Fuel drainback through lift pump?

[Koch13351]

So I upgraded my 6.9 to the 7.3 filter head and return system, using 1/4" clear fuel line on injector and IP returns, along with 3/8" clear IP feed line to eliminate all olives, to be able to track air intrusion and system function, and to get rid of the seperate fuel/water seperator. I also cut down the lift pump to filter head hard line just below the factory 6.9 fuel heater and have 3/8" rubber fuel line up to the filter.

I had no issues for several days after I installed everything, then all of a sudden I had the infamous start then stall air intrusion issue. With the clear fuel line it was instantly apparent where the air was coming from because i could see bubbles being drawn in. The clear fuel line was leaking at the barb fitting on the outlet of the filter head. Even with a clamp on it, it wouldn't seal, as if the ID was too large for the barb. So I replaced it today with regular black rubber fuel line and it fit much more snug. After finally getting all the air out of the system and firing it up, I let it run for about 20 min while I was checking for leaks, purged the filter head through the schrader valve, revving it here and there, and all seemed well.

When I shut it off from the engine bay, I watched as air just flooded the return lines from the filter head return! And I could hear gurgling down by the lift pump. Pump is new, no leaks from any of the lines or outlet compression fitting. I immediately started it back up and let the air purge out until it was running smoothly, then shut it down again. Nothing. No air intrusion, small bubbles in return lines stayed still, showing the fuel wasn't moving.

So fuel can drain back through the lift pump, I'm guessing if it's stopped in the middle of its cycle? Is there a feed line check valve in the system? I thought I saw a mention of one before, but I looked and didn't locate one. If there is, I'm guessing it's irony that as soon as I mess with the fuel system, problems start to arise, as I didn't have any issues before.
Thanks in advance!

 

[Shawn MacAnanny]

The 7.3l filter head has an air vent on it with check valve I just took one apart yesterday and posted pics on the fte forum for a user.

I had the same issue on the parts truck I picked up. I ran an electric fuel pump to diesel can, it'll run. Run it to tank it runs and dies. I ran all the way to the top of the fuel sender to eliminate everything. Left pump on and listened, found pin hold leak in top of steel fuel sender elbow. I could only hear it sucking with an electric pump. I patched it with quick steel and it's been working fine since. I'd check your sender elbow.

There is also supposed to be a backpressure duck bill check valve on the return line in in the tank. This prevents the return lines from draining out.

 

[Koch13351]

It's not drawing any air in while it's running. Once the filter head is purged of air, no more will come out, even after 20 min of run time. And it doesn't always drain back, it's pretty random. When I had the leak on the IP feed line, air came in very slow and It took awhile for the injector return lines to drain out. Now with the air leak I'm having, the injector return lines purge out in seconds. It's crazy. I know there's a check ball in the filter head return port, but that doesn't seem to be the issue. Air is coming from the other direction. I'm gonna check it out again right now to see if my eye catches something I missed earlier.

 

[Shawn MacAnanny]

Fill your tank above half full and see if the issue is still there. If you lost the check valve in the tank it can allow the return line to drain. Filling the tank up will submerge the return line and act as a check valve. If there is air getting in somewhere else it will still be there with a full tank

 

[Koch13351]

Well I started it and shut it off about 15 times just now. Couldn't get it to replicate the massive flood of air into the system. I've only seen it do it twice. And not every shutdown leads to a hard start due to air intrusion. However, I do have air coming in at the #1 return cap now, not from the o-rings but from the barbs, which definitely wasn't happening earlier as I spent plenty of time watching the lines like a hawk after a shut down to try and locate leaks. It also forms a drop about every 20 seconds or so while the engine is running. Probably burrs on the barb from casting. I had this issue with a couple of the caps when I installed them, so I must've missed this one. I'll de-burr the barbs on the cap tomorrow and continue the search for this ghost. Thanks for the replies!

 

[Koch13351]

Fill your tank above half full and see if the issue is still there. If you lost the check valve in the tank it can allow the return line to drain. Filling the tank up will submerge the return line and act as a check valve. If there is air getting in somewhere else it will still be there with a full tank

I have one tank with 1/4 and one with a hair under 1/2. I switched to the tank with 1/2 while it was running after I replaced the line, and that's the tank I had it on when it purged quickly. Truck is on level ground. However, I'll keep this in consideration since it does make sense. Just odd that it doesn't happen every time, so I'm guessing the stroke position of the lift pump has part to do with it. After I handle the cap leak and everything holds, I'll manually turn the engine over to check out this theory. If that's the case, I'll just put a check valve inline of the pump and filter.

 

[junk]

Maybe check the fuel heater o-ring? super simple to change if it's leaking. I just a Viton O-ring from a cheapy Viton o-ring kit I had.

 

[Koch13351]

Maybe check the fuel heater o-ring? super simple to change if it's leaking. I just a Viton O-ring from a cheapy Viton o-ring kit I had.

I took care of the fuel heater with a pipe plug prior to installing it.


Anyways, I got busy on the leaking return issues this morning and those are fixed now. System is holding, no air anywhere. Checked back just now and still no air after several hours of sitting. Also, I couldn't get it to flood with air from the lift pump side, so I'm still unsure about that. Tried it on both tanks and got nothing. I'll keep an eye on things and if it happens again, I'll just put a check valve in and call it good. At least I can drive it now!

 

[towcat]

pull your engine oil dipstick and take a look and smell of the engine oil. if the oil is runny and smells like diesel fuel, you found the leak source of the drainback. hell, it's a $35 part. if I suspect it for causing problems, I change it out now than wonder and let it wipe out my motor from filling the crankcase with diesel fuel.

 

[Koch13351]

Already checked that as a possibility, just forgot to mention it. And I replaced the pump in January. Thanks though. I've been driving it for the past few hours running errands and haven't had an issue yet.

 

[Koch13351]

Well, it happened again. Popped the hood before I started it this morning and there was air in the return lines, but only near the tee I have the filter head returning to in between #1 and #3. Started, then stalled, then started up after only a couple seconds of cranking, and ran rough for about 15 seconds. Let it warm up and then drove about 40 miles to pick something up. Returned to my truck after it had only sat for maybe 10 min and when I started it, it ran rough from air in the system, but didn't die. Drive back towards home and made a quick stop for a drink. Truck sat for maybe 3 minutes, started it, and it died. Cranked again with it floored and it started back up, running rough while purging air again. This happened a couple more times today after that, but not as bad, and not every time. And of course, when I finally get the chance to take a look at it, it's fine. Let it sit for an hour and there's still no air. Started it from under the hood and a decent amount of air came out of the filter head return, but it ran smooth. Something seems to be draining the filter head out.

 

[Shawn MacAnanny]

Were you parked on any incline at any of those locations?

 

[Koch13351]

Parked facing uphill overnight on a FULL tank, and relatively flat everywhere else.

 

[Shawn MacAnanny]

Just curious if you could see any wet spots if it were a line, or anything. I've heard of the schrader valves leaking, some put a tire valve stem over them.

 

[Koch13351]

No leaks in the engine bay and no wet spots on the ground. I used the schrader valve to purge any leftover air in the filter head, and it was dry to begin with. I also keep a sealed valve cap on it.