Duramax Diesels Forum Truck of the Week
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Old 08-12-2017, 04:00 PM
zvacman zvacman is offline
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Hi everyone, new to posting but not to lurking.👍🏻 Hopefully somebody is way smarter than me. I have a 2004 that started leaking fuel and I'm pretty sure it was coming from the fuel sending unit. I pulled the box and changed it out with a Delphi unit. After priming the heck out of it I got it to run for about 10 seconds. After pumping it up again, nothing. Then I changed the fuel filter, primed it and it ran fine, about a mile test drive and 30 min idle time. Parked for 2 days and it won't run for more than 10 seconds after pumping it up again. What gives?

Z
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Old 08-12-2017, 08:18 PM
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I'd look into rebuilding the filter head. Any codes?
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Old 08-12-2017, 08:37 PM
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Moved to LLY. So you replaced the sending unit in the tank first, then it started having the issue? Could be the lines from the tank. Could be filter head.
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Old 08-12-2017, 08:52 PM
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Air leak. Trace your steps back.
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Old 08-14-2017, 12:29 PM
zvacman zvacman is offline
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Quote:
Originally Posted by TheBac View Post
Moved to LLY. So you replaced the sending unit in the tank first, then it started having the issue? Could be the lines from the tank. Could be filter head.
Not exactly. I had the truck on the hoist, climbed up and started it to try to see where it was leaking fuel. Before I could get back down it quit running. I never got it to start again until after changing the sending unit and fuel filter. Then the 1 mile test drive and 30 minute idle. Two days later it won't start without priming the heck out of it. The primer never gets any firmer, I pumped it for 10 minutes straight. I haven't been able to find any fuel or air leaks, but it's kind off tough priming and being underneath listening for an air leak at the same time.

Z
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Old 08-14-2017, 02:40 PM
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Could his fuel lines be collapsing? I don't recall the symptoms of that.


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Old 08-14-2017, 03:07 PM
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since all of the fuel lines all the way to the CP3 are under suction/vacuum, you will likely not see any fuel leaks. if there is a leak air will get in and cause it to lose prime.

try replacing the fuel line coming out of the filter housing with a clear one and see if there is air in the line while priming / running
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Old 08-14-2017, 04:13 PM
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I could see it loosing prime overnight if it had a small leak but it sounds like he can't even get it primed. I'd put the piece of clear line in but I'd look into the filter head.
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Old 08-14-2017, 05:44 PM
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Even with a bad filter head most can still get it started via priming.

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Old 08-14-2017, 06:32 PM
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my LB7 had a leak on the filter housing and it took maybe 100 quick presses of the primer to build just enough pressure to get it to start. when i added a lift pump to it, it started without hesitation. and the leak was small enough that with the lift pump pressure it didn't even drip.
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Old 08-14-2017, 06:56 PM
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What are you calling a sending unit? I've never heard of a fuel basket going bad? What did you actually replace?
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Old 08-14-2017, 07:18 PM
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Quote:
Originally Posted by zvacman View Post
Not exactly. I had the truck on the hoist, climbed up and started it to try to see where it was leaking fuel. Before I could get back down it quit running. I never got it to start again until after changing the sending unit and fuel filter. Then the 1 mile test drive and 30 minute idle. Two days later it won't start without priming the heck out of it. The primer never gets any firmer, I pumped it for 10 minutes straight. I haven't been able to find any fuel or air leaks, but it's kind off tough priming and being underneath listening for an air leak at the same time.

Z
You arent going to "hear" an air leak or see a fuel leak. Most of the time, its a very very small pinhole that causes the issue. 2004LB7 hit on it.

Quote:
Originally Posted by Dozerboy View Post
Could his fuel lines be collapsing? I don't recall the symptoms of that.


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Possibly.

Quote:
Originally Posted by Dozerboy View Post
Even with a bad filter head most can still get it started via priming.

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Yeah, but it sucks ass having to prime it that many times to get it to start.

OP, you need to pressurize the fuel system to find the problem. It only needs a few pounds pressure. I'll bet its a connection from a hose to a hard line.
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Old 08-14-2017, 07:24 PM
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If you have a shop vac connect the hose to the blow side and use it to pressurize the tank from the filler neck. You could also use an air compressor with a blow gun, use a rag to "seal" around the blow gun while blowing into the filler neck. You will likely find your leak near the fuel tank in either one of the connections or a rusted out bend.
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Old 08-14-2017, 07:30 PM
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Quote:
Originally Posted by NC-smokinlmm View Post
What are you calling a sending unit? I've never heard of a fuel basket going bad? What did you actually replace?
i had the "sending unit" on my LB7 loose the squirter thingy on the bottom of the basket. resulted in the "running out of fuel at 1/4 tank" problem.

on my 08 LMM the actual sending unit resistor went way out of calibration resulting in half tank empty light on problems.

while these may both be called sending unit problems none prevented starting of the engine
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Old 08-14-2017, 07:31 PM
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Quote:
Originally Posted by bmc1025 View Post
If you have a shop vac connect the hose to the blow side and use it to pressurize the tank from the filler neck. You could also use an air compressor with a blow gun, use a rag to "seal" around the blow gun while blowing into the filler neck. You will likely find your leak near the fuel tank in either one of the connections or a rusted out bend.
just make sure to limit your air pressure to a few PSI
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