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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old January 24th, 2012, 08:43   #1
Keven
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Default Air bubbles in fuel line coming from fuel filter?

My car has always done this and I am just now starting to wonder if its normal. On my car, there are two hoses coming from the fuel filter. One black and one clear. In the clear one, I can see air bubbles flowing through towards the engine. Is this normal? I checked the fuel line as far as I could and it seems to be in OK shape.
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Old January 24th, 2012, 20:52   #2
c2020again
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There are probably more, but I know of a couple of causes.

You could have an air leak in the fuel system, probably at the o ring on the fuel filter, leaking and letting in air.

Another cause is when the timing is way advanced it will caused air bubbles. It did in my car. Once I got the timing set the bubbles cleared up.

Hope this helps.
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Old January 24th, 2012, 21:13   #3
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Is there an explanation how "way advanced" timing can cause air bubbles in the fuel line?

My 2000 Jetta has had air bubbles going thru that same line for over 300k miles!
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Old January 24th, 2012, 22:29   #4
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As AndyBees indicated; it is a normal occurrence, around here, to have a small stream of "soda bubbles" flowing through that line, likely caused by the thermostatic T o rings not sealing 100%. It bothered me enough to try a new T and o rings, with no major improvement. I recently added a lift pump and they cleared up.
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Old January 24th, 2012, 23:56   #5
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the theory for air bubbles: there may be algie in the diesel fuel tank. The alge farts, resulting in bubbles. Since the system is sealed, the bubble move from tank to fuel filter and pump and discharged via injectors in engine.
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Old January 25th, 2012, 08:15   #6
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There is air dissolved in fuel always.

I've seen on the Dodge forums where they make a big deal about it and you can buy devices to remove it before the pump. I've wondered why it is ignored here except in the context of bubbles in the lines.

There was an Article quoted here a while back from a filter company that went into it.

Here it is, (I hope):

http://pics.tdiclub.com/data/500/Mob...iesel_Fuel.pdf

Note the last paragraph under "The Problem"

That applies to pre PD VW's. I do wonder what the inside of the pump looks like while running. The vane pump whips the air/fuel up pretty good. If you have a lot of air going through the fuel in the return line looks like foam. I wonder if the inlet to the high pressure plunger is also getting foam or if it separates enough that gets pure fuel.

Also note that PD guys can add the bleed discussed as a solution.
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Last edited by KLXD; January 25th, 2012 at 08:36.
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Old January 25th, 2012, 10:27   #7
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Quote:
Originally Posted by KLXD View Post
There is air dissolved in fuel always.

I've seen on the Dodge forums where they make a big deal about it and you can buy devices to remove it before the pump. I've wondered why it is ignored here except in the context of bubbles in the lines.

There was an Article quoted here a while back from a filter company that went into it.

Here it is, (I hope):

http://pics.tdiclub.com/data/500/Mob...iesel_Fuel.pdf

Note the last paragraph under "The Problem"

That applies to pre PD VW's. I do wonder what the inside of the pump looks like while running. The vane pump whips the air/fuel up pretty good. If you have a lot of air going through the fuel in the return line looks like foam. I wonder if the inlet to the high pressure plunger is also getting foam or if it separates enough that gets pure fuel.

Also note that PD guys can add the bleed discussed as a solution.

The outlet on the VE pump is higher than the inlet of the pump, and the low pressure lift pump and the high pressure head/cam etc is at about the lowest part of the IP where it constantly is immersed in fuel. I would tend to think the air bubble would be constantly rejected out of the IP since the oulet is at the highest point. I could be wrong though.
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Last edited by josh8loop; January 25th, 2012 at 10:28. Reason: spelling
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Old January 26th, 2012, 08:15   #8
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The inlet to the high pressure plunger is at the top of the head at the inlet to the shutoff solenoid.

The return flow is through a fairly small hole in the bottom of the QA.

Maybe since flow to the plunger is low compared to the return flow and its inlet is lower than the the return in the bottom of the QA any foam or bubbles are carried out a and the plunger gets clean fuel.
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Saying no to gas for 25 years:

Current: 02 Jetta, Auto; 98 Jetta, 5 Spd; 98 Dodge, 5 Spd, SB, 4x4; 84 Grand Wagoneer with Nissan SD33T, NV4500, Dana 300, Reverse Cut Dana 44, Dana 60

The Black Sheep (Only gasser): 85 CJ, 4.2 w/4.0 Head and Mopar FI.

Past: 85 Mitsubishi PU, 4D55T; 81 Rabbit, 1.6; 80 Dasher, 1.5; 79 Rabbit, 1.5
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Old January 26th, 2012, 09:13   #9
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Example of how much "suction" the Injection Pump has:

Several years ago, I was delivering my brother's '84 VW Jetta diesel to him in Columbus, Ohio from here in southeast Kentucky. I filled up before hitting the interstate. When I got near Cincinnati, OH, I noticed the fuel gauge had not dropped, still on full. Being a little worried I stopped and filled up the tank.........something under two gallons. Wow, that was awesome MPGs.....almost 200 miles!

Well, long story short, something happened to the venting system. The Injection Pump's suction force collapsed the fuel tank!

So, the IP creates a tremendous amount of pumping force to drawn the fuel from the tank! Anywhere along the fuel line system that is comprimised is subject to allow air to enter!

The algae thing is a new one on me ............producing the gas! Heck, maybe it's flamable! Need to trap that stuff and use it in a controlled manner to improve fuel economy!
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Old January 29th, 2012, 18:45   #10
Qui-Gon_Gene
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Default More bubbles...

I am having the issue with air bubbles in my fuel line. It can cause the engine to stall out and not want to start again. It's a 2000 Jetta. I vacuum tested all fuel lines, except for the little ones between the injectors. I also put new O-rings on the Fuel Filter Check Valve (the T-shaped valve that plugs into the fuel filter). I pulled the sending unit and it looks okay, though I did not pull it apart. I am planning to replace the little fuel lines between the injectors, and get a new Fuel Filter Check Valve, because I don't know what else to do at this point.

Can anyone tell me, what is the point of the return line going back into the filter? If I get desperate, can I get a different fuel filter (without the hole for the valve) and just do away with the Fuel Filter Check Valve? (So the return line would go straight from the pump back to the tank.) I know they must have run the return back to the filter for a reason, but why?

Also, what is the best way to bleed the air out of the fuel lines? It has to be done every time I change the fuel filter, so I'm hoping there's a better way than just loosening an injector line and cranking the engine. Anybody? I can't understand why WV didn't put in a bleeding port somewhere.

If anyone has any advice for me, I would much appreciate it. Right now I can't trust my car to not leave my wife stranded somewhere.
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Old February 5th, 2012, 11:02   #11
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Okay, I think I figured out the bubbles. I installed a new Fuel Filter Check Valve, and eliminating some iffy clear fuel lines that the previous owner had installed. (The clear lines don't want to seal properly to the fittings.) This seems to stop air from entering the lines, but I can't get rid of the air that is already in the system. I foresee this problem recurring every time I change the fuel filter, so I have devised a way to easily force the air out of the fuel lines, and send it back to the tank. I haven't done it yet, because I'm having a hard time finding the parts to do it. But, I'd like to hear some input on my idea.
Basically, the idea is to put a manual selective bypass on the return line from the IP to the filter, that would bypass the filter and send the return line from the IP straight back to the fuel tank.



You can probably see easily how this would work. To eliminate any air in the fuel line, all you would have to do is:
Step 1: Close Valves 1 and 3, and open Valve 2. This sends all return fuel back to the tank.
Step 2: Start the engine and let it run for a few minutes, (maybe bring up the RPMs a bit) until all the air bubbles have been purged from the fuel lines.
Step 3: Open Valve 3, and then open Valve 1, and lastly close valve 2. (following that order to allow unimpeded flow of fuel throughout the switch back to the filter.

This should be a rather easy and effective way to purge all air from the fuel lines, yes?
Any thoughts? Anyone?

Last edited by Qui-Gon_Gene; February 5th, 2012 at 11:10.
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Old February 5th, 2012, 12:33   #12
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Quote:
Originally Posted by Qui-Gon_Gene View Post
I am having the issue with air bubbles in my fuel line. It can cause the engine to stall out and not want to start again. It's a 2000 Jetta.

If anyone has any advice for me, I would much appreciate it. Right now I can't trust my car to not leave my wife stranded somewhere.
Restriction in intake/pickup in sending unit. Heres what I did.

Original thread:
http://forums.tdiclub.com/showthread.php?t=68872

I had some issues over the last few weeks were the car has left me stranded and after about 15-20 minutes would restart and quit again later, sometimes within a mile, sometimes 30 miles.

Lots of bubbles in the clear fuel line as it ran. Would Idle as long as I wanted, but at speed or under load it would sputter and die.

Changed the fuel filter and Tee and problem still persisted. Then I decided to pull off the Tee after it died to look into filter and it was dry, and made a huge sucking sound when I removed the Tee. Car would start right up after that, which it had never done before.

Temps are just above freezing and a whole bottle of 911 and winter Power Service in the tank, so I doubted it was fuel gelling. Plus it had done this in 50 degree weather a few weeks ago.

Removed the cover for the sending unit and noticed it was an H revision parts, but decided to pull it anyhow. Even though there is no check valve, the structure for it remains, and stuff gets caught up in it. In my case, it was completely blocked by Algae.

Notice how dark the far tube is, thats the pickup tube.



This is only a quarter of what I got out total:



My suggestion, fuel gelled or not. If you have any running or stumbling problems, or any bubbles in the clear line at all when running, pull the sender and have a look, even if you have the new part number. And while its out drill out that restriction.

FWIW, I firmly believe that if your Tee is good and the o-rings are good, the air is from the pump having major suction and able to pull air past these due to the fuel line restriction. After I cleared the obstruction I am 100% bubble free.

Hope this helps someone else. Good, luck.
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Last edited by randyre; February 6th, 2012 at 09:16.
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Old February 6th, 2012, 14:13   #13
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Okay, I think I should check that out. I looked at the sending unit, but didn't actually take it apart. I'll do a followup about how it goes. Thanks for the info!
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Old February 6th, 2012, 18:35   #14
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Quote:
Originally Posted by c2020again View Post
There are probably more, but I know of a couple of causes.

You could have an air leak in the fuel system, probably at the o ring on the fuel filter, leaking and letting in air.

Another cause is when the timing is way advanced it will caused air bubbles. It did in my car. Once I got the timing set the bubbles cleared up.

Hope this helps.

You read my mind exactly. There is a blue and black o-ring and you gotta get them exactly right. Few air bubbles for 20-30 seconds after the procedure is fine but "always" is a problemo.
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Old February 6th, 2012, 18:41   #15
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I pulled out my fuel sending unit and took the entire thing apart. It's still the original one, btw. Ii did not have any algae in it, although there was a layer of sediment in the bottom. I cleaned it all out, checked the o-ring, which is still in great shape, and drilled out the out-going check-valve. 3/16" bit fit perfectly. It took some work to get all the bits out of the tube, but I got it cleared out and put everything back together. I think that did the trick, because I took it for a drive and had no issues. Since then I have started it 4 times, and it fired up immediately each time.
I had dismissed the fuel sender, but apparently that was a big part of my problem! I have no clear lines anymore to be able to see if I have bubbles left or not, but it's running great right now, so I'm crossing my fingers and going on about my business.... I have a sight glass for the fuel line coming in the mail, so we'll see whether or not I have any more bubbles after I get that in.
Thanks again for the details about that sending unit!
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