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Discussion Starter #1
Hi everyone. So i have an issue of my truck dying when the fuel tank just reaches 1/4 tank. At first, i hadnt put that fact together yet, and i was looking at fuel lines and the filter housing for leaks. When i filled the tank, it had no issues and ran perfectly.....until it hit 1/4 tank again, then it just drops off like its out of fuel. I pulled the tank to see if the sending unit had a crack in the pick up yube, but it looked fine. Could the Cp3 pump cause this, like maybe getting weak? I should also add that this is a totally stock truck with no tuner. Tia for any suggestions or advice.
 

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Gauge might just be off tbh. Cp3 shouldnt cause it to die at 1/4 tank.
 

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Discussion Starter #3
I thought about that, but i used to get close to 400 miles to a tank of fuel, but since this has started im only getting around 300.....which would make sense if i still have 1/4 tank of fuel left. I didnt know if the Cp3 would be possibly on its way out, therefore not able to pull fuel up that far anymore, causing it to die. Hell, i dunno?
 

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I pulled the tank to see if the sending unit had a crack in the pick up yube, but it looked fine.
Did you test it?
Visual inspect won't catch a leak in the fuel module.
 

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Discussion Starter #6
Ill have to really check next time i fill up. I know it doesnt take as much as it used to but ill get a good number tomorrow.
 

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Discussion Starter #7
I did not test it other than look for cracks in the pick up line. I didnt see any place that it could be sucking air though.
 

LHN...We ARE the Joneses
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Cracks in the welds on the top side of the module, suck air at lower fuel levels.
 

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Discussion Starter #9
Cracks in the welds on the top side of the module, suck air at lower fuel levels.
Hmmmm....maybe ill drop it again and look. Is there a piece of specific test equipment or method to test this other than sucking off my tank? Is there some sort of pressure tester or vacuum tester?
 

LHN...We ARE the Joneses
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Pull a vacuum on it, or apply psi to it.
Honestly, with the age and labor involved to get to it, I'd swap in a new one from Rock.

It can also leak from the seal failure in the quick connect line attaching to the nipple on top. This is normally removed and replaced with bulk fuel line and a clamp, when installing most lift pumps.

You might consider installing a JK pump at the same time you're into it. Positive pressure pushing forward will help overcome issues with vacuum finding leaks, or hoses collapsing in the bends. $30 more for a filter head, you can add a FWS filter to the setup too, greatly prolonging the life of the stock filter under the hood and adding more filtration protection.

Some ideas here;
https://www.duramaxforum.com/forum/diy-tech-discussion/806281-my-lift-pump-kit.html
 

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Discussion Starter #11
Pull a vacuum on it, or apply psi to it.
Honestly, with the age and labor involved to get to it, I'd swap in a new one from Rock.


It can also leak from the seal failure in the quick connect line attaching to the nipple on top. This is normally removed and replaced with bulk fuel line and a clamp, when installing most lift pumps.

You might consider installing a JK pump at the same time you're into it. Positive pressure pushing forward will help overcome issues with vacuum finding leaks, or hoses collapsing in the bends. $30 more for a filter head, you can add a FWS filter to the setup too, greatly prolonging the life of the stock filter under the hood and adding more filtration protection.

Some ideas here;
https://www.duramaxforum.com/forum/diy-tech-discussion/806281-my-lift-pump-kit.html
I almost ordered a new sending unit from Rockauto, but after i pulled the tank and didnt find anything, i kind of dismissed that idea. If it had a leak near the welds though, wouldnt that cause it to suck air at all fuel levels? It may still be the sending unit, but ill test it first when i have time to go through that again.
 

LHN...We ARE the Joneses
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If you've had it long enough to know your range, and you've suddenly lost 100 miles on a tank, I don't think it's the sending unit with bad info.

At higher tank levels, the draw pressure isn't as much and any air mixed in goes unnoticed. But at lower levels, the beast raises it's ugly head. Like a drink straw with a split in it.

Usually this shows up as a fuel limp 1st, instead of just dead on the road though.

Another thing to consider too.....was there anything in the basket when you pulled it?
It's not unusual to see an additive cap seal get dropped in the tank, float around for years until it finally gets trapped in the basket at lower levels, randomly being sucked to the teat and causing problems.
 

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Discussion Starter #13
If you've had it long enough to know your range, and you've suddenly lost 100 miles on a tank, I don't think it's the sending unit with bad info.

At higher tank levels, the draw pressure isn't as much and any air mixed in goes unnoticed. But at lower levels, the beast raises it's ugly head. Like a drink straw with a split in it.

Usually this shows up as a fuel limp 1st, instead of just dead on the road though.

Another thing to consider too.....was there anything in the basket when you pulled it?
It's not unusual to see an additive cap seal get dropped in the tank, float around for years until it finally gets trapped in the basket at lower levels, randomly being sucked to the teat and causing problems.
All i saw was just a little bit of stuff on the screen, but it was 97% clear. No signs of any type of blockage. I think there may be a small crack in the tube that i missed when i looked at it, but i felt pretty confident when i put it back in. But its worth a second look. I sure dont want to do it a third time, so i may just get a new sending unit and at least i can eliminate that as a possible cause down the road. Luckily, i keep a 5 gallon can of diesel in my truck box, so at least im not stuck for long!
 
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