1998 300 Tdi Fueling Issue

[Hammerhead]

Hi,

I'm having a problem with my 1998 300tdi.  Slow to start, sits fine at idle, dies on acceleration. Sounds familiar, simple fix, change the lift pump.

But no! Lift pump changed, no difference. So changed fuel filter, air filter, oil filter (not related but I was in a filter mood). Engine starts slowly runs on idles, dies on acceleration.

Possible dodgy replacement lift pump? Off it comes and another one put on. Tested both for blow/suck on activating the pump lever. All good.

Start it up, fine on idle, (you know where this is going) dies on acceleration.

So I crack the banjo bolts on the fuel filter and there's a healthy flow coming through. Checked the bleed screw, expecting some bubbles then fuel. Nothing coming out at all. I take the bleed screw out completely, still nothing. Out of curiosity I put my finger over the bleed hole and there's positive air pressure building up behind it. So much so that it gives a good hiss when I take my finger off, like if you had your thumb over the end of a bike pump...

There's no obvious drips or leaks, of fuel anyway!

Any thoughts on what could be causing my issue or whats next in the check list?

 

[defender dinky]

Hi, I'm sure my mate had something similar to this, think it turned out to do with exhaust box broken down but not 100% sure, what's coming out the exhaust if anything, just a thought, dd

[Yostumpy]

try taking off the fuel filler cap, blocked vent may be causing a vacuum.

Also try taking off the air intake pipe at the manifold, incase, an intake pipe has completely collapsed internally.

Edited July 2, 2020 by Yostumpy

[Hammerhead]

Thanks for the pointers.

Exhaust is blowing clear, there's only one box, the rest is straight pipe so no issue there.

Filler cap off and air intake opened, still dying on acceleration.

It doesn't die immediately but chugs heavily before stalling so pretty sure it just not getting enough fuel through.

Could there be a situation where its sucking air into the fuel pipes but somehow not leaking fuel out? Using my logical head, that would more likely be happening prior to the lift pump because post pump the fuel would be under more pressure and a leak would occur.

HH

 

[monkie]

It really does sound like air in the fuel system. 

Very carefully and systematically check the pipes, hoses and joints for integrity starting with the leak off pipes on the injectors. Check the seals and banjos round the filter. Go right back to the pick up on top of the tank, they are known to rust. 

You then need to get all air out of the system. Get back to that bleed screw and make sure just fuel comes out. 

It will then take some cranking possibly to bleed the high pressure side to the injectors. Keep your battery well charged. 

[Yostumpy]

can you not rig up a gravity fed system, with a short pipe to the lift pump, to see if there is a blockage in tank / pipework. does it have a sedimenter.?

[kevin50]

Have a read of this Mate, I had similar problem, done my head in for 2 weeks, Turned out it was a tiny split in the Top Union going in to Lift Pump from Tank,. 

Dont buy another LP like I did  before you try everything  I went through 3 and thinking they were all knackered but they were OK .Use some clear hose so you can see whats going on.

[Hammerhead]

Update on this problem. 

Figured it had to be on the low pressure side as any splits between lift pump and injectors would have diesel spewing everywhere. Just to discount a crack in the fuel filter housing, while running at idle I briefly undid the banjo input to fuel filter, this was definitely bringing in a lot of air as it was bubbling and spluttering..

So traced the fuel pipes back to the tank but again nothing obvious. With engine at idle, checked again, bingo! Fuel pipe at tank gently bubbling and on closer inspection the casing is split on the bend, just enough to let air in but very, very little diesel out. Presumably on acceleration it just sucks in huge amount of air in and starves the injectors.

Replacement length of 8mm fuel pipe connected and alls well. Didn't have to bleed system, just started up and ran fine.

Thanks for all the suggestions.

 

Now for the electrics, but that's a whole new thread!