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Discussion Starter #1
I own a 1999, 3500. Recently I have had issues with air in the fuel. I have air bubbles in the return lines on the injectors. I have replaced the sending unit, fuel lines, lift pump, and fuel filter. When I run the truck at an idle or fast idle it is ok, but once I start driving it and putting a load on it, it starves for fuel and dies. When I crack open the bleeder on the filter air comes out. Also when I bleed the air out of the filter like normal it fills up, as well as when I crack open the T by the thermostat fuel comes out. I also have a code po251 which I've heard is related to the optic sensor reading air in the fuel. Any ideas where I should start? FYI the injector pump, PMD ( brand new d tech), injectors are all brand new. As well as a Delphi lift pump.
 

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Look for a leak in the fuel lines between the tank and the lift pump. They may not show up as a fuel leak but as an air leak because there is negative pressure there.
Also make sure your fuel cap is working and not allowing high vacuum to build in the tank.
You may have also installed a sending unit with a gas sock instead of a diesel sock,
most replacements come with the wrong sock.
 
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You may have also installed a sending unit with a gas sock instead of a diesel sock,
most replacements come with the wrong sock.
^^^^ This is what I was thinking
 

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Fuel sock to install is :AC Delco: TS1012 only! as others have said, 99% of the Senders installed have the wrong sock

Also inspect the bottom of the Fuel Filter Manager and lines under for leaking,cracked or rot and replace if necessary.
 

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I had the same air in fuel issue and all my lines where dry. Took apart fuel cap & installed tire chuck, put 5psi on tank crawled under and lines began weeping between tank & electric pump, replaced all hard & soft lines.

Fuel%20cap%20mod%201_zpskjtrsvt3.jpg

Fuel%20cap%20mod%202_zpsc8ftrp59.jpg
 

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Discussion Starter #6
I will check everything later today when I am with the truck. The sending unit was ordered off of Rock Auto. I will get in contact with the dealer today about the fuel sock and order or pick up depending on if they have one in stock. No sense in messing around over 20 bucks. I'll re-check all the fuel lines and I'll try to pump air into the fuel tank to see if I can get any leaks. Thank you for the insight so far.
 

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Discussion Starter #7
Also, is it best to assume that since the FFM is full of air, that possibly the problem is between the fuel tank and the FFM, and not between the FFM and the injectors??
 

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The line between the tank & lift pump (working properly) is under suction and will introduce larger amounts of air if there is a leak. The line between the lift pump/FFM/IP is under pressure & should show a fuel leak.
 

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There are 2 o-rings and 3 hoses at the FFM and they are prone to leaking due to hardened rubber parts.
The leaks are usually undetectable but it can cause air in fuel system.
 

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The line between the tank & lift pump (working properly) is under suction and will introduce larger amounts of air if there is a leak. The line between the lift pump/FFM/IP is under pressure & should show a fuel leak.

One should never ASSUME that just because its not leaking fuel out when in USE that it cant be leaking air in when not.
Fact is, MOST air leaks dont leak a drop of fuel, many dont even exhibit a wet spot to help offer up a clue... IF IT WERE ONLY THAT EASY, maybe GM Techs and other mecanhacks wouldnt be so damn quick to replace perfectly good IPs based on little to half assed troubleshooting (plugging in scan tool) and incorrect assumptions.. Its those same assumptions that continue to fuel sales and Tales of PMD Goblins too but lets slay this myth before tackling another...

Unlike fuel molecules, air molecules can NATURALLY squeeze past cracks and orifices that liquids (diesel fuel) can not... The "low pressure" fuel supply system is constantly fighting with atmosphere thats relentlessly searching for a means (way in) to equalize the fuel LEVEL just as atmosphere constantly replenishes the lift pump inlets "low pressure" zone that developes from the lift pump pressurizing the system ..
LIFT PUMPS BLOW (create pressure) S'IT, THEY DO NOT SUCK (create vacuum) IT! Thats how a breached "suction" line introduces air into the fuel system, atmosphere pressure rushes in in an attempt to equalize system pressure. Exactly the same principle applies when the lift pump fails or is unable to keep up with the IP's transfer pump demands that OFTEN expose air leaks that otherwise would go unnoticed.

Also, once exposed and the fault (defective lift pump for ex) repaired, the new "Air-ports" dont usually fix themselves and if left unchecked, will continue to sneak in and cost you further headaches and/or costlier unnecessary repairs, or until the engine has done a self purge of all the air trapped in the system...
If for whatever reason(s), air creeps into the system and starts to displace the fuel, the otherwise "SEALED" fuel system is no longer capable of holding the elevated fuel (FFM) in suspension (as it normally does) when the vehicle is not in use, allowing fuel to drain back to the fuel tank, how fast or slow depends on the size and/or location of the system breach..


Did I mention that a restricted but otherwise completely SEALED fuel supply system is capable of harboring air leaks too.. OH MY..


The Phenomena Of Air Separation In Diesel Fuel
 

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6.2 thread, but it explains how to find air leaks. http://www.dieselplace.com/forum/63-gm-diesel-engines/20-6-2l-diesel-engine/87582-how-remove-air-fuel-system-find-air-fuel-leak-6-2l.html Fuel will not necessarily leak out or weep on an air leak. Most air leaks will be before the pump, as when pressured, a fuel leak will result.
If the engine valley is wet, my guess at the culprit would be the FFM, as was suggested and well illustrated :thumb:. I fought one for a long time, most of it turned out to be my IP, but that was showing other issues with the governor as well. Sock my give drivability issues that your seeing, but it shouldn't show any air bubbles until it starves maybe.
 
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Don't forget, bmxdkj, foamy fuel at the fuel manager can be vacuum also, not necessarily air but lack of.
 
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