Suddenly won't start after recent tandem pump change

It could still be a dodgy float valve, and an empty swill pot of course, and not a leak.
Again, dip in a shallow liquid of your choice, and see if the swill pot fills up.
Mac.
 
johnyfartbox said:
Here is a copy of a guy on Facebook who is having the same problem with air in the fuel supply on his TDI A2.
I asked him what is the problem with his sender unit because he was asking if anyone has one for sale as he's convinced it's the sender unit causing the problem.
Here's his reply.

John .
Morning, from the out let of the fuel supply from the sender unit it it drawing up air. I've run a clear pipe directly from the sender unit to the engine compartment with a temporary inline filter. Checked and replaced all the fuel pipes from the filter housing to the fuel cooler and they're fine. But from the feed of the sender unit it is drawing up air bubbles. Also put the clear feed pipe directly into a separate fuel container and there's no air. Put it back on the sender unit and there's bubbles.Everything is pointing to the sender unit. Regards.

Thoughts guys?

Someone suggested replacing the O ring, I'm assuming that they mean the large one that the sender sits on when it's dropped into the tank.
Click to expand...
There is a small O ring inside the sender unit, it can be clearly seen in one of the photos above near the mucky digit :) Its about the sizr of a 5 pence piece.
 
DaveySprocket said:
There is a small O ring inside the sender unit, it can be clearly seen in one of the photos above near the mucky digit :) Its about the sizr of a 5 pence piece.
Click to expand...
Ha, I was looking for an "O" ring near the part number, "mucky digit" Just twigged.
Fickle finger of fate?
Is that "O" ring in the flow path from the swill pot end to the outlet on the top half?
Mac.
 
1000008091.jpg
 
1000008075.jpg
if you zoom in on the hole where the sender actually sits, you can see the large gasket that might be allowing air into the tank.
I think that is what the guy off Facebook is talking about, but I'm not absolutely certain
 
johnyfartbox said:
View attachment 121828if you zoom in on the hole where the sender actually sits, you can see the large gasket that might be allowing air into the tank.
I think that is what the guy off Facebook is talking about, but I'm not absolutely certain
Click to expand...
I wouldn't worry about that mate, there has to be air in the tank as the fuel level goes down. That seal is not critical, it's just to prevent leaks into the car.

The little O ring could well be an issue though..

I can try my spare sender in my 2004 car if that helps, just to confirm the fuel gauge reading.

At least some kind of progress is being made.. 🤔
 
This one is 2004 so should be fine, I'm guessing that the sender is the same but the electronic gizmo is different.
 
Ok peeps.
Update time.
I've just done a diesel purge on the car, white one, the one in this thread.
I hope I've done this right, I followed instructions so I think so.
When I connected the two clear pipes to the feed and return pipes submerging them into a glass jar almost full of the purge making sure that they were on the bottom so no air could get in I started the engine, cracked first time and then laboured for a couple of seconds then ticked over fine . I began to increase the revs slowly up to around 3000 RPM then back down, repeat and rinse for around 30 minutes, keeping an eye on the level.
The return fuel started to get warm obviously and then decided that would have put enough through to do a clean of the jets and whatever else it does.
The thing I did notice though, there was a constant stream of bubbles going through the return pipe back into the jar, the feed pipe was solid fuel, no bubbles whatsoever.
Thoughts?
1000008363.jpg

Top pipe feed.
Bottom pipe return to the jar
 
Hi,
I have a question for you.
Was the air stream made up of roughly the same sized bubble and same amount of bubbles in the 1st minute as in the 30th minute of the purge?
Depending on the answer yes or no, we can theorise what may be happening.

The rig you have set up is only testing a few hoses and clamps in the run.
Did you replace each rubber hose for new and fit new spring clamps when you suspect they could be past best in this area?

In addition there is an o-ring in the fuel temp sensor housing.
Hoses, gaskets, o-rings and seals in no particular order will be the first to check/renew. Do the cheapest and simplest repairs to a high standard before we delve onto fuel pumps and injector seals.
⛽🦭

It's a long old post sorry if I missed any relevant information that has already been posted.
 
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Once the fuel supply enters the pump, it is pressurised, so pressure side a leak would be visible, I think.
Air is separated out by the mesh, and passes via the tiny restrictor to the return, where you're see it.
Air is only separated by flow through the mesh, and on idle, most of the flow will be straight into the return, which doesn't pass through the mesh, so the volume of fuel actually going to the injector circuit is small. To me, that means it'll take quite a while to purge all of the air in the pump.
@rotifer is right, check the bubble size and flow rate. Hopefully, it will reduce, 🤞
Mac.
Edit: Long time ago now, but this saga started with a tandem pump replacement, all was well till then so things like injector seals seem unlikely to be a problem now.
 
Ok guys, just to clarify, this is how I connected things up, clear hose to metal feed tube, clear hose to metal return tube on top of the engine as you look at it, both ends of tube in the jar full of DP.
So my thinking and why I asked about the air bubbles which incidentally were the same from start to finnish was I thought that the fuel going in from the jar and the fuel being returned to the jar would be bubble less, just solid fuel. But although the fuel was solid fuel being sucked from the jar, the returning fuel had lots of bubbles in it , I was thinking that because the fuel was basically going around in a loop, it would come out as it went in.

Regarding swapping hoses, the only one I have changed is the short feed pipe to the inlet to the pump and clamped with proper quality squeeze clamps.
Regards.and thanks for your input.
John
 
Brilliant reply,
My questions are to try stop us overlooking a simple but possibly crutial detail.
I take it from your reply that your hoses and quality spring clamps are good. Perfect👍
Regarding the fuel temperature sensor O-ring, when was that last renewed please?


As @PlasticMac says we may or may not still be witnessing bubbles that were caught up in the system. Which means we may not be looking for an air leak. But if we assume there is an air leak then following the precess of elimination of each part is the best way I know to fix stuff.

(For those reading this trying to fix their own fuel problem the temp sensor and housing is in the return fuel line).
 
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@johnyfartbox you can probably guess my next question.

Are you happy that the fuel pump metal gaset and the aluminium mating surfaces of the fuel pump and cylinder head are sound?

Especially around the area where the fuel flows inward and exits from the cylinder head ports.

😁👍
 
I've been thinking about that, I was really careful putting the pump on at the second attempt, I even bought a brand new torque wrench and followed instructions to the letter, but you never know eh.
 
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