Air bubbles in diesel line: Audi 2.5 TDI hard start

Hello.

Six weeks ago, the timing belt broke because a piece of metal detached from the valve cover.

Since the repair, the car has been difficult to start if it's been sitting for a few hours. Before, it ran perfectly, but now it doesn't respond to the accelerator and shakes violently when it's cold. After a few seconds, it starts slowly and smokes like a tank until it runs properly.

The workshop foreman says that the fuel injection pump is likely broken because the engine control unit sometimes cannot find suitable values for the fuel injection map.
Today, he replaced the (transparent) diesel line between the injection pump and the fuel filter, but without success.

Sometimes, small air bubbles can be seen in the diesel line leading to the fuel injection pump.
What could be the cause of air bubbles in the diesel line?
Is it possible that the problem has a different cause than the fuel injection pump, or has anyone else experienced something similar before?

It's a 1993 Audi 100 TDI 2.5 with an AAT engine, featuring chip tuning from *** (147 hp), and it has an engine/fuel injection pump replacement with 60,000 km on it.

Thank you for the information.

Bernhard.

Hi,
Small air bubbles don't have much to say.
"It's likely that after the timing belt replacement, the pump setting is no longer correct. There's a valid and common reason for this: shoddy workmanship by the mechanic during the timing belt change. It's quite coincidental that the fuel injection pump should fail now."

It would still be a good idea to check the O-rings on the diesel filter, just to be safe. Try searching for 'O-ring' when you search. Another user had a similar problem.

Okay, I'm also leaning towards a shoddy repair job from the workshop. If it doesn't start or run as well as before after the timing belt replacement, that's a bit suspicious.

Thank you for the answers. They were very helpful to me.

It also seems strange to me that the car is having trouble starting right now. That's why I'm taking it to the Bosch service center next week.
I told them to check the valve timing and everything else.
If the fuel injection pump is supposedly broken due to air bubbles, I now know of a different cause and will first replace the O-rings.

Best regards, and thank you again.

Bernhard.

Deleted upon request.

Hello diesel.gustav,

When he drives it, the car runs like a dream. It has better performance and uses about half a liter less fuel than before the cylinder head repair/timing belt replacement.
The workshop said it was the fuel injection pump. I have an appointment with the Bosch service tomorrow. It's good to know that it's unlikely the ESP (Electronic Stability Program) is broken. That gives me something to argue against if they try to sell me a new one.
By the way, I've been diligently reading through old forum posts, and I had a suspicion that the issue might be similar to haehlein's cold start problem with the 1Z (incorrect fuel injection timing).

Thank you for the information, and have a nice day!

Bernhard.

Hi,

@diesel.gustav:

Find the leak that is causing air to be drawn into the system, fix it, and the problem will likely be resolved.

If the pump setting were incorrect, as is suspected here, you would almost certainly also experience torque and/or power problems due to the deviation in the injection start timing.
One would think so.
However, that was NOT the case with my AAT.
The performance was absolutely correct, but the injection start was completely misaligned.
It barely started and coughed out a huge cloud of smoke.
After that, however, it worked perfectly fine.
I also had (and still have) air in the diesel line. Apparently, this doesn't cause any harm!

@bernhard:
I suspect that a faulty injection start is the cause.

The problem has been solved.

The Bosch service said that the timing belt for the injection pump was misaligned by one tooth. That's why the fuel injection timing was incorrect. They have now adjusted it correctly (25 mm stroke).
Now it starts up perfectly again (even without preheating).

Thank you for your answers. They have significantly reduced the time it took to troubleshoot the issue.

Regards,

Bernhard.

Hello Bernhard,

Hopefully, the Bosch service also checked the dynamic booster starting system (which can only be done with a diagnostic system).

Hello Michael II,

Yes, they initially set the ignition timing statically using a timing light, and then adjusted it dynamically. Additionally, they performed a test on the dynamometer, resulting in a power output of 143 horsepower (it should have been 147).

I was referred by the Bosch service center in my hometown to their central facility. They said that TDIs are difficult to adjust, and some centers can't even do it, so they suggested I go to their central location (Nuremberg). They perform TDI adjustments on a dynamometer there.

I am satisfied.
The mechanic from the workshop called me and said my car was ready. I asked him what was broken, and he gave me a knowledgeable answer.
Price: €230, but cheaper than a new fuel injection pump.

Best regards,

Bernhard.