Return Line Bubbles

[Big T]

I think you are reacting to something that is not a problem.

The return lines from the injectors only carry excess fuel when the injectors cycle.
When the injectors are not popping there is no fuel flow so air takes it's place-you can't get away from having air in the injector return lines no matter what you do.

Where you put the clear line from the IP return is where you need to place your concern and since you have already proven that there is no air coming from the IP at that point all is well.

Had you found bubbles in the IP return it would indicate air intrusion into the fuel system between the fuel level in the tank and the LP.

My injector rebuild guy said the same thing: there will always be air in these return lines. He's seen 6.5 engines with clear return lines and they always had bubbles in the return lines, yet they were running great. He said the area to be concerned would be the line at the IP that Bison had me test and that was fine.

 

[bison]

On the pressurized side I would agree with you on that statement,however the return side has no closure to it-the vent in the filler cap back through the tank itself,the return line and all the way to the injector return lines is open to the atmosphere.

The topic has been posted before when folks use clear or tygon line for injector return lines when changing to new or properly working rebuilt injectors with no driveability problems-most all return line kits even as sent from the factory are opaque lines that you can't see whats going on thus no idea to be concerned and no reason to be concerned either.

Excess fuel sent out on an injection pulse that is still pressurized even after the injector closes needs to go somewhere,but it is not a constant stream since the injector is not always fed pressurized fuel when not on a power stroke-that means a sporadic flow from the return nipples.
You cant have a full return line when the flow is that intermittent and in between the pulses the line back feeds with air.At least as far as my understanding goes.

The return side terminates in the bottom of te tank and is always submerged in fuel so air can't backfeed in the line(unless the tank is bone dry)

Notice the bubble flow is towards the tank and not towards the injectors.

I agree it won't affect the running but it can cause fuel bleed back after shut down,if air comes in trough a bad injector if the nozzle leak is severe.

That's my take anyway.

 

[racer55]

Actually the return dumps into the top of the tank-the sending unit only has the suction side that extends to the bottom,the return is an open hole at the top.

 

[bison]

Then what prevents fuel bleed back after shut down?
The checkvalve in the LP is notoriously unrelyable.

 

[racer55]

Nothing needs to be in place to prevent fuel bleed back on the return side unless there is extreme vacuum in the tank and that problem needs to be dealt with on it's own.

Otherwise the checkvalve in the LP is what prevents the pressurized side from having drain back.

 

[WarWagon]

Actually the return dumps into the top of the tank-the sending unit only has the suction side that extends to the bottom,the return is an open hole at the top.

Your tank is missing the black tube that goes from the top of the tank to the bottom on the return. Otherwise the fuel system would drain back out backwards from air at the top of the tank allowing the heavy fuel to fall back into the tank through the pickup. The IP pump and the lift pump leak. The lift pump taking longer to leak. This is why air leaks cause hard starting after being off a long time - overnight.

 

[racer55]

Could well be,mine never had a return into liquid fuel,nor did it ever have any siphon effect or hard start problems.

 

[WarWagon]

The black rubber/plastic tube shown here in this "mess":

 

[racer55]

Yea,I did a bit of research myself and did come across a picture of a 2000 sending unit with the return line tube-could be mine is incomplete.