Engine Failure: Causes and Solutions

Hello TDI community!
I experienced a breakdown with my Audi A3 TDI (engine code: AGR, 66 kW, no modifications, manufactured in December 1996, approximately 125,000 km) last autumn, and the cause of the problem is still unknown.
Up until this point, my TDI has never let me down, but I suppose there's always a first time for everything. I absolutely don't want to experience anything like this again, so I'd like to get some opinions on the matter.
At that time, I had just driven about 8-10 kilometers on a federal highway within a traffic jam (so the engine was still cold), which was moving at a speed of 65 to 70 km/h due to a slow-moving truck.
At this speed, the TDI engine doesn't seem to be running smoothly in 5th gear and tends to slightly jerk at very low loads. However, this jerking seems to originate from the drivetrain (torsion spring) and the high torque unevenness at this RPM (approximately 1400-1500 rpm).
On that day, the engine misfire was noticeably stronger than usual. I thought, "Okay, just shift into fourth gear." However, as soon as I pressed the clutch, the tachometer was already at its lowest reading: The engine was stalled, but the vehicle was still rolling forward. I pulled over to the right and turned on the hazard lights. I was quite surprised and immediately tried to restart the engine. But the sound it made didn't sound good at all. It sounded like the ignition was firing during the compression stroke, as if the starter motor was struggling against the engine.
Many thoughts raced through my mind: Did the timing belt jump a tooth? Is the pump setting misadjusted? Is the control unit defective?
I should also mention that the vehicle was at the Audi service center approximately 2000 km before this incident for its 120,000 km inspection, which included a timing belt replacement.
After waiting for half an hour and being completely chilled inside the car, I decided to try starting it again: and this time, it worked. As if nothing had happened, the engine started up normally.
The error memory was checked at the Audi workshop, but no errors were found.
Meanwhile, I've driven over 5000 km since that incident without any problems whatsoever. However, the uncertainty is still bothering me.

Does anyone here recognize this problem? Does anyone have a plausible explanation for the malfunction?

Thank you in advance.

Regards,
Alex.

Hello,

When was the error memory read after the breakdown? After more than 50 engine starts, it's empty again...
Otherwise: Replace relay 109 preventatively for the power supply to the engine management system. Are the battery cables securely attached?

Best regards, Rainer.

Hi Alex,

I completely agree with Rainer. I've been experiencing something similar lately. My car, which is located right in the middle of the city (of course, at the most inconvenient intersection), has been randomly shutting off. Sometimes it happens three times in a row within a distance of 100 meters.

Since I replaced relay 109, the problem has never occurred again.
-> It cost approximately 10 euros at the dealership.

Best regards.
David.

Thank you for your tips regarding Relay 109.
Can anyone tell me where I can find relay 109 and where it's located in an Audi A3?
Is it located in the "standard" fuse box on the left side of the dashboard, or is there another designated space for its installation?

Regards,
Alex.

Oh, right, I almost forgot.
"Rainer, of course, I had the error memory read out the very next day, so about 2-5 engine starts after the incident."

Perhaps someone has another explanation or suspicion besides relay 109. I can only reiterate that the starting noise was extremely loud immediately after the error occurred.

Regards,
Alex.

Hello Alex,

You need to remove the lower section of the left cockpit fairing.
You'll need a Torx socket wrench for that.
You will see the relay yourself. The relay is not expensive, you can resolder cold solder joints, but a new, correct part is safer.
You need to make sure that the dealer doesn't sell you the same faulty design; there is an improved version available.
Relay! Search for "109" in the TDI error code database.

Best regards, Rainer.

My old one was black and very square (manufactured in 08/96), while the new one is gray, smaller, and has rounded corners.

But it will still get just as warm as it used to.

Hi!

Rainer K. wrote:

You need to remove the lower section of the left cockpit fairing.

"On my A3 (year 1999), the SIDE dashboard trim needs to be removed. You need to pry off the cover and loosen a few screws; I think it's easier than it looks."

Best regards,
Uwe