Siphon effect preventing air incursion?

[Shern]

What condition would allow an IP to lose prime only while using the cigar hose?

If using said cigar hose, each morning I get a 5 inch long air bubble in the clear line from the IP and a bunch of air working its way out of the pump for the first 5 mins of running. No bubbles in any other clear line.

If I switch the cigar to 5/16 fuel hose, I do not.
Obviously I have an air leak. Just not one big enough for fuel to breach. But here's where it gets weird. It's not on the suction side of the pump (-), it's on the
pressure (+) side.

Ooowee have I been through the paces with this one.

I have replaced the tank strainer,
the rear rubber fuel lines,
the clear hoses,
all of the filters,
all of the crush washers,
I have rebuilt my lift pump,
Switched to worm style clamps,
Rebuilt my OFV,
Replaced the DVs in the IP,
etc etc etc

I drew a ridiculous diagram yesterday attempting to understand the physics at play. There is some sort of siphon effect that keeps the lines full only when using a narrow return line (5/16). As soon as I switch to the cigar, that effect seems to disappear and bubbles begin to form in the clear return line.

Over the years there have been tons of posts about this. I even found one on benz world where a guy had a pinhole in an injector hardline, which, insanely, did not leak fuel despite being totally pressurized.

Now that I have fresh clear lines I can see what's happening while the engine is running. The clear line from the Lift pump is bubble free. The clear line to the IP is bubble free. The clear line from the IP... bubbles until I've driven a bit. They return after shutdown.

After a 3 year hunt, I can feel it. I'm close to finding the culprit.

What condition would have to exist to allow such an odd phenomenon?

[vwnate1]

I can't wait to learn what you find .

Long ago I had an old VW convertible I was resurrecting, every night the brakes would gain air and go soft .

It turned out to be a pin hole corroded on the longest steel pipe in the car, right behind the driver's seat .

Oddly, no matter how hard I stamped on the brakes after bleeding it lever leaked a drop until I wire brushed the out side of the pipe .

[ROLLGUY]

Just a wild guess- Could you have a bad cigar hose?

[Shern]

Originally Posted by ROLLGUY

Just a wild guess- Could you have a bad cigar hose?

I wish... I’m on my third one.
Def not the cigar hose.

Even if that was the issue. How would air find it’s way past the ball check in the ofv?

[Maximan1]

I had the same issue on my rebuilt OM617, turned out the overflow valve on the IP had vibrated itself loose (the bolt that retains the spring, that is) and was leaking air into the system when it parked. If yours is a 2 part one like mine, that's a possible leak location.

[Shern]

Originally Posted by Maximan1

I had the same issue on my rebuilt OM617, turned out the overflow valve on the IP had vibrated itself loose (the bolt that retains the spring, that is) and was leaking air into the system when it parked. If yours is a 2 part one like mine, that's a possible leak location.

Interesting. I’ll have another look there.
Say that’s it, I wonder why the smaller diameter return hose would prevent leak back?

Was yours leaking fuel as well?

[Maximan1]

I think it was leaking a small amount of fuel onto the motor mount arm and oil pan. No real driveability issues, but I was not able to get a before and after comparison after tightening it up (I noticed the ofv was loose while swapping the injection pump).

[Shern]

I wonder if there’s a small amount of negative pressure within the pump. Just enough to equalize with the outside air after shutdown. Trying to imagine a scenario im which air would be sucked in, but no fuel would dribble out. It’s about as much as I can do atm while this blizzard blows.

[Maximan1]

I don't remember that well, but perhaps the ball/spring will prevent fuel from dripping out but air can still make it's way in.

[Shern]

Okay, I believe I understand this a bit better now. I ran a clear hose in place of the cigar hose for a week and found that I had no bubbles whatsoever while running or otherwise. More interesting I suppose, was that when I would check the line after a prolonged shut down, it was always full of fuel. Solid column.

Threw the cigar hose on today, and once more,, after prolonged shut down found the clear line coming from the IP with a 4” bubble in it..

The question I’m now asking myself is whether this is simply
normal, or better still, if it truly matters?

I think the reduced diameter of the temporary clear hose keeps it full of fuel both while the motor is running, and when shut down, like a drinking straw with a finger over it. The cigar hose with its wide diameter and room for surges is never completely filled with fuel (drinking straw with no finger). After shutdown, it must drain back into the tank. The air cavity and like of siphon pull, allows the IP return line to drain back into the IP.

I guess when I’m running the clear line, that siphon effect keeps the IP return line from draining back into the pump. All that’s keeping it from draining a after all, is a ball and spring which I doubt would keep anything completely liquid tight.

My question now, I suppose, is shouldn’t the IP be completely full of fuel? Why is there room for drain back to occur? And is this drainback normal? In other words, does this happen to everyone?

[Shern]

No takers? I realize a lot of you guys have brown return hoses but surely someone must know if there’s a bubble in there come morning?

The other suspect is pitted ball bearing in the overflow valve.

[barry12345]

I will bite. Logically you have hit upon it is the cigar hose involvement. If there is contained air in there I would not have expected it. Are the pulses strong enough to create it? Or is it just trapped in there like the air bubble in the pre filter?

Anyways. It might be interesting to close off the fuel line around the small filter area as soon as you shut down. To see if the valves in the lift pump are allowing some drain back overnight.

If starts are very easy after a prolonged sitting of a day or more. Personally I would not bother with it currently.

I have had this theory that when new the fuel system was so tight. As soon as you hit the key. Full element loads where injected into the engine. So the engine had plenty of fuel to start on. Rather than some delay in getting enough full elements injected. I do also think there might be a small leakage factor present with the relief valve. As you also mentioned.

Again does all this really matter depends on how the engine starts up. Again just my opinions. To me it is more critical to examine the fuel systems when delays in really cold starts are present.

[Diseasel300]

If there isn't any air in the system when a standard/skinny hose is installed, it isn't the relief valve and is unlikely to be air ingress. More than likely it's just down to any air that was present in the system settling out in the wider hose (that's higher than the tank) and collecting there, just like you see in the prefilter. A skinnier hose has a higher flow rate and will tend to sweep any air along with it back to the tank while the "wide spot" of the cigar hose will have a significantly slower velocity through it compared to a skinnier hose and will tend to allow any air to settle out and collect rather than flow on through.

The lack of any air with a standard hose fitted says the system is tight, or certainly tight enough not to cause issues. If the car starts right up and isn't skipping or sputtering, hook the hose up and let sleeping dogs lie.

[Shern]

Originally Posted by barry12345

Anyways. It might be interesting to close off the fuel line around the small filter area as soon as you shut down. To see if the valves in the lift pump are allowing some drain back overnight.
.

I went through this process some time ago. No issues there. I also replaced those valves a couple of years ago.

Originally Posted by Diseasel300

If there isn't any air in the system when a standard/skinny hose is installed, it isn't the relief valve and is unlikely to be air ingress. More than likely it's just down to any air that was present in the system settling out in the wider hose (that's higher than the tank) and collecting there, just like you see in the prefilter. A skinnier hose has a higher flow rate and will tend to sweep any air along with it back to the tank while the "wide spot" of the cigar hose will have a significantly slower velocity through it compared to a skinnier hose and will tend to allow any air to settle out and collect rather than flow on through.

The lack of any air with a standard hose fitted says the system is tight, or certainly tight enough not to cause issues. If the car starts right up and isn't skipping or sputtering, hook the hose up and let sleeping dogs lie.

This is the most cogent explanation. Someone on here was convinced that line was full of fuel at all times, which makes no logical sense to me.

What's odd is that the cigar line can be removed without the t connection leaking out. I watched it for 10 mins once (with everything hooked up) and it wasn't bubbles coming up from the pump, the level just slowly dropped.

I do have a pretty scored up ball bearing. Curious to see whether a new one will affect anything.

I'm terrible at letting dogs sleep. Terrible.

[vwnate1]

? You want me to walk outside and pop the hood in the cool morning air ? .

I'm too.....(lazy) busy or some other lame excuse .