TDI T4 AJT: Rough Idle, Erratic Injection Timing.

Good day.

I need urgent help!
TDI T4, 1999, 280,000 km, not modified, fuel injectors replaced and working properly, mass airflow sensor (MAF) is okay, fuel consumption is normal, hoses replaced, engine runs smoothly and starts well, performance is normal, EGR system deactivated, no error messages according to VAG diagnostics! I've been driving like this for months now. Enough is enough.

'Engine idling roughly during normal operation, even worse when warm. RPM fluctuating between 755-850 (is this normal?)'
According to the Vagcom TDI timer, the injection timing is fluctuating several times per second, oscillating between values of 34 and 79 !! And that's what it sounds like.

Is the VP37 turnout mechanism dirty? I know the instructions for disassembly and cleaning, which can be found in this forum.
Could it be due to a problem with the mechanical part of the pump?
What else could it be?

Please provide an expert answer. [email protected]

Thank you very much, Michael Karrer.

Hi Michael,

Take a look at the fuel injection amount at idle; it should be above 2.5mg per injection.
If the opening pressures are too low or too different (also check the values of the idle speed control), that would explain the effect you're seeing.

Best regards, Rainer.

PS: For nozzle kits sold in the shop, the opening pressure is included in the price and will be checked and adjusted.

1. Thank you.
The amount varies between 0.0 and 2.5mg, but it tends to be too low (less than 2.5mg/H). It fluctuates, just like the speed. Nozzle pressures are okay and consistent.

Questions:

How do I adjust the injection amount? Manually by moving the cover, or via the engine control unit? A short answer is sufficient.

Can you tell me how I can obtain the numerical code to potentially modify the engine control unit? Is it VAG-specific, or is there a better source? (I'm referring to the 5-digit code).

I'm having trouble understanding the idle speed control. Sorry for all the questions.

Thank you very much.

Hi,

You can adjust the idle speed by moving the pump adjustment knob.
Be careful, without a diagnostic tool, you won't be able to achieve a proper setting. Take a look at the technical articles about the VP37.

Best regards, Rainer.

The cause of the error was rust particles in the metering valve. After disassembly, cleaning, and adjustment, the engine is running smoothly again.

You didn't search for that exactly 11 years ago, did you??? It's nice that you were able to resolve it }!!!

matthiasTDI96 wrote:

You didn't search for that just 11 years after the date, did you??? It's nice that you were able to resolve it anyway!!!

Good things take time.

No, it didn't take 11 years. Thanks to the technical article on disassembling the interlocking system and VCDS for measurements, it was easily done in just four hours. I unfortunately forgot to share the results in this forum.

The fine iron particles settled into the air gap of the flow meter, deflecting the magnetic field and the small voltage. The result was a sawing, erratic idle. Water in the diesel fuel caused the particles to rust further.
The T4 was sold to Ukraine with 370,000 km on the odometer and was intended to be used as a taxi.

Recommendation: Regularly replace the fuel filter.