Suddenly won't start after recent tandem pump change

Ahh man, I can feel your pain Johnny, the job is a menace to do. Especially muck about with swapping pumps on and off. Need an early start and a clear head, not to mention a charged set of batteries. Have you a fuel primer pump?
 
Thanks Edd.
I did have a primer but it was crap.
I need to get another, why Audi didn't put one on is beyond me.
I'm now looking at it and thinking that I should throw the towel in and zip it down to my mates very large lock up and strip it for parts for my other two A2s.
 
Thanks Edd.
I did have a primer but it was crap.
I need to get another, why Audi didn't put one on is beyond me.
I'm now looking at it and thinking that I should throw the towel in and zip it down to my mates very large lock up and strip it for parts for my other two A2s.
Don’t throw the towel in. If you don’t want to mess with your other two cars Ill send over this pump I have. If the fuel is full of air again at least you know you have a good new Bosch pump.
 
Take a break good nights sleep might feel better about the situation then. I've got a 12v oil pump, peristaltic thing, should work well drawing fuel, I've never had to try though. Hopefully a mate can help you out.
 
I can confirm there should be absolutely zero bubbles in the fuel return line. If there are even tiny bubbles, then air in the fuel is the problem and don't waste your time looking elsewhere. The tandem pump is good at bleeding air out of the fuel once it is running but won't start or will only start after much cranking. The air in the return line shown on your video with the car running is showing just how well the pump is bleeding air out of the fuel once started. It is also showing the extent of you fuel/air leak.

Since the fuel feed is always under negative pressure it is quite normal to have an air leak in without having any fuel leak out. The last time I had this problem (and I've experienced it quite a few times) it turned out to be the fuel feed pipe connections. The rubber deteriorates over time and though you may think you have done the jubilee clip up nice and tight it will still leak in air. I suggest you replace the fuel feed line with a nice new supple one.

If this doesn't fix it I would insert a one way valve in the feed line just before the pump. This will tell you if your tandem pump is leaking air in as it will stop gravity sucking the fuel back out of the pump (and air in). If it then starts but still shows air in the fuel return line, then the leak is upstream of the one way valve. It may start and then only have a little air, in which case the leak is in the pump.

I am assuming the "rocking pump" issue is now resolved and that you are aware that you should use a vacuum pump on the fuel return to suck the air (through) out of the pump after opening the fuel system.

Trevor
 
Many thanks Trevor for your detailed response.
I will try the fuel pipe replacement if I can get a new one.
I take it that the top short one is the feed pipe?
Yes, the pump now sits square with no movement and all four bolts have been torqued up to the recommended values, 20 top, 10 bottom.

I've heard about a non return valve being suggested but also seen people say don't bother so I'm unsure, but then again you seem to know what you are talking about.

Has you might have read, I changed the pump because of hard stating when parked facing uphill or sometimes when left on the flat for a few days.

I know that I will have to re attach the clear tube with the non return valve to check for air, this will mean a longer piece of tube to accommodate the valve, will that have an effect in any way?
I will also have to put a piece of clear pipe on the lower return pipe to check for air.

As you state in your reply, if there is still air getting in, where would you look? filter housing?

Regards.
John.
 
Longer pipe won't hurt as once it's hit the pump it's all the same.
A non return valve will highlight a fix, it isn't necessary as not in original design. What it may do is point you to fault.
 
The detail advice from Trevor is similar to what I would have suggested if the pump checks out to be good.
I wouldn’t have thought to question the inlet pipe though. It has been off so many times recently I assumed you would have noticed if was lose fitting? A new bit of pipe though is an easy fix if that pipe is the problem. If it is leaking then fuel will be able to flow back to the tank and be replaced by air.
I’m going to side with a NRV not being necessary. The fix is to find and repair the air leak, in the pump or pipes.
If the car can be driven I’d be tempted to fill the tank to the filler neck. See if less air gets in, particularly if parked facing down hill. Be careful though, fuel may also weep out ruining you cooling pipes.
 
I can confirm that a one way valve could solve the fuel returning problem.
I had a leaky tandem pump (needed a long crank to start after being parked for more than 12h) and the valve solved that.
It was mounted before the metal pipe on the alternator side of the engine.
 
The detail advice from Trevor is similar to what I would have suggested if the pump checks out to be good.
I wouldn’t have thought to question the inlet pipe though. It has been off so many times recently I assumed you would have noticed if was lose fitting? A new bit of pipe though is an easy fix if that pipe is the problem. If it is leaking then fuel will be able to flow back to the tank and be replaced by air.
I’m going to side with a NRV not being necessary. The fix is to find and repair the air leak, in the pump or pipes.
If the car can be driven I’d be tempted to fill the tank to the filler neck. See if less air gets in, particularly if parked facing down hill. Be careful though, fuel may also weep out ruining you cooling pipes.
If I parked it facing down hill it was easier to start, did you mean facing uphill?
 
Another thing you could try to rule out the pump is to use a temporary fuel supply as you would do when using diesel purge, something simiar to the following arrangement utilising the clear hose so you can check for any air. Thanks to @nod for the photo:

Screenshot 2023-02-28 at 10.56.58.png
 
I can confirm that a one way valve could solve the fuel returning problem.
I had a leaky tandem pump (needed a long crank to start after being parked for more than 12h) and the valve solved that.
It was mounted before the metal pipe on the alternator side of the engine.
A one way valve is only necessary if there's a leak of air into the system. No leak, vacuum in system, no drain back, no leak, no air. Depending where the leak is, a one way valve may not help. The one way valve has to be on the tank side, (before) of the leak. If there's a leak between the valve and the tank, the line will still drain back, and air will enter.
Mac.
 
If I parked it facing down hill it was easier to start, did you mean facing uphill?
Facing downhill. it raises the height of the fuel relative to the pump so makes it easier for the pump to draw fuel. I'd have to check but facing downhill with a tank full to the filler neck may create +ve pressure at the pump, That's the reason I advise you to be careful you have an air leak somewhere allowing air in and fuel to flow back to the tank. A full tank and parking facing down hill may result in a diesel leak and all of the mess and damage that goes with it.
 
Another thing you could try to rule out the pump is to use a temporary fuel supply as you would do when using diesel purge, something simiar to the following arrangement utilising the clear hose so you can check for any air. Thanks to @nod for the photo:

View attachment 105111
I've done this with petrol engines, connect a pipe to the pump then a funnel to the pipe above the pump and fill with petrol. Petrol pumps deliver what is required, not much at idle, whereas diesel pumps deliver excess fuel at idle. Such a test would confirm or rule out the pump though, just need a decent amount of diesel above the pump.
 
To be clear, the one way valve on the fuel feed (starting as close to the pump as reasonable possible) is to help diagnose where air is leaking in. It's not intended as a permanent fix. It will give you one of three results:

1. If the leak is between the tandem pump and the one way valve then the car will start and very little air will be seen in the clear fuel return pipe. This is because the one way valve will prevent the head difference between the leak site and the level of fuel in the tank from sucking air in at the leak site.

2. If the leak is between the one way valve and the tank then the car will start and lots of air will be seen in the clear fuel return pipe (or the engine will splutter to a stop). There will be no air between the valve and the pump hence it starts, but then air will be drawn in through the leak site further upstream. This will be bled out by the pump and seen in the return, or it will stop the engine.

3. If there is still a very poor or no start then the leak is in the pump.

Given the history of the problem you have described, I would suggest that the original pump was probably not at fault and it was an air leak in the feed pipe all along. You have disturbed this air leak site in the fitting of the new pump making it worse. This gives a clue as to which fuel feed pipe to replace first. philward suggested the feed pipe an unlikely source of the leak (while acknowledging it's an easy fix) but I have found the more they are pulled off and replaced, the more likely they are to leak. I once spent about a month (not full time) chasing around a slow starting issue. I clamped up that feed pipe nice and tight every time I refitted it but that's what it turned out to be. If the pipe "clunks" into position when you push it on you might think that's a nice fit, when it's actually indicating the rubber has gone hard and is no longer malleable enough to seal.

Answering some of your questions directly:

Your plastic tube clearly shows air and fuel coming out of the pump, therefore the feed pipe is the other one.
The lengths of the pipes are irrelevant provided they are air free.
You don't need a clear pipe on the fuel feed necessarily, just the return to see what air the pump is dealing with and when.
Any 8mm diameter diesel fuel pipe will do for testing and indeed replacement if necessary.
If the leak were at the filter housing it would likely leak diesel out given the filter housing is low down in relation to the pump level. This kind of (no diesel out) leak tends to be high up in elevation hence starting near the pump.
philward suggestion of brimming the tank and parking on a (steep) downhill, or car service ramps might well yield results in the form of a physical diesel leak. You would have to go round and carefully check every joint in the feed line (perhaps using kitchen paper) for signs of diesel damp.

I have had about 12 audi fuel delivery systems (not all A2's) in many pieces and adapted them to run vegetable oil over a period of about 20 years. The oil hardens the rubber hoses quicker than diesel and the extra viscosity of the oil creates a bigger vaccuum in the feed pipe than diesel (which is why i fit a lift pump at tank level). Thus I probably have more experience in this area than many. I apologise if this experience might be clouding my assistance.
 
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