Audi A6 Air in Fuel System: 2.5 TDI AKN Fix

Hello,

"For the past few weeks, I've been having a problem where my A6 won't start overnight. The issue appeared suddenly, from one day to the next. There seems to be air in the fuel lines, especially in the fuel filter. I've repeatedly bled the fuel system to get it running again."
I believe that the problem is worse when I park my A6 on an incline (with the engine higher than the fuel tank). When parked the other way around, it starts without any problems for a few days.

In the meantime, I replaced the fuel filter, the black fuel preheating valve on the fuel filter return line, the fuel lines between the filter unit and the ESP (Electronic Stability Program), the leak oil lines, and the fuel lines between the filter unit and the fuel tank.

Since I've already replaced everything between the fuel tank and the fuel injection pump, the only components left are the ESP (Electronic Stability Program) and the fuel pump. This morning, when the car wouldn't start again, I noticed about 5-6 cm of air in the transparent fuel line near the top, close to the filter.

Have there been any issues with leaking ESP systems causing air to accumulate in the fuel filter?
One indication is that the engine doesn't start at all (it doesn't run for even a second), which, in my opinion, suggests that there's air in the ESP system.
Or could it be that the air is coming from the fuel pump? That seems illogical to me, because the air would have to overcome a downward slope to reach the undercarriage after being dispensed from the pump.

I've already found similar cases of air in the fuel system online, but I haven't found a solution to my problem in those posts. In those cases, the problems were resolved by replacing check valves.

I would be very grateful for any advice.

Regards,

Stephan.

It's possible. Someone else had a similar problem here, and they replaced the valve that's located after the return line. I don't really understand why, though, because there's also an overflow hole there.

The problem with check valves is that they often have a certain back pressure, even in the flow direction. This increases the overall pressure loss in the supply line, and if the pump is already slightly leaky, it could more easily draw in air during operation.

The pump in the tank only fills the reservoir; if there were a leak, there would be constant air bubbles in the clear hose.

I had the same problem with a car. I then installed a standard fuel pump in the fuel line between the tank and the fuel filter, which is powered by terminal 15. Since then, I haven't had any more problems with the car not starting overnight. We'll see how long it lasts, until the ESP completely fails.

I found the problem. The Bosch technical hotline suggested that I check the O-ring on the locking screw. And indeed, the O-ring was literally "flattened," which was causing air to be drawn into the fuel injection pump.

See item 392 here http://eu-auto-2000.de/downloads/Ersatzteile%20%208778.pdf.

christians wrote:

The pump in the tank only fills the reservoir; if there were a leak, there would be constant air bubbles in the clear hose.

Hello, good day everyone.

In my Octavia 1.9 TDI 81kW (AHF) (year 2000), there are constantly air bubbles in the transparent hose connected to the ESP system.

The car starts really poorly; some days, I have to crank it for so long that the battery runs out.

Okay, here's the translation:

"I'm not an expert, so I'll ask a simple question: What is the "Stautopf," and where is it located? If I know where it is, I might be able to take a look and see if there's any visible damage."


I just checked the hoses from the filter to the ESP, and they look fine. I also visually inspected the small valve in the return line, right on the filter, for any leaks. You can't really see anything obvious, not even any diesel stains around it or anything like that.

Do you have any ideas about this?


And another question for the experts: When the engine is warm and running, if I disconnect the MAF sensor connector, what should happen? In my case, nothing seems to happen... well, maybe a very slight increase in RPM, but nothing else.

The expansion tank is located in the tank, if the Octavia even has one.
Most often, it's the valve, even if it looks fine from the outside, or the diesel filter. Also, the hoses connected to the diesel filter don't last forever. You'll never find any diesel leaks there because there's always a slight vacuum in that area.

Okay, so the best approach would be to first replace the hoses from the diesel filter to the ESP (Exhaust Gas Recirculation system), and also replace the filter completely, along with the two sealing O-rings for the return line... right?

The car has now driven 260,000 km. At 270,000 km, it's going to need [something] anyway, so I'll get that done sooner rather than later.



Thank you for the response.

Hmm, I ordered the parts from the local dealer yesterday. They don't have Octavia parts in stock, but they say they can order them... at least, that's what they told me.

I ordered the connecting hoses between the diesel filter and the ESP (Exhaust Gas Recirculation system), as well as the two O-rings for the preheating system. I also ordered the diesel filter itself.
Unfortunately, I forgot about the fuel lines that run from the injectors back to the pump at that moment.

The whole thing should cost around 85 euros. Does that price seem about right?

I was somewhat surprised when the salesperson told me about the O-rings: "we never replace those!" And I could seal it with regular hose clamps. }

I'm curious to see what happens.

Quote:

I was somewhat surprised when the salesperson told me about the O-rings: "we never replace those!"

O-rings are usually included with a new diesel filter... it depends on your fine motor skills whether you scratch the plastic part with the screwdriver, in which case you should leave the O-rings in.

Rainer K. wrote:

O-rings are usually included with a new diesel filter... it depends on your fine motor skills whether you scratch the plastic part with the screwdriver, in which case you should leave the O-rings in.

Yes, they are included. I have also replaced them.
The initial impression: there are fewer air bubbles in the "sight glass" of the return line, and the vehicle starts more easily, even after being idle for a longer period.

However, if it's already starting to feel warm (after 3-4 km), it could still be even better.
I mean, I think I read something about this somewhere, that it might be the battery, because the control unit also expects a higher RPM as the temperature increases.

Is that so?

Starters often fail! I've already replaced several starters in Golf 4s! So my advice: check the starter motor's speed and current draw!
I'm pretty sure the problem is with the starter.
And regarding the air in the pipes: replace the black valve (the one where you replaced the two seals when you changed the filter)! It costs a few euros!

Best regards,

Hello Tedi,

Check if your fuel injection pump is leaking anywhere.
I had the same problem with air in the fuel line last week.
When I disconnected the hose from the filter, it was dripping out of the pump located beneath the high-pressure section (where the hoses connect to the nozzles).

Hm, thanks for the tips.

Drops won't do anything.


I'm not going to put much more effort into the car anyway. It's going to be scrapped in 6 weeks.


The idle speed was definitely too low, and the battery was almost completely discharged. After nothing worked at all at the beginning of the week due to the cold, I borrowed a new battery for the past few weeks. It's working well now.

Now, the preheating indicator light is staying on longer, and this morning, when it was 0°C outside, it started up perfectly.

Tedi wrote:

In 6 weeks, it's going in the trash.

I hope that is a typo and you meant "sun" .

Hello, I'm new to this forum. Please excuse my lack of knowledge. I've been having the following problem with my TDI ASV 110 hp engine, with approximately 120,000 km (non-PD). When the car stands for a longer period, such as overnight, it starts poorly. It usually takes about 10 seconds of cranking with the starter. This problem gets worse when it gets colder.
I replaced the following parts: glow plugs (without success); fuel injectors (without success, although one was not quite right).
Okay, I'll deal with the issue of air in the diesel line. Then, I installed a check valve. No success. Next, I installed an additional diesel pump; still no success.
"Then, in this forum, someone suggested that it might be the starter motor (insufficient starting speed, the battery is new). I bought a starter motor with 2kW of power, which is the original type. And wouldn't you know it, I could hardly believe it, but my ASV starts up like a dream. It's noticeably easier to start the engine now."

I probably have my old starter motor to blame for this. I changed the diesel filter about two years ago. And according to the owner's manual, the system should bleed itself automatically. But that didn't happen, the liar. My starter motor got hotter and hotter, but my car still wouldn't start. Then I bled the system the way I used to, and my car started right up again. And since then, I've been struggling with my TDI.

The whole situation was further complicated because the headlight switch was also faulty and drained my battery after three or four days. So, I was always looking in the wrong direction. Only after the headlight switch was replaced did I finally get on the right track.

Ich war mit meinem TDI bis zum Kilometerstand 85000 sehr zufrieden. Then it started.

3 LMM sensors. First, the one from Pierburg (45 €) worked.
2 window lifter hinges (of course, while on vacation in Italy).
1 light switch.
Fuel injectors.
Spark plugs.
Stabilager.
Brake discs: The old ones were driving me crazy with the vibrations.

Since then, I've been doing nothing but tinkering. My girlfriend drives a cheap Japanese car. It just keeps running and running... but there are no spare parts available.

So, if you're experiencing exactly that problem, where the engine just doesn't want to start, the starter motor might be the culprit, even if you don't want to believe it.

Tagessuppe wrote:

Tedi wrote: In 6 weeks, it's going in the trash. I hope that's a typo and you meant "sun."

It's been a while, but... no, it wasn't actually a typo.
But actually, it's only because my dealer, this guy named Schlufffi, who was supposed to scrap the car, ended up selling it anyway, somewhere in the east, and gave me the €3,500 "scrappage bonus" himself.


By the way, I now have a gasoline-powered Octavia 2 FL.