Audi A6 TDI: Intermittent Starting Problems

Hello specialists,

My Audi seems to be losing its affection for me lately. The car has approval for biodiesel and was therefore, whenever possible, also run on it. However, this year it's no longer as cost-effective, but the engine now refuses to run on diesel.
When you fill up with diesel, this part usually has starting problems that manifest as follows: It cranks for about 3-6 seconds, then it starts, but doesn't respond to the accelerator for about 10-15 seconds and sounds like it's only running on 3 cylinders. Sometimes it also stalls.
Now, these problems are increasingly occurring with biodiesel as well, although not always, but mostly when the vehicle has been standing in the sun for a day at temperatures above 30°C or when it hasn't cooled down properly yet.
First, I checked the error memory, which was empty, and replaced the filters. Unfortunately, that didn't work. Then, since it mainly happens with a slightly warm engine, I thought about increasing the initial fuel injection amount. However, the control unit won't let me do that; none of the login codes I found online are working.
Does anyone have any ideas without immediately replacing the pump or finding the correct login?

Best regards,
Kai

"Especially with biodiesel, seals could be affected. Do you have a transparent hose before the EGR valve to check if it's drawing in air? That would be a quick way to check. Is the injection timing correct?"

Hello,

'As far as I can tell, everything is fine with the car when it's running. The values are correct, and like I said, the filter is new... I've basically checked everything. Because the problem with diesel occurs consistently and in almost every temperature range, while it only happens with biodiesel when the engine is 'lukewarm,' I suspect a problem within the pump itself. It uses hydraulics internally, and maybe it takes too long to build up enough pressure to respond to the accelerator pedal. I tried logging the position of the control slide after starting, but in those instances, it always starts perfectly.'
I've already checked the simple things like air supply, filters, and the initial injection amount. I would ideally like to try increasing the starting amount, but unfortunately, the login codes (12233, 262x2...) that I found don't work.
Which temperature sensor is relevant for determining the initial amount of fuel? Is it the one located at the front, near the thick coolant hose, or the one located at the back, in the heating circuit, or perhaps another one?

Hi there,

That sounds like hot air; somewhere, the pump might be running dry.
back into the tank, and that's why you don't get a proper start.
Gas intake occurs until the internal pressure of the pump is within the acceptable range.

A new bypass line with a check valve is cheaper than a new ESP (Electrical Submersible Pump).

Best regards, Michael.

I also thought about air, but why does it only seem to happen with diesel fuel going in, and not with diesel coming out? And why only when the engine is lukewarm? I don't believe it's air anymore because it's reproducible: if you leave a warm car parked for about 4 hours or in direct sunlight, it won't start properly (it takes about 10 seconds before it responds to acceleration). However, if it sits for 8-20 hours, it starts normally (using biodiesel). The return lines have also been replaced recently, and there's no visible air in the transparent hose.
I believe the pump takes longer to reach the correct internal pressure at high temperatures or when running on diesel, simply because the fuel's viscosity is better in those conditions. If the engine is also idling below 700 RPM, it takes even longer.
Can someone please help me figure out how to increase the starting amount? None of the login details from this forum seem to work.

Regards,
Kai.

A colleague of mine with a similar Volvo V70 (same engine) experienced the same problems. After replacing the timing belt on the fuel injection pump, his car started up normally again.

Okay, some time has passed again, and I've been dealing with my starting problem once more, and in the meantime, I had the fuel injectors overhauled (adjusted and fitted with new nozzles). The problem remains .
Today, I finally tackled the temperature sensors because the car starts quite well in temperatures around 10°C (both outside and engine temperature). I identified the sensor located at the top of the coolant inlet as being relevant to the starting fuel quantity, and I did a test with a 3.3k fixed resistor (resulting in an 8.1°C engine temperature) while the actual engine temperature was 24°C. The result: it started perfectly . So, I just need to increase the starting fuel quantity, and it should work properly again for now. I'll probably replace the pump soon, when the belts are due for replacement.
'Could someone perhaps help me figure out how to access my control unit to 're-calibrate' the settings? I don't want to install a resistor switch if there's another way to do it.'
Here are the control unit data:
Control Module Part Number: 4A0 907 401 P
Component and/or Version: 2.5L R5/2VT EDC HS D13

Regards,
Kai.