Strange Diesel Fuelling!? Problem

[lochan2]

Just got back from a 3 week/2,830 mile trip in our Bongo and the engine has starting behaving strangely. With a tank of fuel anywhere between the Full mark and the halfway mark the Bongo runs perfectly. As soon as it reaches the halfway mark it begins to play up, misfiring. no acceleration on kick/change down and wont easily go above 50ish.
If I stop and release the pressure on the fuel filler cap it sometimes helps a bit until I can get to a fuelbank and top it back to a full tank and then away bongo goes again!
Any ideas people?
regards Lochan2

[teenmal]

HI lochan,It could be a few things causing this fault,the first thing I would check is the fuel cap.
The fuel cap has a valve in it.
Mal

[dipstick]

If you check your filler cap you should notice a small one way air vent, this allows air into the fuel tank as the fuel is used up otherwise you will end up with a vacuum in the tank and the fuel pump cannot 'suck' any more fuel through. An easy way to check this is to not tighten the cap properly and run around fore a while and see if the problem is cured, if the problem is still there then at least we can tick this one off as checked. The other problem that can occur is the small filter under the 'banjo' connection on the fuel pump can get blocked. Follow the fuel line from the filter to the fuel injection pump, uno the hex nout and carefully remove, under the bano but in the fuel pump is a small gauze filter - use a cotton bud to remove it, clean and replace. This may help with this problem.

Sorry for using so many words.

[dreamwarrioruk]

it certainly looks to be the fuel cap problem, if it was the banjo filter it would do it all the time.

[dipstick]

Worra bout the the water drain?

[thedogsbollox]

I am no expert but wouldn't a failing, not failed, fuel pump do the same as it would be getting help from the weight of fuel in a full tank.
Sorry, just my tuppennyworth if all the good advice fails

[brorabongo]

http://www.igmaynard.co.uk/bongo/forum/ ... ER#p214670

If you look at the bottom of the thread, you will see that I was having the same syptoms. I've changed the fuel filter and checked the strainer under the banjo bolt, which was clean, and it has not misbehaved since........... Looking into the fuel filter, I could see some c**p, so I quessing that was my problem. Could of been that the filter could not let the thicker veg oil through well enough, yet the diesel was thin enough ?

I then thought it could be as you said, a vacuum in the Tank, as there was a hissing noise as I opened the cap, but it started playing up again, straight after.

Please keep us posted on your findings Lochan.

[lochan2]

Thanks all for your advice and suggestions, but they still dont seem to answer it completely in that whilst on short runs without the engine really warming up, say about 20 miles and 20 or so minutes ( to work)the engine behaves perfectly despite having less than half a tank of fuel.
If she gets upto full operating temp after say about an hour and 60 miles or so she will start showing the previously mentioned symptoms!
Is there a possibility of either fuel line pressure loss and or somplace that could cause evaporation or some other problem causing fuel starvation when bongo gets warm?
Really a strange one this and probably a (hopefully) simple answer
thanks lochan

[2sticks]

While this may not be the answer to the problem, the implications are certainly
worth consideration.

The boiling point of diesel is normally greater than 150 Celsius at atmospheric
pressure depending on the additives included by the producers. Around the
boiling point the fuel will vaporise and the fuel pump cannot pump vapour.

All fuel will contain a small proportion of dissolved air and vapour depending
on how the excess fuel is returned to the fuel tank and how much is present in
the suppliers storage tanks.

The boiling point of fuel is considerably reduced when the fuel pump reduces the
fuel pressure on the suction side, the dissolved air and vapour will be the first
to vaporise at low pressure (high vacuum) which will cause the pump to cavitate
and reduce the delivery volume.

If there is a blockage on the suction side of the fuel pump, and the fuel is cold,
the fuel pump may be able to cope with the high vacuum on the suction side and still
supply enough fuel to feed the injectors, if the fuel delivery temperature rises to
any extent then the combination of both of these circumstances may well be enough to
cause the fuel pump to cavitate with considerable loss of power.

If it were me, I would make absolutely sure that there is not a blockage in the fuel
delivery system to the fuel pump before proceeding any further. But then, again, I
am an old fart and could well be wrong !!.

[scarlet]

to me it sounds like you are pulling a vacum in your fuel tank

[helen&tony]

Hi
Try running round for a week using one of those cheap plastic emergency fuel caps that just wedge in. I had a car that did precicely what your Bongo is doing....I cleared the vents on the fuel cap , and was convinced it wasn't the vents, but I still got a vacuum....I seem to remember that a new cap did the trick.
Cheers
Helen

[lochan2]

Thanks 2sticks, very informative, as I understand it, your explanation means suction/back pressure in fuel tank as previously mentioned and by scarlet and helenandtony.
very many thanks all, will try the temporary fuel cap route me thinks
regards lochan2

[missfixit70]

continued here - http://www.igmaynard.co.uk/bongo/forum/ ... =3&t=26243 - it was the filler cap