Return Line Bubbles

[Big T]

Decided to start a separate thread on this to get an answer.

Installed new injectors on my son's '94 Suburban. While working through leak issue on two injector bodies, I install clear pvc tubing for all the return lines. The truck starts right up and has no shudders as it would with air in the fuel system. So the IP is having no problems popping injectors which were set to pop at 2300 psi, which tells me there is no air in the fuel system from the IP to the injectors.

The clear fuel return lines never clear the air bubbles that are in them. One injector, which is one of the older injectors replacing a leaky new on, has a constant stream of bubbles in the return line going back to the hard return line. The other 3 injectors show occasional bubbles.

How is this air getting into the system? Does not seem it's before the IP or the injectors, otherwise, I'd expect to see the engine running rough. I was told by the injector rebuilder that he's seen a 6.5 with clear return lines and it always had bubbles in those lines. He said that the air is introduced as part of the opening and closing of the injector during the operation of the engine. Is that true? Is it standard to have some air bubbles in the return lines? I thought there was never to be air bubbles in the return lines.

 

[bison]

I would say the injector nozzles are leaking,compression blows air back up the nozzle and out in the return line.
How is the IP return line,is that one bubble free?

 

[Big T]

IP return line? I ls that the black metal line going up the front of the engine to the IP? If so, that line connecting to the first injector is the one with the constant stream of bubbles, but that first injector is one of the old ones. When it was the leaky new one, it would slowly let loose a 1/4" bubble every 10'seconds or so. Now it's a fizz stream.

 

[bison]

IP return line? I ls that the black metal line going up the front of the engine to the IP? If so, that line connecting to the first injector is the one with the constant stream of bubbles, but that first injector is one of the old ones. When it was the leaky new one, it would slowly let loose a 1/4" bubble every 10'seconds or so. Now it's a fizz stream.

Yep,.is that hose clear tube too?,if there's bubbles there then you have air intrusion upstream of the IP.

The first injector will pass all the acumulated air from the other three on that bank as well

 

[handcannon]

I'm sure what Simon is talking about is the return line coming out of the IP. Do you have a clear flex line there? There should not be any air bubbles in the return line from the IP, other than a very tiny bubble that may stay at the top of the bend in the clear line.

The return lines from the IP and the injectors all attach to the steel line that goes across the front of the block. All the returned fuel merges in that line and then returns to the tank.

I'm not an expert on this, but I'm guessing the bubbles in the returned fuel from the injectors is not a big problem. But, in my experience any bubbles that do continue to flow through the returned fuel from the IP is telling that there is a potential problem there.

Don

 

[Big T]

Yep,.is that hose clear tube too?,if there's bubbles there then you have air intrusion upstream of the IP.

The first injector will pass all the acumulated air from the other three on that bank as well

Again that hard line is at the front of the bank. I have clear line connected from it to the first injector and each subsequent injector in the bank. That first injector is fizzing, but every line has space filled with large air bubbles at the top and the injectors continue to produce an occasional tiny bubble. If that means there's a leak ahead of the IP, then I better start inspecting that line from the tank to the lift pump.

My first priority is to address the leaky injectors and that is being addressed. My rebuilder had his computer crash and he changed e-mail address. I re-established contact and spoke with him on the phone. Leaky injectors were sent back yesterday.

If it ever warms up here (currently 43 F), I will change out the water pump today for the 2000 HO model and Dmax fan.

 

[bison]

I am talking about the 1/4' short black hose from the top of the IP to the hard line running across in front of the IP and is connected to both left and right bank injector return nipples.
Swap that hose out with clear tubing and observe for air bubbles when running

 

[Big T]

I am talking about the 1/4' short black hose from the top of the IP to the hard line running across in front of the IP and is connected to both left and right bank injector return nipples.
Swap that hose out with clear tubing and observe for air bubbles when running

Got it! I'll do that today. Thanks.

 

[Big T]

Alright, retrieved teh '94 from my son's place so I can install the 2000 HO Water Pump, spin on fan clutch and Dmax fan tomorrow.

Per Bison's instructions, I installed some clear hose on the return line from the IP to the hard line going to the injector return lines. NO AIR BUBBLES.

I took the following video of the clear return lines with the engine running:



The engine starts and runs fine as you can hear in the video. That first injector is one of the old ones that I had to put back in to replace one of the leaky new ones. Same with the third one. As you can see, most of the bubbling is from the first injector, though the air never clears from the other lines.

 

[bison]

Vid wont load.
As for the injectors bubbling air trough the return tubes,i never checked that myself on my trucks,it could well be normal altough i doubt it.
No air in the IP return tube means fuel system is healthy upstream.

 

[colbythekid]

[video]http://thewakemans.net/IMG_1981.mov[/video]

Hope this is better.

Colby

 

[Big T]

Thanks Colby. Though the video in my post works on both the PC and the iPhone for me. Somehow I thought I got a shot of the clear line installed on the IP with no bubbles in it, but that would have been at the front of this video. Oh well, take my word for it, there were no bubbles in that line which makes sense given how well it's running. Hopefully the remaining issues are addressed when we get the repaired injectors back. I'm still thankful that none of this occurred on the passenger side.

 

[WarWagon]

Not fun, but, see if you could pin it down to a specific injector.

 

[Big T]

Not fun, but, see if you could pin it down to a specific injector.

That's what I'm trying to do here. At the moment, can't really do this until I get the leaky injectors back and installed. That first injector is creating the most bubbles and it is an old injector. When the new leaky injector was in there, it did the same. So I guess you could say I've narrowed it down to that one injector and we'll see if the new, non-leaky injectors solves the problem.

 

[colbythekid]

[video=youtube;SiiSn8PBy3E]http://www.youtube.com/watch?v=SiiSn8PBy3E&feature=youtu.be[/video]

Had time to load it again. This one should work for everyone...

Colby

 

[racer55]

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.

 

[bison]

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.

Explain were that air comes from if not trough compression backfeeding into the injectors?

 

[racer55]

The atmosphere,the return lines are not an enclosed pressurized system beyond the IP outflow valve.

 

[bison]

AFAIAC The fuel system is a closed system,once the air has initially bled out there is no other way for air to get in unless there's a leak somewere up or down stream.

 

[racer55]

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.