Bubbles in Fuel

Thomas M · Post by **Thomas M** » Sat Aug 22, 2009 5:38 pm

I haven't explored all circumstances, but it seems that if my 1991 Jetta Diesel sits unused for a couple of days start up is difficult and requires a lot of cranking until (I assume) fuel is input to the pump. Once it starts, a lot of bubbles can be seen in the input line to the pump. Slowly they subside but do not go away completely and the car runs just fine. Some small bubbles are also in the return line. I want to check out the simple causes first.

I have pulled the fuel sender and it was 100% clean. No grit or dirt at all.

I inspected the input and output banjo fitting on the pump. Clean and tight.

I will put a new fuel filter on which has not been changed since 2004. I don't expect any dirt in the filter but will just do some preventive care.

My question is: can air enter the system though the o-ring of the return line fitting (I don't know what it is called) that is on top of the filter body and secured with a pin insert?

Any other simple suggestions that I can try on my own would be appreciated. However, if you wish to comment with shop type solutions please feel free. Thanks. Thomas

Quantum-man · Post by **Quantum-man** » Sat Aug 22, 2009 6:52 pm

Forget your return lines... If you have bubbles running down the supply line; then you have a leak to air under vaccuum. So It's a joint check either side of the fuel filter, and the screw on top and maybe the water drain at the bottom then check for leaks on joins in pipework back towards the fuel tank. By-pass the tank and create a feed from a bottle into the fuel filter to check that part...

tawney · Post by **tawney** » Sat Aug 22, 2009 7:54 pm

Five year old fuel filter??? Change the filter; your bubble problems will probably go away.

82vdub · Post by **82vdub** » Sat Aug 22, 2009 10:07 pm

tawney wrote:Five year old fuel filter??? Change the filter; your bubble problems will probably go away.

I was thinking the same thing. Even those these filters are somewhat expensive, they are cheap when it comes to replacing an IP. I change my fuel filters each year, no if's, and's, or but's. I even change the factory fuel filter on my 82 that has a Frantz fuel filter in the line before the factory setup.

Thomas M · Post by **Thomas M** » Sun Aug 23, 2009 2:04 am

Here is the in-returnline fitting that sits on top of the filter and is held in place with the pin. Does anyone know what the function of this gadget is? As you see, I added small hose clamps on the main fuel lines.

Here is the O-ring. I'm wondering if air could be leaking in here and making its way into the input fuel line. I have never changed the o-ring and am the original owner of the '91 diesel with 247,000 miles. Any comments would be appreciated.

surfcam · Post by **surfcam** » Sun Aug 23, 2009 8:32 pm

Basically there is a thermostat inside that gadget. When the fuel warms up from the injectors it opens so the warm fuel can warm the incoming fuel. If your not planing on moving you could just remove that gadget and stick a pipe in there. You could then replace your filter with an older two connection one. I have one of each and have not notice any difference in the way they both run in the winter. I live in South Alberta which is fairly mild but we do get are cold snaps. I haven't check the mileage with and without the thermostat.

Thomas M · Post by **Thomas M** » Sun Aug 23, 2009 11:06 pm

Surfcam,
Thanks a lot. Makes sense. I'll probably leave it. Although Vancouver, WA is relatively mild, we too get our cold snaps and winter trips to the mountains can be in sub-freezing temperatures.
I'm not sure, but I think the o-ring is renewed with a new filter. I'll make sure I replace it one way or another to prevent any air leakage at that source. Now on to the new filter. I'll report back on this thread for those interested.
Thanks Again, Thomas

Post by **Fatmobile** » Sun Aug 23, 2009 11:08 pm

Do you still have the check valve, under the car by the water seperator?
Air can be pulled into the injection pump while the car is sitting,.. past the "OUT" banjo bolt crush washer.
More likely it is coming in past one of the small lines between the injectors.
Hopefully not past a worn out mainshaft seal, a finger swiped behind the injection pump sprocket, near the shaft will sometimes come back wet or damp if this is the case.

If there is a working check valve under the car; fuel won't be able syphen back to the tank and air won't be pulled in.
It often clogs and is removed.

Thomas M · Post by **Thomas M** » Mon Aug 24, 2009 12:45 pm

Fatmobile,

I am the original and only owner of this vehicle. I hope the valve is still there..............................

Let's see if I understand this.

If the checkvalve is not working and stopping the fuel from returning to the tank via the siphon effect and there is an air leak, air could be drawn in from a leak in the small fuel return lines between the injectors and into the pump via the return banjo fitting allowing the siphon effect to draw air into the pump and allow the main fuel line to be emptied back into the tank. Or something to that effect.

I assume this is the suspect check valve oriented up as in the car with directional arrows on the valve body.

Sounds like a couple of simple things are in order before I replace the checkvalve. New fuel filter and new injector return fuel lines all the back to the pump.

Many thanks to all contributors of this forum. This kind of support validates my desire to keep this simple vehicle forever.

Post by **Fatmobile** » Wed Aug 26, 2009 12:09 am

Right, either way the fuel syphening back to the tank is a symptom of air leaking in, being pulled in by fuel as it trys to drain back to the tank.
It could be the o-ring you pointed out in the fuel filter, I've heard of it causing problems.

A side note, this might have nothing to do with your problem:
A vacuum gauge on your fuel line will let you know if your;
filter,
that check valve,
the tank screen,
or tank vent are clogged.
Before they cause a problem.

Thomas M · Post by **Thomas M** » Thu Aug 27, 2009 7:14 pm

Fatmobile wrote: A vacuum gauge on your fuel line will let you know if your;
filter,
that check valve,
the tank screen,
or tank vent are clogged.
Before they cause a problem.

Fatmobile,

How does one interpret the vacuum gauge indications to diagnose a problem? Engine on or off?

The pump is sucking fuel from the tank, so I guess there should be a vacuum indicaton while the engine is running???????????????

Please explain.

PS: filter (comes with new o-ring) and injector rubber hoses soon to be replaced.

Thanks..............Thomas

tawney · Post by **tawney** » Thu Aug 27, 2009 9:08 pm

A couple years ago I took Fatmobile's suggestion about the vacuum gauge; it's been a huge help, especially since I'm running mostly on WVO. When the filter starts to get clogged the vacuum will start to rise from the usual near zero to three or four or more; you know it's time to change the filter before it gets worse and the engine stalls because it can't get fuel, or it starts pulling air into the fuel from a vacuum leak.

Steve

Post by **Fatmobile** » Thu Aug 27, 2009 10:16 pm

Yep, vacuum should be near 0 while the engine is running. I worry at 5 and do something at 7 in hg.
And the syphening back to tank doesn't read as any vacuum either. so I never see a vacuum when the engine isn't running.
I probably saved my life last winter.
It was too cold to be driving around -10F or something. I didn't have enough antigel or something on a trip to see some family. After family I was in the area so was going to drive into the country and see a friend. The vacuum started climbing on the way there as the fuel started to gell.
I decided to head for home instead. The vacuum got pretty high by the time I got home. Had to keep driving slower and slower to keep it down.
If hadn't known there was a fuel line restriction, I would have gone out to my friend's house and been stranded; at night, by the road on the way home.

Thomas M · Post by **Thomas M** » Sat Aug 29, 2009 12:37 pm

What kind of vacuum gauge is appropriate?

Sounds like a low range 0-10 in. hg. or so would do it.

What about it being in a fluid filled line? Are there vac gauges for liquid and different ones for gases(air)????

Any pics of your install available????

Post by **Fatmobile** » Sun Aug 30, 2009 12:47 am

A vacuum gauge with a large space between the numbers, one with a wide sweep.
Some of them have very little movement between 0 and 5.
I have a post about a vacuum gauge install around here somewhere.

Fluid does move up the line. A vaccum will pull the air out of the line and when you shut it off diesel will be pulled up to replace it.
...so I put my vaccum gauge up high.
On the dash in the driver's corner, near the windshield. The line comes up around the rear corner of the dash.