C250 fuel system symptoms... fuel starvation...

[chizzel89]

Hi all, been having some annoying fuel delivery related symptoms with my newly aquired C250 TD Sport.
When I picked it up it was running B100, and seeing as its had the common prep work needed to run bio/ wvo blends (new O-rings on the push-fit pipes, shut off valve and decent glow plugs, as well as a spot-on engine which starts almost instantly from cold after the glow light has extinguished) I decided to put in 20ltrs of my finest WVO with 10% petrol mixed in.
There was just under half a tank of B100 in there when I poured in 20ltrs of WVO blend. Obviously I've researched the idea of running WVO blends in an unmodified OM605 extensively, and talked to several people who run this exact blend of WVO/Petrol long term with no problems at all.
Anyhow, the symptoms are mainly during kickdown. When the box changes down, the car initially picks up well as it should, but at 3-3.5k rpm it starts to surge back and forth a bit (just like my old pug did when it was struggling to pull in the fuel). Taking it up to the kickdown change point (4. something rpm) is a labourious and worrying task and I just avoid it now as I don't want to damage anything (lift pump for example).
It will do it a little after pushing it hard up a steep hill (giving it some welly, but not on kick down) too.
So far I've changed the pre-filter (and after market disposable thing just before the main filter) which was quite cruddy, so I'll change the main filter tomorrow and check the small witches hat pre-filter under the bolt.
I'm reliably informed these cars don't have a tank strainer, so atleast I won't have to get underneath the car just yet!
Is there any tell tale signs it could be anything else?
I imagine if it was the lift pump, the car woulnt be starting so very well as an ailing lift pump would let the fuel drain back/ leak?
Thanks in advance

 

[Dieselman]

It sounds like the viscosity of the WVO is causing too much drag so the lift pump is drawing in air past the seals or joints.

Try connecting a vacuum gauge to see the amount of negative pressure and try running from a can and short oversize hose.

 

[chizzel89]

I suppose so, though it is running less than 50% WVO/ petrol blend. About 60-70% is Bio which will have the same viscosity as normal petro diesel and the rest was 20ltrs of WVO which had 3 ltrs of petrol mixed thoroughly in with it.
That WVO blend would be nearer the viscosity of pure Bio than WVO, so the overall fuel will be very very liquid, little different from Derv only more lubricity. Vapour lock due to the petrol can be ruled out due to the fact the fuel heater isnt connected.
My old Peugeot 405 does just what you say... sucks in air from dodgy seals rather than pulling fuel in when under heavey demand but if you then let off the throttle when its playing up, the engine will struggle and splutter due to the air in the pump for some moments until its cleared the air to the return pipe. The merc behaves normally as soon as you let off the throttle, no rough running at all.
I changed the pre-filter today which got some air in the system, and it ran like a dog for about 15 seconds or so and stalled once, so you'd expect it to do a similar thing after it sucking in air during a prolonged effort with kickdown.
Thanks for the tips anyway, I'll get someone to hold it at 3.5-4k rpm whilst I inspect the clear pipes and pre-filter for bubbles.

 

[chizzel89]

A vacuum gauge sounds an excellent idea. Is there any particular type I need? Any links much appreciated.

 

[tim.100]

. check the small witches hat pre-filter under the bolt.

Sorry to hijack your thread, but is this pre-filter a part of the main filter assembly on the front of the cylinder block?

 

[chizzel89]

Sorry to hijack your thread, but is this pre-filter a part of the main filter assembly on the front of the cylinder block?

It is near there, on the right of the I.P as you look at it. You have the big rubber pipe coming up on the right (from the metal pipe on the passenger side inner wing), and then a junction where it goes down to 8mm I.D pipe. It changes direction 90 degrees here, and theres a little 200 micron filter in the right angle plastic bit I believe.

Changing the main filter today improved things, and I also discovered that the feed pipe running from the metal pipe on the inner wing to the filter is breaking down and oozing and sticky. Got some new hose to change that with.
Also the pipe running from the pre-filter to the main filter assembley is also hard, micro-perished and not sealing properly. Actually saw air bubbles creeping in there whilst I was bleeding the system. Whilst I was giving the starter a rest air bubbles would get in.
Only saw bubbles in one of the push-fit pipes- the one running from the filter assembley down to the lift pump, probably because of the issue further back in the system.
All the O-rings on these were changed about 20k miles ago, so they should be fine.

 

[chizzel89]

Problem sorted! Or it seems to be sorted (touches wooden table)....
Kicked down twice, once from 5mph and the other time from 40mph, let off at 70mph. No issues at all, all systems go. These cars are fun:bannana:
The fuel line from the inner wing was so rotten that it was oozing fuel and partly crushed in one area. The hose from there to the main filter was micro-perished and hard.
The witches hat filter wasnt spotless but pretty clear so I left it.

tim.mcd-
I misinformed you accidentally about the location of this pre-filter. Immediately to the left of the main filter is a black plastic gubbins with a 10mm hex head bolt on it. This is where the fuel feed goes in to the housing before going down to the lift pump.
If you take out the 10mm hex head and the small retainer plate underneath it, you can carefully prize the fuel feed hose tail out. With this out, you can carefully prize the other black plastic gubbins up and out, and the pre-filter is part of this. Like an upside down witches hat with a slightly square end.