Audi CAGA: Engine Failure & Complete Breakdown

"I have an A4 that starts to jerk and stall around 3000 RPM. If I keep my foot on the gas, the engine speed drops to about 1300 RPM, then it hesitates briefly before revving up again, and the problem repeats. If it stalls and I just slightly ease off the gas, it recovers and I can continue driving until I reach 3000 RPM again. However, if I'm still pressing hard on the gas after it stalls and then press the clutch, the engine dies."

I drove the car and recorded the injector voltage, but that doesn't show that the voltage is disappearing; I also need to use a clamp meter.

The attached log shows that the lambda value increases to 32.77 after a drop, even though the average fuel injection amount and dwell time do not decrease significantly.

Fuel pressure doesn't just drop; it's often too high before it drops. The logs are difficult to analyze because the connection is usually disrupted at the same time that the problem occurs.

I thought it might be the engine control unit, but I've already tried using a different one, and it still does the same thing.
I also tried bypassing the main relay, but that didn't help either.

Hello,

Why not leave the .csv files untouched? This makes the work much harder than it is with KDataScope...

Based on the high lambda reading during acceleration from low RPM (not your peak value), I suspect a leak in the charge air system (air leak). With the mid-range fuel injection setting, are you sure that's actually what's being injected, or is it being limited by airflow/lambda?

Best regards, Rainer.

Upload the CSV file tomorrow.

"Smoke reduction is controlled via boost pressure; I believe lambda is just a correction factor."


Why does the engine stall when I release the clutch and immediately floor the accelerator after it has already stalled? But I'll try cleaning the carburetor.

Normally, they can detect air leaks as a fault even before you see or hear anything.

Here is the CSV file without any modifications.

Hi,

Quote:

Smoke reduction is achieved by controlling the boost pressure

.
then, with a static pressure of 1300 mbar, it is certainly not possible to inject 42 mg/H. Unless the smoke limitation is controlled via lambda/air mass, or the data set from the engine control unit has been corrupted, or the 42 mg/H is not the actual amount injected.

Do you see the logical contradictions?

Best regards, Rainer.

"With the measurement blocks used in the past, it was described as 'fuel injection quantity limitation based on detected air mass'."
This value should definitely be included in the measurements. The average injection quantity is certainly not a direct measurement, but rather calculated by the engine control unit (ECU) based on parameters such as injection duration, rail pressure, and other factors (just like the idle correction values).
I've noticed that the boost pressure seems to be lagging, and after the "drop," a more pronounced limiting effect appears to be coming into play. It would make sense if, at high lambda values, the normally map-controlled EGR system is opened even further.
For the engine to stall, either the fuel injection must stop, or the air supply must be cut off (by closing the throttle), or both.
Are all the cables and connectors in good condition? Does the motor have a semi-professional history?
hg
Herbert.

dieselschrauber wrote:

Hi, Quote: Smoke reduction is achieved by controlling the boost pressure . then, with a static pressure of 1300 mbar, it is certainly not possible to inject 42 mg/H. Unless the smoke limitation is controlled via lambda/air mass, or the data set from the engine control unit has been corrupted, or the 42 mg/H is not the actual amount injected. Do you see the logical contradictions? Best regards, Rainer

Here's a smoke map from CAGA, and it matches the measured values quite well.

Herbert wrote:

With the measurement blocks used in the past, it was stated that "fuel injection quantity limitation based on detected air mass." This value should definitely be included in the measurements. The average injection quantity is certainly not a direct measurement, but rather calculated by the engine control unit (ECU) based on parameters such as injection duration, rail pressure, and other factors (just like the idle correction values). I've noticed that the boost pressure seems to be lagging, and after the "drop," a more pronounced limiting effect appears to be coming into play. It would make sense if, with a high lambda value, the normally map-controlled EGR system is opened even further. For the engine to stall, either the fuel injection must stop, or the air supply must be cut off (by closing the throttle), or both. Are all the cables and connectors in good condition? Does the motor have a semi-professional history? hg Herbert

.

Yes, the owner says the car has a history, but I'm not sure if everything that was supposedly done to it was actually completed.

"According to the owner, it has a turbocharger, DPF (Diesel Particulate Filter), all injectors, a high-pressure pump, a mass airflow sensor, and an exhaust temperature sensor."

Here's another log file (in CSV format) where I gradually increase the speed, but it still cuts out at 3300 rpm.

Hi there,
The behavior looks almost like a limp mode. It reminds me of when my old Audi 80 switched to limp mode due to turbocharger problems ("limit exceeded"). The power would cut out, the speed would decrease, until I briefly pressed the clutch and released the accelerator, then it would start working again.
Perhaps the removal of the lambda value is a secondary effect: when the emergency mode or protective function is activated, the exhaust gas recirculation system is shut down, which results in an excess of air.
The problem here doesn't necessarily have to be related to the boost pressure. You also don't see any errors in the MSG memory (?) .
"Exceeding a temperature threshold could also trigger it. Does the vehicle have an automatic transmission (torque converter temperature)? Is there an excessively high value coming from one of the temperature sensors in the exhaust system?"
hg
Herbert.

Hello,

Quote:

Here is a smoke map from CAGA, and it matches the measured values quite well.

"You're right. Is that crankshaft from a standard engine? I wouldn't have expected that."
Once the engine starts running along the smoke curve, the turbocharger intercooler is likely to be clogged.

However, the question remains: why can't he achieve the desired boost pressure? Is the VTG (Variable Turbine Geometry) being properly adjusted when needed (e.g., during an actuator test?), and what is the differential pressure across the DPF (Diesel Particulate Filter)?
If nothing can exit from the back side, there will be no significant pressure difference across the turbine side of the turbocharger, and therefore no drive force.

Best regards, Rainer.

Hello everyone,

I recently experienced a very similar phenomenon in a friend's Audi A4 B8.
Engine is also a CAGA. No response to the accelerator pedal, jerking at partial throttle, sometimes rough running when starting. Power loss occasionally. Noticeable and strange exhaust smell.
Error memory is empty!
The LL-Ruheregulation was noticeable, but I couldn't pinpoint a faulty injector because the values varied drastically from one cylinder to another. Initially, I was at a loss and had already prepared the owner for the possibility of replacing all four injectors.

By chance, while I was working on the service, I noticed a damp EGR unit. I removed it and opened it up, and the PA6 bracket that lifts the valve was completely worn out in one spot. This caused the valve disc to get stuck intermittently in various places. Consequently, the actuator motor couldn't close properly.
No error was recorded because the valve does not restrict the flow; instead, it is the curved track that causes the limitation.

The valve was from Valeo, and I replaced it with a new Valeo valve.
After installing the completely different engine, the engine runs smoothly, the throttle response is direct, and the exhaust smell from the tailpipes is normal again. Since then, I've driven 2500 km without any issues, the long-term fuel trim values are almost the same, and there's no more hesitation.

It might be worth removing the EGR valve, loosening the small Torx screws and taking a look inside. Nothing can go wrong; you can disassemble it and reassemble it. If the actuator lever no longer has a slot but instead an oval hole or notches, it will likely get stuck.

Regards,
Mpire

Hello.

First of all, thank you for your help; I've finally found the errors.

It was the pressure relief valve in the oil leak line.

Hi there,
How did you come up with that?
hg
Herbert.

I measured the return flow rate, which was fine. Then I wanted to measure the pressure in the return line, but the plastic connector on the valve broke off. I then installed a hose without the valve and cranked up the speed while it was running, and then it stalled, which wasn't happening before. Yesterday, I had hoped that the pressure relief valve might be the cause, and that has now been confirmed today.

Thank you for the feedback!