Audi A6 TDI Starting Problems: Common Causes

Hello,

Unfortunately, I can't seem to overcome my starting problem without some help.
The car is an A6, manufactured in 2004, with engine code BAU (2.5 TDI 180hp), quattro all-wheel drive and automatic transmission, and approximately 114,000 km on the odometer.

Problem: If the car sits for more than 4 hours, it takes approximately 5-10 seconds to start. Occasionally, after starting, a medium-sized cloud of grayish-white smoke comes out of the exhaust.
However, if it's left standing for longer than 24 hours, it starts up again after about 2 seconds (not perfectly, but it's better).
Once it runs for 30 seconds, it then starts up cleanly within 1 second.

I've owned the car for 6 months now, and I bought it with this problem. About 3 months ago, the AGR (Abgasrückführung) valve was replaced, but it didn't fix my problem.

I have reviewed the following so far:
Air bubbles in the supply line: none observed.
Oil leak lines: Visual inspection revealed no findings, no abnormalities.
Pass-through values based on instructions from the forum: no abnormalities.
It also seems to correspond to the subjective impression that it runs quite normally from around 2000 rpm. There's no hesitation, just smooth and consistent acceleration, etc.
Visual inspection of the LMM: no findings.
Last week, I disconnected the engine temperature sensor, and after that, it started without any problems, despite the excessively long preheating time. However, even with a new temperature sensor, the problem persists.
TimingChecker: Values within the limits, but at the lower end.

Renewed so far:
Battery, air filter, EGR valve, engine temperature sensor.

Other phenomena that may be related:
Subjectively, there's not much torque below 1500 rpm. The automatic transmission downshifts immediately, even with slight changes in the accelerator pedal. I wouldn't necessarily expect that from a 2.5-liter diesel engine.
Momentary hesitation when starting. That is, I have to give it quite a bit of gas for the car to accelerate quickly. But then it really picks up speed.
"With its performance, any Golf with 75 horsepower will outrun me at the traffic light. This makes the Quattro system pretty useless."
Both issues could potentially be related to the transmission, but this is my first car with an automatic transmission, so I don't have anything to compare it to.

I'm looking for a tip or suggestion on what else I could check. I have VCDS available, so I would be grateful if we could narrow down the problem through measurements before starting to replace parts randomly.

Best regards and thank you in advance.
Ingo.

What does the injection start time indicate?
It seems like he's going to be late.
Was was the timing belt recently replaced?

Hello,
Injection start at idle: Between -1 and -2° BTDC (Before Top Dead Center), timing checker see appendix, just within tolerance.
Einspritzbeginn Vollast : Zwischen 9,5 und 12° n. Off-topic.

I have no idea if the borderline values could be causing my starting problem.

Question: To adjust that, would I need to check the tension of the timing belt on the ESP and, if necessary, adjust it on the camshaft pulley? Is it possible to do this without having to remove the front panel for service access?

The timing belt was supposedly replaced by the previous owner (or by a friend of a friend who works at VW) at approximately 90,000 km, but there is no proof of this.

If it's the cause of my problem, I don't understand what the injection timing has to do with the dwell time or the preheating time.

Best regards,
Ingo

By how much does the fuel injection start timing "retard" when the vehicle is running normally at idle?

Best regards, Rainer.

Can you maintain a constant speed of 2000 RPM in either "P" or "N" mode? Or does the speed tend to increase or decrease?

Hello,

@Rainer: Actually, not really. Sometimes it "jumps" between 3.6 and 3.8, but not often.

@RedR32: Yes, no problem, both cold and hot.

I created another log from the MWB4 while it was starting today. The car had been parked for about 18 hours, and it was a typical starting sequence that represents the problem I'm experiencing. I'm not sure if it will be helpful.

Best regards,
Ingo

Hello,

I have no idea if anyone is reading this, but my problem has gotten worse since yesterday. Now, it's noticeably jerky at a constant throttle and during acceleration, starting around 1800 rpm. It's similar to what you'd experience with misfires.
"I attended a driving safety training course yesterday, and I didn't experience these problems throughout the entire training. However, at the end of the training, I had to park for a while on a 20% incline (with the driver's side lower than the passenger's side), and the issue started after the initial strong acceleration."

Upon arriving home, I first read the error codes and performed a measurement run, and I saw that the measured value for the actual injection start time was not what it should be; I would call that noise. See the log file 2011_10_15_MWB4_1.
Error entry found:
00550 - Injection start control.
27-00 - implausible signal.

The pump status was at 0.

After clearing the error code, the stuttering/jerking/hesitation persisted.

Today I took some more measurements, once with the NHG (negative heat generator) disconnected and once with a full tank. However, it's still misfiring. Therefore, I'm ruling out the NHG as the cause. The misfire is less pronounced when the engine is cold, but it's still present. The error code hasn't reappeared.
I checked the tension of the ESP timing belt, and it is within the acceptable range.

After doing a little research online, I've come up with three theories, and I'm wondering if anyone might have something to say about them.
a) Fault in the ESP (Electronic Stability Program) system, possibly due to a faulty solenoid valve or similar component.
b) OT sensors.
c) Vacuum pump or booster pump, or something else related to the diesel fuel supply.

What I noticed during yesterday's measurement, almost incidentally, is that the vibration level with 3 cylinders running is above the threshold. However, if I subtract the NHG (noise, heat, and gas emissions) value, the readings are within the acceptable range.
Unfortunately, I don't know enough about how this works, so I'm not sure if there's a connection.

I hope someone can give me a tip on how to narrow down the problem further.

Best regards,
Ingo

SuperIngo wrote:

Error entry found: 00550 - Injection start control. 27-00 - implausible signal.

Air in the fuel injection pump.

That was also my first thought when I only read "starts after a long time playing the organ."
Try changing the fuel filter first, bleed the pump again, and if it's still the original one, there's an 80% chance you'll need to buy a new one.

EDIT: Have you actually checked the timing belt tension?

Hello,
The shaking/jerking stopped on Monday morning. And it disappeared without me doing anything.
Starting problems persist (unless it's been pre-heated).

Best regards,
Ingo

Well, then just keep on shining and don't do what you were told.

I'm just saying: O-ring on the fuel injection pump?