Car runs poorly when I get down around 1/4 tank of gas?

ghunt81

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I swear I am not imagining this...

I only drive my car 2-3 days a week and my commute to work is not even 10 miles so I don't put a lot of miles on it.

I noticed several weeks ago the car was running weird below 2500 RPM, felt like it was missing a bit especially starting from a stop and you could hear it from the exhaust. I filled the tank just to see if it was something weird with the gas, and within a mile or so it cleared up and was running normally.

Well, again this week, I get down to around 1/4 tank and it starts running like crap again. Drive it with it being like that for a couple days, so today I stopped and put 10 gallons in it and again within a mile or so, no more evidence of missing or anything. WTF?
 

AHaze

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Mine started doing the same thing recently. In my case, fuel isn't properly transferring from the passenger side of the tank to the driver's side. There's a fuel level float on both sides and I believe the gauge shows an average af the two so if the passenger's side is half full and the driver's side is almost empty, the gauge shows 1/4 but the pump starts picking up air.
The only obvious issue I found is a crack in the rubber duckbill valve on the fuel pump assembly. I can't see how that would cause this issue but I'll replace it and see.
 

OLD 96

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Junk in the bottom of tank? That is getting picked up? When fuel level gets low? Fuel filter need changing?
 

ghunt81

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Doubt it's the fuel filter, I don't think it even has 5000 miles on it. What AHaze mentioned sounds plausible.
 

Dino Dino Bambino

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It looks to me like you have sediments getting stirred up in the bottom of the tank. Next time the tank is near empty, pull out the fuel pump assembly from under the rear seat and take a look inside. Also check if the fuel pump filter sock is dirty.
AFAIK there's a fuel gauge sending unit on each side of the tank. Since the passenger side fills up first, the sending unit on that side reads from E to 1/2, and the driver's side sending unit reads from 1/2 to F.
 

86GT351

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It looks to me like you have sediments getting stirred up in the bottom of the tank. Next time the tank is near empty, pull out the fuel pump assembly from under the rear seat and take a look inside. Also check if the fuel pump filter sock is dirty.
AFAIK there's a fuel gauge sending unit on each side of the tank. Since the passenger side fills up first, the sending unit on that side reads from E to 1/2, and the driver's side sending unit reads from 1/2 to F.
Good points
 

Iceman62

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Waaaaaay back in 1985, I had a Dodge Laser that wouldn't start when fuel level was low (<1/4 tank). Ended up being issue w/ fuel pump where it would lose prime & drain lines back into tank. Replaced pump & no more issues.
 

Laga

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Let’s look inside the tank. EA41A7C3-F1A4-449C-85E6-86ECE7430C9D.jpeg 34E05C61-3C40-4054-81E5-71878FD5AE41.jpeg
The top photo is the drivers side. The bottom is the passenger. The black hose inside the tank is the crossover tube. On the drivers side it is connected to the fuel pump where a Venturi opening pumps gas from the passenger side to the drivers side. On the passenger side, it is connected to the fuel level sensor and sucks fuel from about a 1/4” off the bottom of the tank. I think AHaze is correct and something is preventing this transfer. Whether it’s a pinched or broken tube. This is know to happen. You would then drain the drivers side while still having fuel in the passenger side. The “curb” on the drivers side is where the fuel pump sits and allows for the pickup of all the fuel in the tank. ( So much for the myth that a low fuel level burns up a fuel pump) There is a hole at the bottom of the curb in the photo to let the gas in.
These photos were taken last December when I was having fuel level sensor problems. The car is an 2005, so the tank was almost 17 years old. Do you see any sediment? That stopped 30 years ago. Plus, the level of the fuel would not make any difference IF there was sediment because the fuel pump sits on the bottom of the tank.
Open up both sides of the tank and inspect everything. Make sure you can blow air through the crossover tube and everything is intact.
Putting the locking rings back in can be a real pain. If you look closely, there are tabs on the tank that the rings slide under. Bend the front edge of the tab up SLIGHTLY, with needle nose pliers, and the rings go in much easier.
 

WJBertrand

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Another thought. The fuel pump is cooled by immersion in the fuel. Is the pump becoming exposed above the fuel level at 1/4 tank? If so it may be heating up and starting to fail.


Sent from my iPhone using Tapatalk Pro
 

Laga

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Another thought. The fuel pump is cooled by immersion in the fuel. Is the pump becoming exposed above the fuel level at 1/4 tank? If so it may be heating up and starting to fail.


Sent from my iPhone using Tapatalk Pro
The fuel pump is cooled by the gas passing through it
 

WJBertrand

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The fuel pump is cooled by the gas passing through it

Not sure if that’s true for all designs. I know these are common failure symptoms for the fuel pump used in some motorcycles, the ST1300 for example. The fuel flowing through the pump is evidently not providing enough cooling. They exhibit the same symptoms; work fine with plenty of gas in the tank and begin to fail as the fuel level drops low enough to expose them. Mine started failing that way and the pick up screen and tank bottom were nice and clean. A new pump fixed it.
 
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Laga

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Thats not entirely true. The pump needs fuel around it.
Do you remember Pat Goss? He was the mechanic on “Motorweek” for 40 years. He had his own repair business for over 50. Sadly, he passed away earlier this year. Before he did. This was his answer to the question, “Can driving with less than a quarter of tank of gas damage your car”? 6BAA82A5-3ED2-4079-BF91-D15F83BE58AB.jpeg
I’ll stick with his opinion.

Plus, look at the path the gasoline take in a fuel pump.
9A60975F-9BEF-42B9-9FBA-421A1E408189.jpeg
The gasoline passes over the areas where the heat is generated.
If the pump needed to be immersed, there would be a cut off float, and/or the tank not designed for the last drop of gas to be picked up. Like above.
Ever hear of the Hydramat made by Holley? It allows the in tank pump to draw gasoline without even touching the gas.
F1B5D2E2-D3B6-4885-B1FE-468CE347689B.jpeg

2A688CCE-B2EF-463A-A872-8AF18CE56BFE.jpeg
 

Pentalab

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With < 1/4 tank of gas....and it sloshes around pretty good, esp back and forth. Some have posted on S197 that the eng goes temp lean, when gas mashed, and remaining fuel in the tank get sloshed towards the rear. You could clearly see that on the data logging, where the AFR ratio went straight to hell..then went back to normal.

On my 2010..with small roush M90 blower, a few summer's ago, as soon as I tried to get into boost, it would gag. Fuel pressure dropped from 40 psi..down to 5 psi....and FPDC skyrocketed. Below boost, it ran fine. WTF ?

Turns out there was a lot of gak and crud, floating around the bottom of the tank..on the driver's side. The 'sock filter' at bottom of oem fuel pump was completely clogged up with crud. Once removed, and cleaned, and put back in, it ran fine. Note the issue was with 3/4 to full tank of gas...and not < 1/4 tank.

I suspect the OP's issue is that interconnecting tube that flows gas from pass tank, to drivers tank.
 

Pentalab

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I can watch fuel temps via my aeroforce gauge plugged into the OBD port. But I believe that fuel temp reading is done upfront, in the eng bay, or fuel rails etc.

The story I got was these pumps are cooled by immersion in the tank. I believe that came from Ford, who also insisted not going below 1/4 tank.
 

Laga

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The fuel temperature sensor is part of the fuel pressure sensor on the fuel rail. (At least it is on the F150 4.6) . So that is immaterial.
My Walbro 400lph pump is fully encased in plastic around the metal pump. This would insulate the pump and prevent heat transfer. Why doesn’t it overheat?73F8C83B-74BF-4ED6-8843-43881A643994.jpeg
Ever go WOT with a low fuel level, or take a long , hard left hand turn on a track day? What happens? The pump sucks air and you get fuel starvation. No fuel passing through the pump ( and no lubrication for the impeller) will definitely overheat the pump.
 

Ret

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Reading the Rs197 post is like reading the morning paper for me. I learn something new every day about our Mustang's. The car posted having the fuel pump issues is a 2005. I'm not having any issues but curious does the 2008 GT/CS come equipped with duel fuel pumps also.
 

Juice

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I have run mine down to having the light low fuel light come on on track. My car never sputtered. And the fuel light went out when I came in the pits. So, no slosh effect for me.

Its easy enough to monitor fuel trims and pump duty cycle to see whats happening.
 

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