Audi 3.0 TDI ASB starts very poorly, no more ideas!

Hello everyone,

I would be very grateful for your expert assistance.
I have been trying to find the reason why my Audi A8 4e 3.0 TDI ASB starts so poorly for a long time. Cold and, if he was driving, even worse.
The fault memory only contains G28 as an implausible signal.
He will only return when he has worked very hard, without taking breaks. I suspect it's a result of an error, especially since a new sensor has been installed and the actual values are always plausible.

HDP, injectors, DRV, rail pressure sensor is either new, overhauled, or has been tested with cross-connection.

When monitoring the rail pressure value during startup, one observes that it drops to approximately 50 bar during unsuccessful attempts.

Does anyone have any ideas, or a good tip, about what else I can do?

I can't understand how such a clear and symptomatic problem is so difficult to diagnose and resolve.

Therefore, I am extremely {GRATEFUL} for any suggestions and assistance!

Thank you in advance!

LG

With my old Touareg with the 3.0 TDI, the injectors were leaking, which was the cause. I then sold the car far away...!

The injectors have been completely overhauled and have been re-inspected by a Bosch service. Unfortunately, I have to decline.

I still need to add that the Audi runs perfectly once it has started. He really doesn't want to, or simply does very poorly...

Hello,

The fundamental principles regarding leaks and fuel supply are the same:

Starting problems and pre-heating system at TDI

...should be reviewed.

With a CR-TDI like yours, I would use VCDS to monitor the fuel pressure regulation in the rail during start-up.
Below a minimum pressure, the engine control unit does not authorize starting.

Best regards, Rainer

Thank you for the information, the rail pressure actually drops to around 60 bar! However, the DRV is not.
The DRV and Rail pressure sensor have been checked and replaced multiple times!

Hasn't anyone else had a "brilliant" idea yet!?

Hi

Air intakes in various locations, diesel filter, fuel hoses, return hoses... including their connections.

I would check that first.

Does the operating speed already reach the target? Minimum pressure in the rail is necessary, prior to no start approval.

Best regards, Rainer

Does it start with a StartPilot? Observe the {KW/NW} sensor during startup using an oscilloscope.
Is the pressure relief valve in the crude oil line OK?
Start approval is only available from 180 to 200 bar, as I recall.

Starting a diesel engine can be problematic. The ignition occurs spontaneously in the self-heating system until the piston moves upwards, causing a backflow.
The sudden drop in rail pressure is a clear indication of where the problem lies.
hg
Herbert

Hi,
If no rail pressure builds up during starting, either insufficient fuel is being fed, or the fuel is unintentionally flowing back or into the combustion chambers.
In both directions, you have already made unsuccessful exchange transactions. Before doing something like that, it's definitely better to first investigate the root cause with careful troubleshooting.

Just a note in between: For this, you'll naturally need a diagnostic tool. However, your profile doesn't mention anything about a VCDS license. Please, follow up on this.

First, I would start on the approach side to prevent the high-pressure pump from running dry.
Is the fuel tank vent clear? Are the filters and lines in the tank clean? Is the diesel filter unobstructed? Do the pre-feed pump and fuel pump supply enough fuel and enough pre-pressure (measured)? It has been properly drained (relevant to initial filling)?
In addition, a test assembly with a fuel metering valve and a fuel pressure control valve is required.
After that, a recording of Rail Druck and speed would be interesting, starting from ignition one up to and including the test drive.
hg
Herbert

Thank you for your helpful tips and suggestions!

I am also a fan of a well-founded and meaningful diagnostic strategy, and I have also tried to follow it to a large extent.

-The air-free supply hose to the HDP was and is, the diesel filter is new

-The starting speed is sufficient, and of course, it will decrease relatively quickly if it runs for a long time without starting (voltage drop).

-The amount of pre-promotion was measured (the pressure in the ASB is likely irrelevant, according to the manufacturer's information, only a volume test per time unit is required)
--> Objection: A few months ago, it completely stopped starting once. Suddenly, there was no diesel flow from the pre-feed line to the diesel filter! A pre-feed pump was vibrating very strongly. I have then renewed them!
Subsequently, there is again a pre-charge, but the starting behavior remains poor.

-Leaking oil line, pressure-holding valve with VW manometer checked, holding pressure is excellent!

-The sieves in the tank were clean, the tank ventilation was unobstructed.

-Not properly vented (initial filling)

-Fuel pump was repeatedly activated by VCDS component test for purging.



Next steps, as far as I can tell:

NW/KW to be checked with Oszi?
Engine start from ignition until engine running and operation (While logging RPM and rail pressure?)


Have I missed something, or is there another tip on how to proceed/start in the smartest way?


I THANK YOU!!!!

Is it really that simple that if the rail pressure is too low to start, there are only the two options Herbert mentioned?
Insufficient funding or loss due to retention or injectors?

Could it be that the rail pressure is being taken back by the engine management system as a result of another problem?

@Herbert: The unsuccessful swap attempts were actually the result of a careful investigation... In some cases, the test results were implausible or did not match the manufacturer's specifications. Then, I was forced to use a workaround with a test by temporarily disconnecting parts... One wants to make progress.

Or what do you think? What would have been your approach?

Herbert wrote:

After that, a recording of the rail pressure and speed would be interesting, starting from ignition point one up to and including the test drive.

Jay7 wrote:

-- Ein objection here: A few months ago, it completely stopped starting once. Suddenly, there was no diesel flow from the pre-feed line to the diesel filter! A pre-feed pump was vibrating very strongly. I have then renewed them! Subsequently, there is again a pre-charge, but the starting behavior remains poor.

Question: Has your problem been going on exactly since then?

If I understand correctly, your primary pump has failed, and as a result, the high-pressure pump has also run dry. These pumps are unfortunately quite sensitive. On the other hand, I am surprised that the start itself is the only problem reported, and that there are no other performance issues.

@Dieselschrauber: I can definitely record the rail pressure and RPM up to the weekend!

@guste100: No, the problem has existed for quite some time and not just since the failure of the pre-pump.
Exactly this symptomology is so strange. It's disastrous when he starts, but when he runs, everything is perfect! Even the relevant real-time values during the drive are all normal!

He just won't start...