Air bubbles in fuel line coming from fuel filter?

Keven

Veteran Member
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Jun 16, 2008
Location
Tampa FL
TDI
2000 Jetta TDI
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.
 

c2020again

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Nov 22, 2011
Location
Winton, NC
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2001 Jetta TDI
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.
 

AndyBees

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May 27, 2003
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Southeast Kentucky
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Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
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!
 

\/\/0J0

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Apr 27, 2010
Location
Knoxville, TN
TDI
Sadly, none anymore
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.
 

Dodoma

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Joined
Mar 30, 2004
TDI
2002 Jetta White
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.
 

KLXD

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Lompoc, CA
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'98, '2 Jettas
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/Mobile_Air_Separation_In_Diesel_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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josh8loop

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Sep 15, 2009
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Vero Beach, Fl.
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2002 VW Jetta TDI Manual(performed 01M to 5-speed swap) - 183,000 miles and climbing!
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/Mobile_Air_Separation_In_Diesel_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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KLXD

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Aug 22, 2009
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Lompoc, CA
TDI
'98, '2 Jettas
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.
 

AndyBees

Top Post Dawg
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May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
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!
 

Qui-Gon_Gene

Member
Joined
Jan 29, 2012
Location
Texas
TDI
2000 Jetta TDI
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.
 

Qui-Gon_Gene

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Jan 29, 2012
Location
Texas
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2000 Jetta TDI
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?
 
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randyre

Member
Joined
Sep 13, 2011
Location
Woodward, Ok
TDI
2000 Jetta TDI Blue
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. :eek:

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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Qui-Gon_Gene

Member
Joined
Jan 29, 2012
Location
Texas
TDI
2000 Jetta TDI
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!
 

2002MK4

Veteran Member
Joined
Jan 24, 2011
Location
Scarborough
TDI
2002, 2003 Jetta TDI
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.
 

Qui-Gon_Gene

Member
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Jan 29, 2012
Location
Texas
TDI
2000 Jetta TDI
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!
 

bucksee

Member
Joined
Dec 13, 2010
Location
West Palm Beach
TDI
03 VW Jetta
I am having same issue with my 03 Jetta. The car can start and run but get stalled. Replaced the Tee and filter tried to bleed out air from filter to IP line then bleed out air from return line from IP to filter. When bleeding out air from return line from IP to filter is when I get a lot of tiny bubbles none stop.

Changed out O-ring few times before changed out the Tee and filter. I'm going home tonight looking into sender now. I'm running on B100 I was going to look at the IP seal but run into this thread. Oh I'm living in South Florida which is not that cold. I'll update after I look at the sender.
 

bucksee

Member
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Dec 13, 2010
Location
West Palm Beach
TDI
03 VW Jetta
Pulled out fuel sender but looking okay. Cleaned out fuel sender all areas reinstalled back fuel sender. Drive test and will update in a few days.
 

BlindViper

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Jul 13, 2010
Location
maryland
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97 A3
My buddies mk4 had bubbles in the line it was the rubber from the hard to the clear line. He put a layer of double wall heatsink over it and fixed 95% of them
 

dieselwins

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Joined
Aug 14, 2012
Location
kalamazoo, mi
TDI
golf 2001 passat 2005
I am noticing varying areas that cause bubbles in the fuel lines. I have been tackling this problem for about 2 weeks now and im getting to my end. If i fill IP up and crack injectors car starts up and runs great. That is....until bubbles slowly take over the fuel (as shown in clear line). Car will still run but im afraid too much air is bad for the pump so i am forced to shut it down. Mighty vac holds vacuum from tank to filter filter itself to pump the pump itself to filter BUT i am losing vacuum on the return line from filter to tank. I have not checked the injector lines i decided to replace them ( ordered from boraparts will be here wednesday). I have put plugged the line to the fuel sender and pressurized the line but cant find a leak. Any suggestions. New filter new thermo T new sender, so i know those 3 are no issue. Im going to dig in further tonight. Im hoping its a connection and no break or crack in line.
 

dieselwins

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Aug 14, 2012
Location
kalamazoo, mi
TDI
golf 2001 passat 2005
Dang injector return lines. I hooked up mighty vac to the return injection line going to the IP......bubble bubble bubble....bubble...bubble 10 mins later bubble bubble bubble...i think its clear now. Im still wondering how the tiny amount of bubbles that came from injector return lines causes the mass amount of bubbles found in the clear line going to IP. I know the bubbles would enter the system, but it seems like the amount wouldnt cause issues so quickly. Any enlightenment on this would be great.
 

Qui-Gon_Gene

Member
Joined
Jan 29, 2012
Location
Texas
TDI
2000 Jetta TDI
The return line from the injectors to the IP gets recirculated back into the filter, as well as being sent back to the tank. So, some bubbles will keep making a loop through the IP, until they get worked out through the injectors or get sent back to the tank. I think it's odd that they set it up that way, but it's designed to make things more efficient, I think. It's a pain when you have bubbles though.

I had 2 problems causing my bubbles, one was hoses that would not seal on the fittings. Do not use the cheap worm-drive hose clamps on the fuel lines, they can't be trusted. Unless you still have all the OEM fuel line clamps, and know they are still trustworthy, go get some made like these:

http://www.autozone.com/autozone/ac...n-systems/_/N-25x2?itemIdentifier=347131_0_0_

They can be trusted not to leak.

My second problem was the check valve in the fuel sending unit. The check valve is on the outbound side, and is simply a bad design. It causes restriction of the fuel flow, and VW actually did away with it in later models. You may not have this problem, depending on your model year, mine is a 2000. If you go that route, just make sure you get all the bits of check valve and spring out! You don't want to clog up your fuel line. Anyway, I drilled out that check valve, got good clamps on all my hoses, and new hoses, and I have been bubble free ever since.
 

TDI Convert

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Ames, IA
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2006 Jetta BRM

Qui-Gon_Gene

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I don't know the best place to get them. That price on Autozone is for a whole box... but I don't know how many come in the box. It might be the dozen that are pictured. I just checked, and what I bought are MasterPro MP52F15. There were 10 in the box, but I don't remember now where I bought them. I probably got them at Autozone, or Oreilly's... or maybe eBay. That was a year ago now. That does look like a good deal on Amazon, if it's for the quantity in the picture, and you get the free shipping.
 

dieselwins

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Aug 14, 2012
Location
kalamazoo, mi
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golf 2001 passat 2005
I replaced all my clamps to really nice ones, replaced my T-fitting, replaced the injector return lines with viton, STILL AIR BUBBLES. Im literally tearing my hair out, this damn car gives me headache after headache. am I the only one who has the bubbles that cant seem to figure out where the hell they are coming from??
 

snowden

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Feb 12, 2013
Location
Norfolk UK
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2001 Golf ALH
I have a steady stream of bubbles in mine. Car doesnt run any different.

Are you experiencing any power loss? Do the bubble cause harm?
 

spoilsport

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Oct 3, 1999
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Houston TX
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2000 Golf GLS Silver (Sold). 2005 Jetta TDI Wagon Tiptronic (daughter's)
I replaced my thermo tee orings with slightly fatter Caterpillar orings. Completely cured my fuel line bubbles.
Can you tell me where you sourced the o-rings and I'm wondering if CAT makes them in Viton as I'm using B100 most of the time.

Had a gelling episode last night in Houston, no less. :rolleyes:
 

mike92105

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Apr 30, 2004
Location
Lemon Grove CA
TDI
2002 Jetta TDI automatic 2015 Passat SEL
Looking for no bubbles

Any ideas as to why a check valve in the pickup would fail after 10+ years? I too have bubbles, so I put a cheap plastic fuel filter inline (which would always be half full of air). Not realizing how much suction there was it eventually collapsed. Now I am trying to figure out where the air is getting in at. Last time I vacuum tested both lines to the tank and they were fine. I've noticed a lot of plastic/rubber inside the engine compartment that seems to me is getting brittle with age. Are the injectors closed when the engine is not running?
 
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KLXD

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Aug 22, 2009
Location
Lompoc, CA
TDI
'98, '2 Jettas
If you collapsed the filter you must have some kind of restriction in the feed line or tank pickup.

High vac in the feed line will do two things: Make any air leaks worse and cause dissolved air to come out of solution and become visible bubbles.

Read the article linked in post #6.
 

Qui-Gon_Gene

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Jan 29, 2012
Location
Texas
TDI
2000 Jetta TDI
The IP does generate a lot of suction, but I agree with KLXD, (who I'm sure knows a lot more about it than I do,) it sounds like you have more vacuum in that line than there should be.
My check valve was causing problems after 12 years. Others have had the same issue, so I'd say don't rule it out. I think you can buy a new sending unit that does not have that check valve, if you don't want to drill it out of your old one. Just make sure there's no valve in the one you buy!
And of course check the line itself for restriction, if you haven't already.:)
I've been driving for almost a year and a half, and around 25,000 miles, since I since I drilled out my check valve, and have had no issues at all since then.
 
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