Author Topic: After engine stalls, fuel backflows from return line thru IP to filter (& tank?)  (Read 762 times)

March 27, 2021, 01:22:21 am

Otis2

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Hi again all, it looks like I spoke too soon in saying the vehicle runs great.

After a test run today, after about 35 minutes of driving with fantastic power, the engine staggered to a stall.

I re-started it and examined the clear fuel lines, while it idled like crap.

There were obvious air bubbles in the clear line between the fuel filter and the injection pump, and there were so many tiny air bubbles in the return line leading from the IP to the tank that it looked like a frothy milkshake in there.

After shutting it down, I could see the air bubbles being sucked *backwards* up the return line and into the injection pump, as well as the bubbles moving backwards from the IP back to the fuel filter.

There is a check valve between the fuel tank and the filter, so I guess it would keep a nice vacuum in the fuel feed circuit, leading to this backflow.

So is this a symptom of a plugged fuel feed at the tank?

I think I have some crappy fuel in there, as the vehicle has not been used much since COVID started, and when I changed the fuel filter when installing the new injectors, the filter was full of some really dirty crap.  I may have algae problems in there.

Another possibly relevant symptom, the stalling initially happened while idling at a traffic light, facing down hill, as I said after about 35 minutes of great power.

Vehicle is a 2wd Vanagon, converted from a gasser, so it has the odd flat-shaped fuel tank up front.

I'm not looking forward to draining about 1/3 of the fuel tank, but will do so if necessary.  Probably about 25 litres of fuel in there.

I would appreciate your comments and analysis.  The backflow of fuel after shutdown is worrying.

Where should I be looking next?

Thanks.

(Edit - I drained a bit of fuel from the bottom of the filter, and it was nice and clean.)
« Last Edit: March 27, 2021, 01:23:58 am by Otis2 »



Reply #1March 27, 2021, 10:30:15 am

Rabbit79

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One thing you can check is to make sure the gasket from the old fuel filter didn't stick to the filter flange. I did that once on the Rabbit and it acted a lot like what your symptoms are.
Current: 1979 Rabbit 4dr
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Reply #2March 27, 2021, 03:06:37 pm

libbydiesel

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It sure sounds like an issue with the fuel flow from the tank.
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Reply #3March 27, 2021, 06:55:21 pm

Otis2

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Thanks. 

I'm going to try running it directly from a fresh jerry can of diesel, and see if I still get the milkshake coming out of the return line towards the tank.

If so, then I fear it is the pump itself leaking air, but given the tremendous vacuum in the circuit going back at least as far as the filter, I am hopeful the pump is still good.

I pulled the brand new fuel filter, with only 35 minutes of run time, and I got this mess from the spillage over the top as I removed it. 

Hopefully none of this crap went as far as the pump, but as I mentioned above, the plastic drain valve in the filter bottom drained out only clean diesel.  I hope that valve drains from the pump side of the filter, and is an indication that only clean diesel reached the pump.  I certainly never saw any of this crap in the clear line feeding the IP when it was running, anyway.

Gasket on the fuel filter was correct, so that's not the issue.

Looks like I am in for at least a new fuel tank.


Reply #4March 27, 2021, 07:24:02 pm

libbydiesel

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Is that rust or is it algae?  Either way a new tank is probably the easy way to go.  If algae, though, it means that water is getting into the tank.  Make sure the fuel system is sealed well.  Also keep the tank relatively full if it is going to sit, in order to avoid the potential of condensation forming.   
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Reply #5March 27, 2021, 08:49:28 pm

Otis2

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I don't know how to differentiate between algae and rust flakes.  Your guess is better than mine... you've seen more VW diesel engines than I've had hot dinners. 

Looks like reddish brown flakes, which suggests rust to me, but maybe algae is that colour too. 

If there is a solution that doesn't require replacing the gas tank, then I'm all ears. But obviously I'd rather need a new gas tank than a rebuilt IP and injectors.  Better safe than sorry.

I'll probably have to take it to a mechanic for the gas tank, because it is effectively impossible to legally dispose of waste diesel fuel here without a commercial disposal contract.  And 25 litres of spoiled diesel is more than I can ask anyone to take from me as a favour.

I am happy to report that I popped the banjo bolt from the top of the fuel filter flange, dunked it into a jerry can of fresh diesel, and cranked up the engine.

The engine idled and revved quite happily.

No air bubbles whatsoever between the jerry can and the IP, and a solid stream of clear diesel flowing out of the IP return line and back to the tank.

No evidence of backflow up the return line (no air bubbles to track it, even if it existed).

So that's a relief. 

Looks like the new fuel filter did its job, maybe plugging up and dying in the process, and so the IP and the new injectors are hopefully all fine.

Hopefully not much crappy fuel backflowed directly from the tank to the IP return port under the influence of that vacuum. 

That would really "suck" (ha!). 

Is the return port on a 2wd Vanagon fuel tank located up high on the tank? 

If so, then I hope & expect the return line was mostly sucking air and the filtered diesel that had already just made its way through the IP.  There was less than half a tank of fuel in the vehicle when this happened.

Reply #6March 27, 2021, 10:30:50 pm

fatmobile

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 Not sure what engine you have in your van.
  Nearly all of my VW diesels have a vacuum gauge on the dash.
 It tells me  how hard the pump is pulling.

 A fuel line restriction can happen when the tank vent is clogged.
 Does it his when you pull the cap after driving?
 The fact that the fuel flows back through the supply line says it's probably only on that supply side
not the whole tank.
 
 I'm not too experienced with the van but if it was a Rabbit;
I would pull the fuel sender and pump fuel through a filter
then back into the tank.
I call it "vacuuming the tank".

 There is usually a screen in the pickup.
 And a check valve somewhere in the supply lines that can plug up.

The diesel algae I see is usually black.
What comes from the filter drain is unfiltered.
Commonly thought of as a water drain.

 Sometimes pumping fuel backward through the supply line can backflush the clog out.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #7March 28, 2021, 12:32:15 am

Otis2

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Thank you fatmobile.  The engine is an AAZ.

I like your vacuum gauge idea.  What amount of vacuum do you normally see on that gauge, as a benchmark?  Any specific recommended gauge that stands up to diesel fuel contact?

I did remove the gas cap to see if it ran better that way, but it did not.  So I don't think it is a clogged vent.

Only when fed from the jerry can does it run properly.

Maybe the screen in the tank is clogged at the pickup, irrespective of any issues with the fuel filter or check valve. 

I'm thinking that pulling the tank out is the only way to see for sure.

Even if I backflush the clog on the supply line, I expect that the crap will stay in the tank, and it will only be a matter of time before a clog happens again.

I have been googling around for diesel tank algae cleaning, and there are plenty of "mechanic in a can" products out there, including "fuel polishing services" and such.

At the moment I am leaning towards pulling the tank out to get a proper look and deal with whatever is in there, be it algae or rust.

In order to access the fuel gauge sender on the van, you have to remove the tank as far as I can tell, so at that point, "in for a penny, in for a pound".

I'm seeing some suggestions to pressure wash the inside of the tank and call it good.  We'll see how it looks inside once it's out, I guess.

I just saw a youtube video where an old Benz guy was cleaning his diesel fuel tank at a coin-op car wash with a pressure wand, which I think might be a little shady. 

No idea how they handle recycled water at those things, but that's a little too close to "straight to dolphin mouth" for my comfort level, without knowing more.  Pretty sure they weren't expecting people to wash out the inside of diesel fuel tanks in the coin-op car wash bays...  :)

Interesting that what comes from the fuel filter drain is unfiltered, because it was pretty clean.  Thanks.  That suggests the main clog is more likely at the pickup, than the filter.

Once I pulled the filter right off, it was clear there was a mess inside it though, as you can see from the photo.

Edit - I just slid a pretty powerful magnet around the bottom and sides of that plastic tub in the photo, and none of the red/brown particles suspended in the diesel fuel reacted to it.  If they were ferrous, then I would have expected them to follow the magnet.  Maybe that's not a fully determinative test, but certainly it more supports the algae hypothesis.
« Last Edit: March 28, 2021, 12:50:16 am by Otis2 »

Reply #8March 28, 2021, 03:24:03 pm

ORCoaster

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I think your best spend of time would be pulling and cleaning the tank. 

You could drain as much of the current "fuel" in the tank into some sort of bucket and possibly filter it.  I wouldn't have a problem with the car wash cleaning.  Do they have all kinds of solvents that blast the oil and gunk off the engine as an option do they not?  The little bit of diesel you add to their separator tank will hardly be noticed.

When the tank is clean and you add new fuel to it I would through some of that fuel cleaner in there to deal with any algae in the lines. 

When you have the tank off you should put some light air pressure down that return line and the intake at the filter.  That will get any of the old crap out and help in the future care and cleaning. 


Reply #9March 28, 2021, 06:38:17 pm

Otis2

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You may well be right about that. 

Youtube videos on van fuel tank re & re make it look pretty simple, except for the issue of what to do with all the spoiled fuel in there.

I will investigate my disposal options this week.

Worst case scenario I suppose, is having a couple of 20 litre jerry cans around my garage for months to come, that I can't get rid of.

Reply #10March 28, 2021, 07:58:14 pm

ORCoaster

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How many weeds do you have along a fence or between bricks.  Diesel is a great weed killer.  The soil will process it just fine in short order.  Microbes in the soil will break it down. 

Reply #11March 28, 2021, 11:03:23 pm

fatmobile

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I bet if you put it in a container all the bad stuff will go to the bottom.
 The rest of the fuel can get used.
 
 On the vacuum gauge; 5 is a little high
8 I plan to do something about it.
 By 10 I've changed the filter or something.

 I'm not sure how hard it would be to have a dash-mounted vacuum gauge in a van.
 At least not with the tubing.
I think they do make electric senders.

 Old gasser rabbits have an adjustable vacuum switch in one of the vacuum lines that could be used for a warning light.
Otherwise for the Rabbits I look for a sunpro or VDO gauge with a needle that has a long sweep.
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Reply #12April 03, 2021, 09:18:44 pm

Otis2

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So I have drained and pulled the fuel tank, and it's a rusty sludge-filled mess.  Time for a new tank for sure.

I hope none of this crap got past the fuel filter, and now I'm super glad that it plugged up and stopped the engine.  It would have killed the IP and new injectors for sure.

This van is a converted gasser, and still has the charcoal cannister and the twin plastic expansion tanks in it.

The charcoal cannister is useless on a diesel, but the plastic expansion tanks apparently got used in the factory diesel Vanagons.

The Bentley manual shows the fuel system for diesel vans (page 20.10), and the whole system is common to the gassers right up until the line leading from the gravity vent valves on the top of the expansion tanks. 

In the gasser, those lines T together and run from the gravity valves back to the charcoal cannister.

On the diesel van, it looks like there is a separate section of hose that just hang there from each gravity vent doing nothing.  The Bentley manual seems to show some kind of flange at the end of those lines, but I can't tell what the purpose of the flanged ends are.  An air filter in there, maybe?

Can I please have a suggestion as to what I should do with the lines leading out from the gravity vents in the expansion tanks?  Just a length of hose hanging in the open?  Maybe some steel wool jammed in the end as a filter?

What are you guys doing with the gravity valves in your expansion tanks?

One of my plastic expansion tanks was cracked, and there was all kinds of mud and crap that got thrown up into it.  On the vanagon, these things are immediately behind the front wheels. 

I am sure that didn't do much good for the cleanliness of the diesel in the fuel tank.  But on the other hand, if I just hang an open line off the gravity valve, it seems like that sort of amounts to the same thing as having the crack!

Reply #13April 09, 2021, 08:42:42 pm

Otis2

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To answer my own question, I have discovered that flange at the end of each vapour tank hose is VW part number 321201890.

It looks like this:



Seems to be NLA everywhere at the moment.

However, the pictures I can find of it don't seem to show any filter inside it. 



So effectively the factory stock design of the diesel Vanagon just ran short lengths of open hose directed from each of the float valves on each of the vapour expansion tanks.

Reply #14April 10, 2021, 12:46:32 am

fatmobile

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I've put a plastic fuel filter on the end of a vent hose before.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block