3.0 TDI ASB: Won't Start - Troubleshooting Guide

Hello everyone,

I've been dealing with a problem with my Audi A4 B7 3.0 TDI for the past two weeks. The engine code is ASB, the transmission is JAX, and the mileage is approximately 233,000 km.
I was having problems with the broken linkage of the throttle valve 2. I then removed it and temporarily installed a fixed bracket for the actuator motor – this resolved the problem for the time being.
After reassembling the intake manifold, etc., the engine will no longer start.
What has been checked so far:

- All connectors/harnesses appear to be in good condition.

- The diesel filter is approximately 6 months old (maximum 5,000 km).

- Measurement of fuel pressure at the inlet and outlet of the diesel filter (inlet: 0.35 bar / outlet before the fuel temperature sensor: 0.3 bar). Controlled via VCDS.

- Bleed the fuel system to remove air, ensuring that diesel is present at the HDP (High-Pressure Diesel Pump) and that no air is visible at the inlet.

- Pressure measurement at the HDP output using the HPT CR HDP test device 3 (a pressure gauge that measures up to 1000 bar) --> The pressure gauge immediately shut off the valve (reached over 1000 bar) after 2 seconds of the start-up attempt.

- Timing belt of the HDP: Visual inspection appears to be okay.

- Measurement of return flow volume for all 6 injectors: Not possible because no fuel is reaching the injectors (6 start attempts, each lasting 10 seconds, were tried).

- Bleeding procedure performed according to Elsawin: Pressure regulating valve disconnected, then start the engine 4 times for 10 seconds each.

Basically, I'm not getting any rail pressure. The reading remains constantly at 0.0 bar!
I'm expecting to receive two "new" (used) pressure vessels with pressure regulating valves and pressure sensors in the next few days. However, I find it hard to believe that the pressure sensor or the pressure regulating valve would simply stop working completely overnight, without any prior warning signs.

In the last attempt, I wanted to check the injectors (return flow measurement). Since my engine won't start, I tried to start it by cranking it. However, I didn't get any results here either: No diesel reaches the hoses (which might also indicate a rail pressure of 0.0 bar ).

I'm now wondering where the pressure is going. All the pressure lines are secure. If I open a fitting during the startup process, a small amount of diesel leaks out.

I have attached the following files.
- Images of diesel pressure measurement.
- Video demonstrating HDP pressure measurement.
- Video of a launch attempt.
- Log file of a startup attempt.
- Autoscan
The diagnostic scan still includes an error message regarding the intake manifold flaps. However, the car is now error-free in all control units.

Okay, I'm ready. Please provide the German text you want me to translate.

Hello everyone,

Unfortunately, the problem still persists.
I have now obtained a support key to potentially get some suggestions or assistance.

Over the past few days, I've taken a closer look at the MSG (Mass Airflow Sensor) and the wiring to the fuel quantity regulator, fuel pressure regulator, and fuel pressure sensor.
The MSG is dry - no water ingress.
All lines show no signs of damage.
I can access and control the fuel pressure regulator valve and the fuel pressure sensor using VCDS.
Unfortunately, I'm still not getting any rail pressure, showing a constant 0.00 bar on the sensor. Either the sensor is really defective (I hope the new one arrives in the next few days), or the pressure is escaping somewhere else.

What I've also checked:
- All temperature sensors (oil, water, diesel) are reading 20°C - so they are within the normal range.
- WFS indicates the key as authorized in VCDS.
- According to Elsa, the pre-lift pump should provide approximately 1 bar of pressure. However, I am only getting 0.35 bar at the inlet of the diesel filter. Could the problem be located here?


Greetings,
Jonas.

Hi, I suspect the problem is that you're not able to bleed the air out of the system, since the issue only started after the repair. Disconnect the fuel hose at the high-pressure pump inlet. Connect the fuel pump to a tester and, as soon as fuel starts to come out of the previously disconnected hose, reconnect it. Then, slightly loosen a high-pressure line between the fuel rail and an injector. Have a second person start the engine, and as soon as fuel starts to leak, tighten it again. Of course, don't open it too much, maybe 90 degrees or a little more.
Otherwise, I would really wait for the pressure sensor first, since you have already ordered it.

Best regards,
Dago

Hi,
Are you measuring the pressure at the HDP (High-Pressure Diesel Pump) at 1000 bar, without being connected to the fuel rail, or with it connected?
hg
Herbert.

Hello Herbert,

Unfortunately, I can only take measurements at the pump (outlet). Sadly, there is no suitable adapter for the thread on the rail included.

@Dago:
I've already gone through the procedure. I bled the system from back to front: starting with the diesel filter and going all the way to the pump inlet.
I haven't bled the fuel rail yet, I'll try that tomorrow. Also, I haven't been able to observe the diesel line at the entrance to the pump (during the starting process) yet (because I've always been alone so far...). Perhaps air bubbles are actually still getting in there...

Greetings,
Jonas.

Hi,
It's quite possible that the pump, without a buffer, damaged the pressure gauge. If that's the case, you'll need to measure the pressure using a pressure regulator and a pressure gauge connected to the rail.
Just forget about the venting attempts for now. Your electronics are reading zero pressure and not activating the valves, which is why it won't start.
hg
Herbert.

Hello everyone,

The engine is running again. It was indeed the pressure sensor. After installing it, the engine started right up...
What surprises me:
I had previously conducted numerous startup attempts with the sensor disconnected. When the sensor is not connected, a fixed pressure of 320 bar is applied (presumably for startup authorization). However, it still wouldn't start.

Greetings and thank you very much for your assistance.
Jonas.

Hi,
Two logics are likely at play.
- If the measured pressure is below 120 bar, the start-up will not be authorized.
- In the event of a pressure sensor failure, a replacement value of 320 bar is used for an emergency function, allowing the system to continue operating (to prevent the motor from abruptly shutting down and surprising the driver).
The system descriptions are not clear in that regard.
I would still investigate the low pre-charge pressure.
hg
Herbert.

Jonas1991 wrote:

Hello everyone, The engine is running again. It was indeed the pressure sensor. After installing it, the engine started right up... What surprises me: I had previously conducted numerous startup attempts with the sensor disconnected. When the sensor is not connected, a fixed pressure of 320 bar is applied (presumably for startup authorization). However, it still wouldn't start. Greetings and thank you very much for your assistance. Jonas

Thank you for the feedback! X

The issue with the pre-filter pressure is also bothering me. However, I can only think of the pump itself as the potential cause. The pressure before the filter is already only 0.4 bar. There should only be the hose connecting the measurement point to the pump, right?

Greetings.

Hi,
That information should be available in the ELSA guide, even specifically for your vehicle model (you have access to it, if I understand the text above correctly).
hg
Herbert.