Bubbles in Fuel Line, Comments Requested

[thomas m]

1991 Jetta MK2 NA Diesel

I've been troubleshooting the "bubbles in the fuel line" between the fuel filter and the injection pump problem.
I've read a lot of the posts about what the problem might be and suggestions on how to remedy it.

I just want to get some comments on what I have found.

I disconnected the fuel feed line from the fuel sender to the fuel filter. So both ends are disconnected and open.
I cannot blow through it at all with my mouth when blowing from the sender to the fuel filter. I am not blowing through the filter.
Should I be able to easily blow through it with my own breath??

Then, I put my manual vacuum pump on the hose end that attaches to the filter. The other end still open.
I can easily pull 10 to 15 inches of vacuum. I've read that the vacuum in the line when idling should be low, like 3 inches !!

Comments.

[ORCoaster]

Sounds like a restriction in the line going to the filter AND the filter itself.  You are correct low vacuum on a working filter.  10 inches? or is it psi? is way high.  I think I get bubbles out of the fuel at like 8 psi on a gauge I have connected between the IP and the filter. 

Try using some compressed air on that line from the filter to the sender.  And any idea how old the filter is?  New car to you might just need a fuel filter replacement.  Blow all the lines out when you have the filter off.  Just to be sure any crap pulled up in the lines is out and you shouldn't have any problems keeping the IP full.  Unlike some folks on this forum that seem to have that trouble and we can't figure out why.

[thomas m]

Thanks, ORCoaster  !!

Filter was new in July, 2016......but I've got a new one on the shelf....

Yeah, I've been focusing on replacing the check valve near the water separator. I see some folks remove both of these.
I didn't think about blowing air in from the filter through the fuel feed line.......maybe a flush with some clean diesel would
make sure all of the gunk is remove.

I guess I would have to remove the check valve anyway in order to blow air through since it doesn't allow flow from the filter to the tank.
Then blow the lines out and may as well replace the valve if it doesn't clean up.

There must be some dirt blocking the tank side of the check valve.

Thanks for the tips.

[srgtlord]

If you want a nice semi permanent fix install.an electric low pressure fuel pump before the filter

[thomas m]

srgtlord:

If you want a nice semi permanent fix install.an electric low pressure fuel pump before the filter

Like this??   
 https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=low+pressure+diesel+fuel+pump+12+volts

Would this eliminate starting problems if the check valve is removed from the line?

[srgtlord]

Yes, a pump similar to that. If you have no restrictions in your fuel system having an electric pump makes life easier. Personally I enjoy not having to track down the source of air bubbles all the time on an aging vehicle.

[ftm1776]

I removed the check valve near the water separator with some difficulty. Still bubbles/foam showed up in the fuel line to the pump.
I could not blow through the check valve. It would pull about 3 inches of vacuum when I sucked air through it with my vacuum pump.
I thought it should not have any resistance at all???

I hesitated to change the fuel filter because I had replaced it last July. Anyway, the bubbles totally cleared up with a fuel filter change!!!
Also, I can feel more power going up grades on the freeway. I cut the top off of the filter. It didn't look too dirty to me. It's a Hengst. I never suspected the filter because of my history of changing the filter ever since I have owned the car. Are the Hengst filter good quality??

I thought there might be some hard starting issues without the check valve, but not so far.

Thanks for the tips and the forum support!!

[ORCoaster]

Sounds like a restriction in the line going to the filter AND the filter itself.


I KNEW IT!!!....  Not much else in the way.  I have no history with that brand of filter so will let others chime in on that one.  Glad you got it running to your satisfaction.