Fuel draining back to the tank

[derrynoose]

My 10 year old Mercedes 250 Turbodiesel failed to start this week. The problem has been diagnosed as fuel flowing back to the tank when the engine is stopped, due to air entering the fuel line. Expelling the resultant air lock by releasing one end of an injector pipe near the fuel filter while the engine is cranking starts the engine again. My garage has been unable to solve the problem permanently so far. Any suggestions? As there is no fuel leak that I can see, is it likely that the air is entering at the front end of the fuel line. The garage has fitted a non-return valve partway along the line. Is this a waste of time if the air is entering the line between the valve and the filter?

[TWR]

Did this problem start all of a sudden? did it start after the car was serviced? If so was the in line filters changed? has this so-called valve been put on the right way around?

[Nkakamasembe]

1.Fuel is returning to tank by the supply line due to vacuum being created as you run the engine,due to the fuel tank breath being blocked or if you had work on the tank recently this tube my have been squeezed.

2.The breather is the smallest tubing coming out from the fuel tank, it will either run the full length of car to engine bay or will be the going up the filler neck BUT NOT connecting into the filler neck.

3.Try running your engine with fuel cap not fully shut. See if the fueldoes return after switching off.If it does not return then you need to unblock your breather.
4.The oneway valve is money wasted.

[Hymie]

I would agree with Nkakamasembe's analysis of your problem. A similar thing happened to me many years ago � on a normally aspirated (carburetor) engine. The symptoms were of fuel starvation, attempting to accelerate over 30mph resulted in misfiring/rough running of the engine.

In my case, the offending part was a blocked fuel filter on the fuel line (inside the petrol tank). I had to drain & remove the tank and throw the filter away to fix the problem.

If the tank breather is blocked, it is possible for the vacuum formed to be sufficient to collapse and damage the petrol tank.

[alavahalf]

Some years ago had exactly the same problem on a Puegeot Diesel....same result, no sign fuel leaking....and it just would'nt start after it had been left parked for a few hours.
Very frustrating...I new it had fuel in it..plenty of compression...battery fine...glow plugs ok...
I asked the local Peugeot franchise, and thy said it needs a new pump...( never did trust them).

In the end I thought about the lack of signs leaking fuel...and went about it backwards....I got a a short length of rubber fuel pipe, similar to the solid pipe running the length of the car...I then removed a schrader valve from a bike inner tube, and fitted into the end of the flexible pipe, and clamped it with a jubille clip.
Got under the car ,as near as I could to the fuel tank. I separated the fuel line at the first flexible joint. Block the pipe going back to the tank, and then fitted the short length of pipe to the pipe going to the engine....at short blast of air was then applied through the valve using a compressor.
Instantly I could hear a hiss of air, all I had to do was follow the fuel pipe back to the filter to find the air leak.... and there just below the filter was a constant stream of bubbles coming from the bleed screw.
The bleed screw was nipped up and the car ran on for over 400,000 miles without a new pump.

Thought this one up myself...should really have patented it !
Good luck...

[Hymie]

I still think it is a fuel/vent blockage problem � the reason that releasing one end of an injector pipe temporarily cures the problem is that this releases the vacuum build up.

If it is the tank breather that is blocked, you should notice a significant in-rush of air when releasing the fuel filler cap � and running it loose, as Nkakamasembe suggests, will cure the problem by providing a breather route for the petrol tank.

My advice is to pass the information supplied by ABers to your garage � better still, find a new garage.

[derrynoose]

Thanks to everyone for your extensive suggestions. I have a feeling that Hymie's advice will be the eventual outcome. My garage has had the car, for the second time, since last Monday and I have heard nothing. The garage specialises in servicing Mercedes etc and has been very good till now. However, serviing is one thing. Fault diagnosis is another! I will be passing on all your comments before finally giving up.
To go right back to TWR's point, the only recent work carried out has been the replacing of the battery on the day before the problems started. The new battery is longer than the original and is fitted in the boot. It would be too simple a solution to find that the breather pipe runs through the boot on its way to the engine compartment, wouldn't it?

[Hymie]

Thanks for giving me the credit for Nkakamasembe�s suggestion!!!

It may well be that the breather does indeed enter the boot space (close to the battery) on your car. Many breather pipes run along the outside of the filler tube to the filler neck � with the tubery within the boot space (linking the filler with the tank).

[derrynoose]

Thanks for the response,Hymie. I actually meant your advice about a change of garage! There has been a lot of interest in my problem (even the AA man who got me under way last week wants to be kept up to date!) I will post the eventual outcome, earlier rather than later, I hope.