Aargh fuel problem

[sparkie]

My Mk 3 Tdi ran out of fuel (doh) and I have put 20 litres in, but it will not start. No fuel coming through, so new fuel filter fitted now, all hoses correctly connected, I have taken off the pipes to the fuel tank, and they are clear. So what's the problem? Battery good, lots of voltage, engine turns over really well. Oh, and there's a really weird thing under where the flow and return pipes go in/out of the tank, the fuel sender has some spring;loaded bellows thingy - what is that about?

[gldgti]

take off the return line from the pump and blow into it or somehow otherwise gently pressurise the fuel tank. Wait until fuel flows out of the return port on the pump. Then you can be sure that there is fuel in there. You can't necessarily always prime it just by cranking. A long hill and a roll start might work, but I would try the first trick first. If you do get it to start after pressurising the fuel tank, remember to afterwards remove and replace the fuel cap (let the pressure out)

If it still wont start, I would suspect immobiliser.

[sparkie]

I managed to get it to run for a few minutes, it seems when I run compressed air down the return line, I do not always get fuel coming back, sometimes I do, and sometimes I get air and/or a mist of diesel. It ran with a fair amount of air in the transparent fuel line just where it enters the injector, but after a few minutes it stalled, and I had to start again. If I put more fuel in the tank and I likely to get a better result? What happens if the lifter, which I gather is built into the injector pump, is cactus? Surely a possibility? Anything I can do to check that, and if it is, is that an injector rebuild?

The fuel sender is weird, that's to say I don't understand it! But the small square grid of holes at the bottom is clear, is it worth me disassembling this to see whether there is something blocking the fuel line very time it sucks/pushes fuel?

[gldgti]

As long as you can get it to re-start you should be OK to take it for a drive and that will get the rest of the air out of the system.

The actual fuel pump is in the front of the injector pump, yes, and it could be cactus....

How sure are you that you actually ran out of fuel? If the fuel pump has stopped working, perhaps that is the actual problem.

You need to completely dis-assemble the pup to get to the vane pump in the front, so its practically a rebuild job.

[sparkie]

Thanks glddti, another 6 hours spent on it today!!

I am not totally sure I was out of fuel, though I have put about 50 litres in the tank, maybe more, so it was certainly low. Now I just can't seem to get decent pressure in the tank, via the return pipe, with a compressor to push diesel through the fuel delivery pipe, I just seem to either get air, or I can hear the compressed air coming out of the fuel cap. If I was able to pressurise the fuel tank and the engine ran, then I could purge all the air. Again I got it started and it ran for a minute or so, so it could be the fuel lifter? I assume that the fuel sender ensures that the fuel is picked up from the bottom of the tank? Is it possible that somehow fuel, or air actually, is being pushed into the fuel pipe higher up the tank?

[gldgti]

urm.... be careful with a compressor! that's why I said gently!
I normally do it with my mouth, since I can't make more than about 1 psi there's no danger of hurting anything. please don't pressurise your tank too much.

when you get it to run, can you drive it? typically if the feed pump up isn't working then you will have no dynamic advance and no power

[gldgti]

Another question - are you sure you are doing it the right way around...? It would be possible to prime the system by pressurising the supply line and getting the fuel into the pump by the return line, but then once it starts running it will suck air from the feed line into the pump. Worht asking I figure, just incase.

The pickup is definitely at the bottom - usually the return is aswell. When you feed air into the tank you should hear it bubbling.

Can you take a photo of what you are doing? It might help a bit. Its been a while since i worked on a mk3 and I cant remember exactly how the fuel lines are laid out in the engine bay.

[sparkie]

So Monday came and I ended up getting a mechanic to look at the car, his diagnosis, the lifter pump is stuffed. (I sort of handed that one to him as I told him what I suspected). But he suggested before I embark upon an expensive IP rebuild, which would involve having the car trucked 30km to a diesel specialist. So the second mechanic came this morning, gt the car stared in a few minutes and checked it over - algae in the fuel tank blocking the fuel lines!! I am so relieved I got a second opinion. Now I just need to empty and clean the fuel tank, run some stabiliser through it and replace the fuel filter!

[gldgti]

Good result if that is your problem.... still a pain to clean the tank though... not sure which I would prefer .

It does make sense though considering your described issues. Most likely, the gunk has been in there for a good while, but its only when you ran it close to empty that it has clogged the filter/strainer.

You noted earlier in the thread that you had checked the strainer in the tank.... and said it was clear?

Perhaps you only need to change the main fuel filter (e.g you sucked up all the fine crap in the tank when you nearly ran it empty, and thats pretty much all in the filter now (hence blocked) but the tank, for the most part, is clear.

Would be pretty simple just to change the filter and even stick one of those cheap petrol clear ones before your new diesel filter so you will see if theres any rubbish coming through once you get it running again without ruining your new filter straight away.

All above would be much easier than cleaning the tank

***edit***

some years ago when I started running my (at the time 24 year old) mk1 diesel on biodiesel, I had a similar problem. The bio started to 'cleanse' the fuel system and all the garbage from the previous 2+decades was coming through. I changed the main filter once, and then 50km later it was blocked again. Learning from this, I put one of the clear filters infront of the diesel filter, and carried another half a dozen in the car toolbox with me. I ended up using the rest of those clear filters as they would visibly clog with rubbish in a short period of time, and the car would start to under-perform, so I would just pull over and swap it and keep going.

After a couple of months the system was fully clean and I went back to just the main filter.

Dad (who's been running bio in his mk1 for about 15 years now) still uses a clear filter infront of the main one, just so when he lifts the bonnet he can visually see the colour of the fuel, if theres air bubbles coming though, if its clouding up in the cold weather etc.

You can add any kind of filter in line from the tank with a VE pump diesel engine, as they don't use pressure feed from the tank unlike most fuel injected petrol cars.

[Ryeman]

Seeing fuel is always drawn from the bottom of the tank, why is a low level problematic?

[brad]

Seeing fuel is always drawn from the bottom of the tank, why is a low level problematic?

Just guessing - because the algae is floating on top?

[gldgti]

algae will hang around closer to the top of the liquid volume for sure.

Also, any solids that settle out of the liquid will become more highly concentrated as the volume of fuel in the tank is decreased.... its hard to explain in text, but basically when there is lots of fuel in the tank, the probability from moment to moment of the pickup pulling in some solids that are washing around in the bottom is lower than when there is hardly any volume of liquid in the tank, but the amount of solids is pretty much the same.

I guess another way to put it, is that when the tanks is full, the amount of fuel being taken out by the fuel pickup is relatively small, compared to the available volume of liquid.

When the avialable volume is very small (tank nearly empty), all the solids are washing around in the remaining fuel, so the concentation is very high, and you are therefore more likely to pull it into the pickup.

Trust me

If you've ever tried gold panning, that might help.... maybe

[Ryeman]

That would be the case I guess if you weren't in the habit of letting it get low.

[gldgti]

Mostly, if you fill up once you get just into the red (in a VAG car anyway) you usually have at least 7 litres in the tank remaining. I know in the Skoda (which I never ran out of fuel) I could do more than 120km in the red. Same in the cabriolet.

[sparkie]

So tank cleaned, actually an easier job than I thought, fuel filtered and put back in, new filter, hard to prime (again), but all good now! There seems to be some agreement out there that the combo of bio and petro diesel can be a problem, plus having the fuel tank low (lots of air).