1.4 Tdi Struggling to start..

[Little Dog]

There is an excellent thread on here where someone took apart a tandem pump (with photos) and iirc concluded it was unlikely/impossible for oil to contaminate fuel by pump/seal failure.

It is one of my unfinished threads, I still have the casting to cut up and report on hidden check valves https://www.a2oc.net/community/index.php?threads/how-does-a-tandem-pump-work.51970/

 

[DaveySprocket]

Jumping back a bit the following information may help those of you who are considering doing your own diesel filter (with the plastic housing) service in the near future.

In 2018 purchased @depronman fuel filter housing drain plug for its functionality and long service life. In essence it's one of the cheapest fixes when diagnosing and repairing a 1.4 TDI thats struggling to start ., I am a fan of no cost, low cost repairs

The link below begins with the FSI filter but further into the thread they discuss the Tdi filter housing.

fuel filter how to change

my 1.6 fsi has a small inline umbrella fuel filter on the off side engine bay next to the brake fluid reservoir i was thinking it might be a aftermarket additon does anyone else have this on there car, i think i will replace it or remove it any thoughts?

www.a2oc.net

As always, good luck!

Having tried to work around this problem twice now, I wish I had just ordered one of these in the first place, would have saved some grief. If they are in stock I will definitely be getting one.

 

[DaveySprocket]

Well the plot thickens, like black slime in a diesel tank maybe..

I was woken up at 2am by the wife throwing up (poor lass), her main concern was that she had woken me up omg, she takes consideration of others to a different level. She thinks she picked up a bug from someone at work, so hopefully nothing serious

Anyway the upshot was I spent the wee hours of the morning wondering how black slimy stuff gets into a fuel tank

A mate called round last night, he has had diesel bug in his fork lift, and it's a right slimy mess swimming in the fuel itself apparently, pretty easy to identify, so I'm pretty sure what I have is not that.

He also has an A2, and recently took out the sender, to find it was as clean as a whistle. Photos of other ones that I have seen on the forum have been fairly clean too, so I don't think it's a case of, that's just what an old diesel tank looks like.

As far as I'm aware, something can only get in the tank two different ways, either through the filler neck, or down the return pipe from the tandem pump. We've all seen the black gunk that tends to accumulate around a cars fuel cap, but does that stuff get into the tank? I guess some will.

Both my cars run pretty well once they are going, so tandem pump performance must be pretty good, that's not to say that they are'nt possibly leaking a tiny bit of oil into the return feed, I guess it would soon build up. He can correct me if I'm wrong, but it seems like @Little Dog findings are not yet 100% conclusive that it's impossible. The black stuff does seem much more like oil than dirt and grime.

 

[Deleted member 20348]

The information that @TAABVW and @Little Dog have provided us with has put weight on the side of the pump not being the source of oil in the tank. @Little Dog has more investigation work to do in the form of deconstruction of the pump to see what the hidden chambers are doing.

@TAABVW has put forwards that the injector holding o rings are causing engine oil to mix with the returning diesel fuel oil.

Hopefully someone will be able to explain how this is happening as I don't know.

The Zip file is the
SELF STUDY PROGRAMME 223
The 1.2l and 1.4l TDI engines
With Pump Injection System
Design and Function


At page 29 Audi start to talk about the injection system.

 

[DaveySprocket]

Jumping back a bit the following information may help those of you who are considering doing your own diesel filter (with the plastic housing) service in the near future.

In 2018 purchased @depronman fuel filter housing drain plug for its functionality and long service life. In essence it's one of the cheapest fixes when diagnosing and repairing a 1.4 TDI thats struggling to start ., I am a fan of no cost, low cost repairs

The link below begins with the FSI filter but further into the thread they discuss the Tdi filter housing.

fuel filter how to change

my 1.6 fsi has a small inline umbrella fuel filter on the off side engine bay next to the brake fluid reservoir i was thinking it might be a aftermarket additon does anyone else have this on there car, i think i will replace it or remove it any thoughts?

www.a2oc.net

As always, good luck!

@Goddas also seems to be suggesting that oil could be coming from the injector seals, so that seems our most likely answer to that conundrum..

Ultimately, as has been mentioned, it is a bit of a red herring as far as why my car isn't starting.

If it stops raining this afternoon, I will try to use the vacuum pump to pull fuel right through the tandem pump, and see where that gets me.

Have a good day everyone

 

[PlasticMac]

As an interested observer, engine oil travelling from the sump to the tank seems, (very), unlikely. If it does, surely it would mix with the diesel, not separate out?
Mac.

 

[Little Dog]

The theory is that top black seal has failed and when the engine is not running oil,which is not under pressure, can seep past it and contaminate the diesel? I would exclude every other possibility before going down that route.
If that seal has failed you would likely be getting diesel in the sump?

Edit, engine oil in diesel sinks:

Give it a good shake and it dissolves into a single combined liquid. I now have a long wait to see if it separates out again.

 

[DaveySprocket]

View attachment 120653

The theory is that top black seal has failed and when the engine is not running oil,which is not under pressure, can seep past it and contaminate the diesel? I would exclude every other possibility before going down that route.
If that seal has failed you would likely be getting diesel in the sump?

Edit, engine oil in diesel sinks:

View attachment 120654

Give it a good shake and it dissolves into a single combined liquid. I now have a long wait to see if it separates out again.

It could be interesting to put a small plastic object in there, and see if the oil decides to cling to it?

 

[Little Dog]

It could be interesting to put a small plastic object in there, and see if the oil decides to cling to it?

Put two plastic items in. Oil is not separating from the diesel after 1 1/2 hours

 

[DaveySprocket]

Put two plastic items in. Oil is not separating from the diesel after 1 1/2 hours

Was it new oil that you put in, or old? Could make a difference

I think really we need a live feed webcam on this exciting experiment

 

[Little Dog]

New oil, shall I put some old in?

 

[PlasticMac]

Flow can only happen if there is a higher pressure at the engine end, than at the tank end, (pressure differential), and for that pressure to be maintained even when relived by the flow? And in sufficient quantity to dump all the diesel in the line back to the tank, and then continue to pass oil. Is this likely?
Mac

 

[Little Dog]

Flow can only happen if there is a higher pressure at the engine end, than at the tank end, (pressure differential), and for that pressure to be maintained even when relived by the flow? And in sufficient quantity to dump all the diesel in the line back to the tank, and then continue to pass oil. Is this likely?
Mac

The two top seals seal the fuel return:

So I guess the pressure in there could drop to zero and diesel could run off? If so in theory oil could seep in. Expensive to repair and I think unlikely to be the cause I would explore other options first.

 

[audifan]

If the injectors are leaking diesel into the cylinders this will get past the piston rings and into the sump where it will mix with the oil. This is bad for a few reasons, the diesel will wash off the oil in the bores increasing wear and at some point the quantity of diesel in the oil could very well produce a "run away" situation....

For oil in the fuel would still say the tandem is the culprit.

 

[Deleted member 20348]

Hi,

Is the diesel pump on your TDI manufactured by LUK?

On the TDIclub form page there are photos of a Bosch tandem pump being disassembled.

From the photos the fuel pump mechanism of the tandem pump is housed in an aluminium case that sits above of the vacuum pump. (Seen below)

Picture credit: jolebert clubTDI 4th february2014

The pump intake and output require a seal to protect between the two flow paths/sections in the bosch model.
This seal prevents a demanding placed, such as suction on the vac pump chamber. Thus the engine oil from the vacuum pump will not be drawn into the fuel intake.

If this seal in the intake side of the fuel pump system fails then the pump will draw from both the fuel tank and the vacuum pump.

Totally happy to be way off the mark on this.

I ain't no engine

The bosh pump check valve is limiting pressure that is exiting the diesel pump and entering the cylinder head en route to the injectors at a specified 7.5 bar befor the spring pressure is overwhelm and the fuel is released into the return flow.

This creation of such pressure must have a process that is drawing in fuel. Some form of vacuum?

Anybody skiled in fluid dynamics?

 

[Little Dog]

@rotifer the pin hole in the Bosch is the oil supply for the vac pump. A seal prevents it passing to the diesel side where the pressure will be negative to draw diesel from the tank. If that seal is defective then as @audifan suggested oil could pass to the fuel supply.
A question now needs to be asked; is the contamination in the tank oil?

 

[Deleted member 20348]

Right, so once again seals are the problem?
Just age related I guess, they won't last indefinitely.
EDIT,
If it's the same fuel pump since new then 175k is good milage for it to cover. Plus we're not sure if it's the pump yet.

 

[Little Dog]

Right, so once again seals are the problem?
Just age related I guess, they won't last indefinitely.

If the back gunk in the tank is oil then I suspect that seal.
However I have used oil in my experiment and all I’m getting is dirty diesel. Once the oil has mixed with the fuel it appears not to separate out.

 

[PlasticMac]

@rotifer the pin hole in the Bosch is the oil supply for the vac pump. A seal prevents it passing to the diesel side where the pressure will be negative to draw diesel from the tank. If that seal is defective then as @audifan suggested oil could pass to the fuel supply.
A question now needs to be asked; is the contamination in the tank oil?

I can see how oil could pass into the fuel line, but not how it would then flow to the tank. Surely it would mix with the fuel, and end up being burnt, may upset injectors, depending on the ratio of oil to fuel.
I think the contamination in the tank is more likely to be be paraffin wax thickening and settling out.
By experience, the filters in diesel forecourt pumps need changing more often than the petrol ones.
Diesel quality can vary too. Blend varies summer to winter, with wax dose reduced in cold weather.
Mac.

 

[Deleted member 20348]

I can see how oil could pass into the fuel line, but not how it would then flow to the tank. Surely it would mix with the fuel, and end up being burnt, may upset injectors, depending on the ratio of oil to fuel.
I think the contamination in the tank is more likely to be be paraffin wax thickening and settling out.
By experience, the filters in diesel forecourt pumps need changing more often than the petrol ones.
Diesel quality can vary too. Blend varies summer to winter, with wax dose reduced in cold weather.
Mac.

Afaik,
Diesel mixed with the oil flows back to the tank via leak off from the injectors. I think, I'm not sure, the injectors receive an abundant supply of fuel from the pump and they can't use it all so send some of it back.