Enthusiast
OK, so if I have confirmed it runs with the engine running, it should be running all the time with the engine running. At least I can rule that out. When I activate the priming circuit (pigtail just behind the fuse box) I can watch fuel come out of the center tube in the FFM There doesn't seem to be any bubbles at all, but it just flows very slowly. It does not fill as fast as the engine uses it. I have ordered the in-tank sock, so hopefully that is where the issue is.
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Enthusiast
I pulled the tank to change the sock, and the sock looks fine. I am changing it, but am pretty sure I need to look elsewhere for the issue. I also noticed zero evidence of leaking or places where air would be getting into the system.
While preparing to pull the tank, I tried to use the return line to empty the tank of the remaining fuel. I attached a hose at the rear fitting and jumped the LP. Nothing came out. I opened the FFM and attempted to suck to siphon - nothing. It was as if there was a blockage. The line between the tank and the LP is open and unobstructed. I am not sure why a blockage in the return line would cause a fuel starvation issue, so any suggestions will be appreciated. I am very stumped on this.
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Enthusiast
A blockage in the return line will over pressure the IP and it will shut down-might seem like a fuel shortage when it is a fuel overage.
2005 Chev K3500 CCLB
Enthusiast
Except when the truck dies, I can open the FFM and it is empty. I will run the LP and fill it back up, and truck runs again - for about five minutes, then I have to do it again. This issue has actually gotten much, much worse. When it started, I would be able to maintain 50mph to get home from 20 or 30 miles away. Now, I can not make it a couple miles before it dies.
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Administrator
The return line isn't "open" through the supply side of the system. While return flow is allowed while the engine is running, the amount is small, compared to the capacity of the lift pump. If the ignition is "off", the ESS will not allow any flow from the lift pump to the return, in any case. Engine off return flow, assuming it is allowed, would be very restricted. If you want to use the lift pump to drain the tank, either connect a hose to the LP outlet, or use the water drain hose/valve. To siphon the tank, disconnect the lift pump inlet and connect a hose there.Originally Posted by JTodd
As said, if the return plumbing is blocked, the engine won't run, for a couple reasons. Fuel pressure at the injectors will equalize, preventing a "pop" (actually, they'll pop once, then not reset), and the same will happen in the injection pump. A blocked return results in a lack of (sufficient) differential pressure, resulting in insufficient fuel flow to operate the system. Continued attempts to run with a blocked return will result in significant IP damage.
Enthusiast
If the line is not normally open, how do I check for blockage? Would this return line failure (if there is one) cause the FFM to actually run out of fuel?
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Administrator
Your lift pump isn't supplying fuel to the FFM/IP while it's running. If it is able to fill the filter while not running, and doesn't while running, the pump is unable to maintain pressure/flow to keep up with running demand, or it's stopping after the engine starts. Get a pressure gage on the system and watch for when it actually falls off.
Also, if the filter is emptying, you have an air leak somewhere. Even with a failed LP, the engine should run indefinitely under light load. The injection pump is capable of drawing fuel, at a reduced volume.
Enthusiast
Have you tried running with the filler cap off?
A blocked cap vent could prevent the LP from pulling fuel from the tank.
Another step you might want to take is to make a flexible fuel line that you can connect to the LP inlet and try running from a 5 gallon fuel can.
Last edited by racer55; 01-19-2013 at 10:14. Reason: gallon pail.
2005 Chev K3500 CCLB
Enthusiast
The FFM seems to be under vacuum when I open it to refill.
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Enthusiast
I ran it with the cap off and still same problem
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Administrator
No, the return line will have no affect on the fuel filter level. I don't think the return line is the problem. Focus on the supply side, and when it does or does not actually supply fuel.
Fill the filter. Install a fuel pressure gage on the water drain line. Open the water drain valve. Start engine. Watch fuel pressure. It should maintain about 4 PSI at idle. If it's less than that, or falls off gradually, the lift pump is likely failing. If it drops to zero immediately or soon after start, the pump has likely lost power, for whatever reason. If the pump is still making noise and no pressure, it's shot, or there is a significant air leak between the LP and tank (also supported by the filter emptying).
Enthusiast
Lift pump is a brand new AC/Delco unit However, it is exhibiting the exact issues as with one it replaced. I will get a pressure gauge to check that side.
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Administrator
Silly question, I know, but, how much fuel is in the tank?
Enthusiast
I drained about eight gallons while dropping the tank. Fuel starvation issues have shown themselves with a full tank and everywhere in between.
Jeffrey Todd
_ _ _ _ ___ _ _ _ _
'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Enthusiast
Check the LP harness ground,it should be mounted to the frame in the vicinity of the LP,a bad ground can cause the LP to be under powered even if it is running.
2005 Chev K3500 CCLB
Enthusiast
I will check that, but here are the results of the fuel pressure test after I purged the air from the FFM
Initial glow cycle - 6#
Start and idle - 6#
Run hard until it starts to miss - 0# (This was after about 40 seconds of very hard acceleration, brake, acceleration)
back at idle climbing back to 6#
It seems that even though the pump is bringing it up to 6#, it is not supplying the needed volume when engine under full load.
Jeffrey Todd
_ _ _ _ ___ _ _ _ _
'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Enthusiast
The last thing I can think to try is the re-ground the LP. I am going to splice into the ground just off the LP and run it to a frame bolt. I will replace the fuel filter so everything will be new. That was replaced earlier while having the problem and no change, but that will complete the replacements. A couple years ago I had an air leak in the fuel line right off the tank. To repair, I extended the rubber lines further up the frame to get past the rusted area. Is there a chance that the rubber lines are collapsing? Beyond those, I am out of ideas.
Just to re-state what I have done
New LP - AC/Delco
New FFM last ditch screen
New filter sock.
Run without fuel filler cap
Confirmed no blockage from tank to LP in fuel line
Confirmed no air leaks in fuel line
Getting 6# of pressure at idle
Jeffrey Todd
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'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
Administrator
Definitely get a close look at the lines from the tank. Plan on replacing them, at least. Collapse is possible, as well as a kink, or a crack (air leak).
Everything complicated has been eliminated. Only the simple stuff remains.
Enthusiast
Even later model trucks with a 93 LP will regularly see 0 psi fuel pressure at full throttle applications-as long as the gauge does not drop into the vacuum range you will have sufficient fuel flow.
If the gauge does make it to vacuum range there is a real problem.
2005 Chev K3500 CCLB
Enthusiast
Additional ground for LP did not work. I will try a new filter, but doesnt the filter work when taking fuel from the FFM and not when it putting it in? After that I will drop the tank again and replace the fuel line from the tank to the metal line. Perhaps over time it has degraded and is collapsing enough to restrict flow but still allow LP to maintain pressure.
Jeffrey Todd
_ _ _ _ ___ _ _ _ _
'96 6.5TD GMC Suburban 2500 4x4 - 300,000 miles, JK remote FSD, Dual Thermostats
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