different.

[garre1tt]

I will not claim to be smarter but I do have a theory or two. First I will say that I also notice the sharpness on the top quarter of the tank.
First thought, and this relates more to my driving stile. After I first fill up I tend to give it more fuel. I have a full tank so I don

[DaveNY]

Im' stickin' with the weight of the fuel giving a little extra push to the lift pump. It has to be something simple and common to all vehicles. Not everyones fuel sock is dirty nor do we have the same driving habits. But it appears that a few of us seem to experience the same phenomenon with a full tank. I still remain open to other theories but for now im just happy Im not alone. Dave