low pressure fuel leak got me stumped

OleBlackyLBZ

Active member
May 22, 2020
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During the 8ish years of owning this truck I have fixed multiple fuel leaks, both high and low pressure. I'm 99% sure this one is low pressure since pulling the regulator on the cp3 gives me nothing, also the truck starts fine if I let the lift pump run for 6-8 seconds before I crank it. If I DONT let the lift pump run, it is hard to even get it started, it's never acted this bad starting with any of the other leaks before. Weird thing is, no fuel, nowhere... is it possible it could be leaking backing through my Airdog or have anything to do with something inside the fuel tank? Wherever it's bleeding off it's not visible. I already have all new low pressure lines up by the engine and a fairly new filter head. Whatever this leak is it's bad cause it only take 10 minutes of sitting to take forever to start without help from the lift pump running first.
 

Ron Nielson

Active member
Oct 11, 2009
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Berryton, KS
Is your Airdog actually running and pumping? And is the fuel reaching the filter? Sounds foolish to suggest, but sometimes you need to start with simple. Like James said, try different spots. Choose the easy spots to start with.
 

OleBlackyLBZ

Active member
May 22, 2020
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Weak cp3 or maybe your lift pump is causing aeration. I'd put a clear fuel hose on your filter head and check for air in the fuel.
Truck runs like a scalded ape, no low rail pressure codes. Only issue is getting it started if I don't let the lift pump prime the low pressure side before cranking.
 

OleBlackyLBZ

Active member
May 22, 2020
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Is your Airdog actually running and pumping? And is the fuel reaching the filter? Sounds foolish to suggest, but sometimes you need to start with simple. Like James said, try different spots. Choose the easy spots to start with.
The lift pump works, if it didn't then letting it run for 6-8 seconds to re-prime the low pressure side before cranking wouldn't solve the starting issue.
 

08lmm72mm

Active member
May 13, 2019
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Saskatchewan, Canada
I wonder if your fuel pickup in the tank is getting partially blocked somehow or a fuel connection on top of the tank is allowing air in once the truck is off then when the lift pump is on it seals tight? Getting air somewhere. Sounds pre lift pump to me
 

2004LB7

Well-known member
Dec 15, 2010
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Norcal
are you still running the stock filter head? those are known to leak when they get a few miles on them. could be an o ring, bleed screw, primer bulb or crack in the housing. primer bulb will often seal up under pressure but leak air in when the pressure is off
 

OleBlackyLBZ

Active member
May 22, 2020
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are you still running the stock filter head? those are known to leak when they get a few miles on them. could be an o ring, bleed screw, primer bulb or crack in the housing. primer bulb will often seal up under pressure but leak air in when the pressure is off
Yes, the stock filter head is still on there. It's an almost brand new one that I took from my buddy when he deleted his. If it's leaky it's sure not moist around the seals or showing any signs of leakage.
 

pepper

New member
Mar 23, 2015
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I'm in the exact same situation with my LMM with it bleeding down; started about 9 months ago. Mine is worse after I tow so maybe heat related on my part. Like you, I've replaced everything from my lift pump to the CP3. No drips, no oil increase, rail pressure demanded/actual are fine, balance rates fine and runs perfectly normal when it eventually starts. My next step is to add a one way valve in after my lift pump and see if that helps. It should eliminate that half of the fuel flow.
 

OleBlackyLBZ

Active member
May 22, 2020
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So what are you going to try?
Probably nothing for now, a few months ago it was hard starting but not as bad. Pulled the regulator on the cp3 to max it out and fuel went everywhere, it was a high pressure line leaking but only under pressure when you were in the throttle hard. So it was a pain to find just looking around, until I pulled that regulator off.
 

OleBlackyLBZ

Active member
May 22, 2020
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Does a lift pump hold pressure to the filter head after its shut off. I guess what im asking is what all is supposed to stay primed.
 
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