How to prime my 1.4 tdi after remaking the tandem pump

Guest 4123 · 26 Feb 2022

titusm said:
@RAB this thing looks like it can act as another return valve....and again is on the return line....almost at the end. Not sure if it does...i will test on the new piece
View attachment 91489

The only point of a non-return valve in the return valve is that it will prevent fuel returning to the tank from the inlet line but only with the fuel system is intact (assuming no leaks). As soon as the system is dismantled, the fuel will flow back to the tank in the absence of any other non-return valve.

RAB

titusm · 26 Feb 2022

RAB said:
The valve is a regulating valve. The arrow on it should point towards the tank. Below 15degC, passage to filter is open, above 46degC, passage closed. Make sure that the O-ring for the valve is in the correct position.

Fill up the new filter with fuel before use.

RAB

The fill up is in the replacement procedure? the thing is i do not remember filling it up last time i changed it...but it might have been in the summer.

If the filter is empty - and I use a hand pump shouldn't it get full of fuel? (if the regulating valve is opened on the return line...i guess this is kind of impossible?) AAAAAND that is why they installed the return valve in the tank...so you can prime the filter...otherwise you would have air coming in to the lines when priming:

))) finally some sense. Not sure if the same logic applies for the other model that has plastic housing for the fuel filter.
This means in my case...that because of the "thermal valve" I could not prime the system...(this one had a very subtle crack on the sidewall).

So when you cannot prime the intake fuel line...you have 3 possible causes:
1) some problems with the lines - cracked, or bad o-rings - in my case some bad rubber passages between various plastic tubing.
2) the return valve in the fuel tank does not work and allows air to come (from the tank via return fuel line) in to the system when priming the fuel filter (easy to check - try to aspire(suck) some fuel trough the return line right at the tank level)
3) the regulating valve or thermal valve is broken and allows air to pass around it - I determined this by trying to prime the system right after the filter and after close inspection notices a subtle crack in the valve (because of the dirt the crack is hard to see).

A pump like one in the picture is a must when determining issues when priming...and is also useful for priming procedure(it is affordable).

titusm · 26 Feb 2022

RAB said:
The only point of a non-return valve in the return valve is that it will prevent fuel returning to the tank from the inlet line but only with the fuel system is intact (assuming no leaks). As soon as the system is dismantled, the fuel will flow back to the tank in the absence of any other non-return valve.

RAB

Found the reason for the return valve in the tank...
for priming.
If there is no return valve in the tank then air will go into the lines when priming....At least in the metal filter model - because the return line and feed line communicate together via the filter so when you try to absorb fuel via feed line the return line would return air if it wouldn't have a return valve.
So it is important.

This might be the main reason why in some cases it is hard to prime the fuel system after changing the tandem pump or the filter.
That's why some of the guys had success only when pressurizing the fuel tank.

Guest 4123 · 27 Feb 2022

The tandem pump is designed to pump liquid, not air. That's why it struggles with an empty inlet.

RAB

titusm · 28 Feb 2022

titusm said:
@Cyril13013 any ideea what type of fuel filter you have? Did you change 057127247A on your filter?

@RAB this thing looks like it can act as another return valve....and again is on the return line....almost at the end. Not sure if it does...i will test on the new piece
View attachment 91489

I just boght the new valve aaand surprise...the new valve allows much better flow compared to the old one.
So...this part needs to be changed from time to time...
It might be also one of my lower mileage...

titusm · 2 Mar 2022

Back after installing the new valve: 057127247A -- the one that is installed on the filter.

Now priming is very easy... (it took like 5 or 6 manual pumps to have fuel without air)
The engine runs better and by the looks of it the consumption is much lower.
After longer idle there is no more white smoke(and the idle is not so rough).
The injector correction ...is better(before I had correction of 0.5 when warm)...So that valve is a big pain if it goes bad...
Some pictures from when I had issues too with priming (after having to bleed the petrol out

) ).

1)check priming directly after the filter - if you have air coming in the manual pump then you have an issue (you need a pump with clear tube on the intake side so you can see what is going on). - in my case was some old tubes that go to webasto(first picture), and the above mentioned valve(last picture).
2)prime intake to tandem pump - picture with external canister.
3)prime with output into tandem pump -- last picture with manual pump.

I paid around 10 euros for the manual pump (it has an adaptor that fits the output from filter fitting directly). You can find cheaper pumps but the adaptors might be missing so you would have to invent some. I've seen pumps going for as low as 3euro.