Air from diesel tank?

[CHRIS STARKEY]

Hi, C250 turbo estate '96.
Fuel starvation at engine - all O rings & pipes changed but still loads of air after 1 day.
Removed pipe to fuel tank & sucked & just air coming through!
Fuel in tank (reserve light not on)
Any ideas anyone??

cheers
Chris

 

[bolide]

You've missed an O-ring, not seated a pipe properly or you've fitted a new fuel filter without the washer

Nick Froome
the independent Mercedes Estate specialists

 

[CHRIS STARKEY]

Thanks bolide but nothing has changed since the day before when it was running fine.
The O rings were changed weeks ago & the filter hasn't been moved recently.
It just wouldn't start - as though all the fuel had drained back into the tank.
I don't understand how there can be air coming from the tank without things like O rings leaking - any other ideas anyone?

 

[Ian B Walker]

The rubber pipes from the fuel lines by the inner wing to the pump perish after a while. It may be worth while checking that they are tight and in good condition.

 

[spock500]

The C250D is fitted with a complex arrangement of saddle tanks which straddle the prop shaft.

It sounds as though one of the tanks pick ups are blocked, not a major problem as both sides can be accessed from their respective tanks senders.

 

[CHRIS STARKEY]

Thanks both I'll try & follow the pipes back, need to get her jacked up first.

I've also noticed when filling her with diesel it becomes difficult to get any fuel in after a short while & you have to just let it trickle from the pump - anyone had the same problem & do you think it could be linked?

 

[Stratman]

The tank breather could be blocked. The incoming fuel should replace the air in the tank as it flows in. It sounds like the air can only escape via the small gap around the pump nozzle. If the air can't get out when being filled it's highly likely it can't get in to replace the fuel used when running. The vacuum thus created by the fuel pump would put a greater strain on seal and O-rings than they were designed to take.

Way back when, I had a Daimler V8 (Mk2 Jag shape) which ran beautifully except when I put my foot down at higher speeds when it misfired horribly. New plugs, points, filters and suchlike had no effect. I eventually found the cause when filling up at a motorway service station. I removed the fuel filler cap and heard the tank unbuckling! The end of tank vent tube was blocked with mud. A quick prod with a screwdriver and all was well.

Check your tank venting.

 

[spock500]

Yup, breather sounds likely if it's linked to the nearside tank - there's a bridge across the top connecting both tanks, if the breather is blocked then it will become difficult for fuel to flow across the top.

Either that or the bridge itself has become blocked but this would have to be pretty big.

These are the senders and tank diagram.

 

[CHRIS STARKEY]

Update.
It appears that due to my Father putting veg oil in the tank the pipes/filters/strainers have become blocked - the problem is we can't get the sender units undone - tight as anything.
Is there a seperate tank strainer on these saddle tanks - can't see one but according to pictures on Russian site there is one at the back somewhere?
Also are the two halves joined together - again according to the picture they are but can't see from under the car?

 

[SilverSaloon]

Also dont discount the main 8mm rubber piping from the metal fuel lines going into the engine bay to the IP. I replaced mine with new 10mm hose and after having lots of bubbles in the fuel lines, they are now no bubbles. The ends of the old hoses had gone all brittle. Also take the opportunity to fit new jubliee clips at this time too.

 

[spock500]

The sender units were ok on mine, I just tapped the alloy ring around with a flat chisel.

I never looked for a strainer as i drilled two big 'oles in the bottom of each tank - see here for saga vegetableoildiesel.co.uk - C250D Saddle tanks - Powered by XMB

 

[Uberwagon]

This is normal,

I went through 2 diesel filters in the first few hundred miles of using rapeseed oil in my Land Rover, after that it has been fine.

Dave!

 

[CHRIS STARKEY]

The sender units were ok on mine, I just tapped the alloy ring around with a flat chisel.

I never looked for a strainer as i drilled two big 'oles in the bottom of each tank - see here for saga vegetableoildiesel.co.uk - C250D Saddle tanks - Powered by XMB

Thanks Spock - I have been reading your saga, the problem is my Dad is doing this as he wanted the car, however, now he has seen how complicated it is he has changed his mind & I want to keep it standard to sell it.
He is at the moment taking the sender units out to see if he can find any blockages or breathers.

 

[CHRIS STARKEY]

This is normal,

I went through 2 diesel filters in the first few hundred miles of using rapeseed oil in my Land Rover, after that it has been fine.

Dave!

Thanks Dave, we are looking for the filters/strainers on the tank as the blockage seems to be there.

 

[spock500]

I've got a feeling the tank strainer is on the return line, no idea why but looking at no.53 in the diagram above that's exactly where the return enters the offside tank.

Easy to get at as it's on the outside, looks like it unscrews similar to one in my 300TD

 

[CHRIS STARKEY]

we can't find a tank strainer anywhere!
does anyone know whats going on inside the tank?
There seems to be pipes & other bits inside?

 

[spock500]

What's on the other end of the return line, that's the one that goes into the back of the offside tank?

If you take the pipe off does the fitting not unscrew?

 

[CHRIS STARKEY]

The pipe at the back of the tank behind the drivers side is the mail feed to the pump just a jubilee clip (53).
There is no sign of number 35 anywhere on the tank!
what is the valve on top of the expansion tank where pipe 131 joins?

 

[CHRIS STARKEY]

The valve where pipe 131 joins is apparently a rollover valve - to cut off fuel supply if the car turns over.
Does anyone know where pipe 158 goes?

 

[spock500]

Do you have those fittings shown in the above diagram as that looks like the layout for the C250Cdi?

Does the fuel feed not come out of the bottom of one of the senders (nr side tank)?

Post some pics of the tank and parts if you can.