TDI Performance Issues: Troubleshooting Guide

'I've been having a problem with my Golf IV TDI for some time now - I've even posted about it here before. I'm experiencing a lack of power. Where I used to easily reach 200 km/h on the speedometer, I now only reach 185 km/h. It also feels sluggish, and I'm using about 0.5 liters more fuel because I have to press the accelerator harder.'

'Error message: 'Intake manifold valve, short circuit to ground' - intermittent error. The workshops have not found anything despite intensive searching, and they claim that everything is fine and that it has nothing to do with the lack of power.'
The valve, the wiring harness, and the throttle body were checked. It seems the problem is coming from the control unit... and I'm supposed to leave it alone because it's expensive.

Due to insufficient performance of new LLMs, but no improvement observed with the new LLMs.

The car has been chipped (90 - 115 horsepower).

The VTG (variable geometry turbocharger) was also inspected and found to be in good condition. Additionally, a new diesel filter and a new air filter have been installed.
Disconnecting the exhaust gas recirculation system doesn't achieve anything.

So, what else could it be?

Oh, at full load, it doesn't smoke as much as it used to (which is hard to completely avoid with tuning)... a sign that not as much diesel is getting in, right?

I suspect that the chips might be faulty.
The car still drives better than it did without the chip... but not like it usually does.

Hello,

Without the appropriate diagnostic tool, you will now find it difficult to determine the cause.

Mögl. Source of error a)
If "Saugrohrklappe" refers to the throttle valve (I'm not 100% sure), you should check if it is fully open and that the vacuum diaphragm is properly pushing it open.
To ensure that you can rule out a faulty control of the shut-off valve, you can disconnect the vacuum hose from the sensor and seal the hose leading to the solenoid valve with a screw or similar.
A visual inspection of the valve (you may need to disconnect the intake hose in front of the valve/AGR unit) can also be helpful; it must be fully open.

Mögl. Source of error b)
The EDC (Electronic Diesel Control) is reducing the fuel injection amount due to a fault or a malfunctioning sensor.
Without the ability to diagnose using a laptop or VAG device, there's not much that can be done here.
My suggestion: check the new mass airflow sensor (LMM) and the turbocharger boost pressure. The target values and measurement conditions are listed in the error database and are associated with the engine code.

Mögl. Source of error c)
The fuel injection pump is not receiving enough diesel fuel. The EDC (Electronic Diesel Control) system does not have a way to detect this condition.
The most common causes are usually leaky fuel hoses or defective sealing rings on the return valve that is inserted into the diesel filter.
Almost every time, air bubbles or foam appear in the transparent fuel line between the injection pump and the diesel filter (the engine must be running at the time).

Mögl. Source of error d)
Your tuning chip has likely been "reprogrammed" due to a defect, or some bits have been lost. I find this unlikely, and I don't have any further thoughts on the matter at the moment.

Best regards, Rainer.

Hi!

If your tuning chip used to work, then it's definitely not the problem. The system checks checksums, and if the chip is faulty, the engine usually won't even start. Of course, there's always the famous exception...

Best regards,
Uwe

HI,
Thank you for the quick responses; this is a great forum!

Regarding 'diagnostic devices': Unfortunately, in my experience, workshops are either completely overwhelmed or simply too lazy to properly address the problems that customers report. I've already spent a good 150 euros just trying to fix this performance issue, and it hasn't solved anything. I can't imagine that things will change if I go back to the authorized workshop again. I only encounter complete incompetence and often outright rudeness, with suggestions like 'We recommend installing a new turbocharger...' or similar nonsense.

I'll double-check for any fuel line issues myself... and if necessary, I'll just leave it as it is. The main thing is that this lack of power doesn't indicate a mechanical defect in the engine or turbo. That would be very annoying, as I plan to sell the car soon.

Hello tk3003,

Well, I had a similar experience with the VAG workshops in the Nuremberg area; the ones I visited were actually quite disappointing.
That's why I also bought a diagnostic connector and software. Ulf's circuit should be relatively easy to recreate, and you can do a lot with the free demo version of VAG-Com.
The problem is that you can't perform load tests without a laptop, so it's absolutely necessary, and unfortunately, it's expensive.
If you look at the 150 euros, you could have almost gotten an old computer where Windows 95 is running. In this price range, you can hardly use a laptop for anything else because it's simply too slow for current software (or my patience is too short ).

Perhaps someone in the forum has a laptop or diagnostic connector from your area?

Best regards, Rainer.