TDI Performance Issues: Troubleshooting Guide

Here's the problem I'm experiencing with my 90 horsepower Golf TDI:

"When accelerating, the increase in RPM is slower, even in the first two gears, compared to another Golf TDI with a similar engine. Reaching the top speed is not possible on a flat surface; the speedometer indicates a maximum of 165 km/h."

"Occasionally, when accelerating fully, the engine may exhibit a brief hesitation or stumble at approximately 3500 RPM. It feels as if there is a lack of fuel."

3. The vehicle does not produce any soot, even when accelerating fully.

Hello Diro,

We need a little more information about your car (engine code, year of manufacture, etc.).

VW Golf TDI Estate, 66 kW, manufactured in November 1996, engine code likely Z1.

Diro wrote:

VW Golf TDI Estate, 66 kW, manufactured in 11/96, engine code likely Z1

.

I think "1Z" captures the essence better (typo in the original).

Okay, now let's talk about your problem:

You probably haven't had the error codes read out yet (or had them read out). Please post the error entries here.

The first [suggestion] indicates a faulty mass airflow sensor (MAF sensor) --> check it, see the forum for various posts (use the search function).

The second [possibility] suggests a faulty needle lift solenoid --> for testing, please refer to the forum for various posts (use the search function).

The third reading tells us that the engine is still running with an excess of air, which is perfectly fine.

Error codes read, but unfortunately, no entries found.
Air mass meter checked, OK.
The solenoid valve 028 906 283 F was already replaced as a precaution.
The exhaust gas recirculation system was checked and, for testing purposes, it was temporarily disabled.

Unfortunately, all of this hadn't resulted in any significant improvements up to that point.

Hello Diro,

Have you considered the simplest possibility, which is that your fuel injection pump might not be able to draw in enough fuel?
Possible reasons could include a clogged diesel filter or leaks in the fuel lines, causing air to be drawn in instead (this can be seen in the transparent hose between the diesel filter and the injection pump; there should be no large air bubbles visible in this hose while the engine is running).

Best regards, Rainer.

Rainer K. wrote:

Hello Diro, Have you considered the simplest possibility, namely that your fuel injection pump might not be able to draw in enough fuel? Possible reasons could include a clogged diesel filter or leaks in the fuel lines, which would cause air to be sucked in (visible as air bubbles in the transparent hose between the diesel filter and the injection pump; there should be no large bubbles present in this hose while the engine is running). Greetings, Rainer

... or the pump fails to deliver enough diesel due to a defect .

"Hi everyone, I've now replaced the diesel filter and checked the fuel pump, which seems to be okay. There's also no air in the fuel system. However, it still doesn't want to run properly. The brief interruption that occasionally occurred during acceleration in the first three gears at around 3500 RPM is gone, but it still can't reach its top speed of approximately 165 km/h on a flat road. Only when I'm in the slipstream or there's a slight downhill slope does it reach 175 km/h according to the speedometer. I'm starting to run out of ideas. Oh, and the current mileage is 155,000 km, and the fuel consumption is between 5.5 and 6.5 liters."

Hello,

"Please measure the mass airflow (MAF) sensor values in the higher RPM range (3000-4000) under full throttle and report them here. This requires a diagnostic tool or VAGCom."

Best regards, Rainer.

...as an alternative, another, preferably newer, LMM (lambda measuring module) can be tested. Sometimes, the LMM still provides values that fall within the tolerance range, even though this already indicates significant performance losses.
The conversion can be completed with minimal effort.

Best regards,
Uwe

Hi.

Replacing the magnetic valve (likely the one for turbocharger pressure regulation) doesn't automatically guarantee normal turbocharger pressure --> Check the turbocharger system or perform a test drive with a (temporary) boost pressure gauge.
Below 0.9 bar is suspicious --> Check the turbocharger and the piping system between the turbocharger and the engine, including the coolant, for leaks! Bleed holes usually reveal themselves through external oil traces.

Is the engine drawing in unmetered air between the mass airflow sensor (MAF) and the turbocharger? This would lead to reduced power and fuel efficiency.

Does the AGR (Abgasrückführung) system function normally, particularly the complete closing of the pneumatic valve? If not, this can also lead to a loss of power, although usually in conjunction with abnormally high soot emissions.

Hi!

There is no significant vacuum between the mass airflow sensor (MAF) and the air intake manifold, so any leaks would not allow air to be drawn in. If there were blow-off valves after the air intake manifold, the engine would run lean and produce soot. It appears that the engine is not receiving enough fuel. A common cause of this is the mass airflow sensor.

Best regards,
Uwe

Uwe wrote:

Hi! There is no significant vacuum between the mass airflow sensor (MAF) and the air intake manifold, so any leaks would not allow air to be drawn in. If there were blow-off valves after the air intake manifold, the engine would run lean and produce soot. It appears that the engine is not receiving enough fuel. A common cause of this is the MAF sensor. Best regards, Uwe

Hello,

Okay, then disconnect your crankcase ventilation system from the intake manifold and observe the mass airflow sensor (MAF) values. Small cracks, as you mentioned, might not be noticeable, but they should still be repaired to prevent dirt from being sucked in.

Best regards, Rainer.

Uwe wrote:

There is no significant vacuum between the mass airflow sensor (MAF) and the turbocharger, so any leaks would not allow air to be drawn in. If there were blow-off holes after the turbocharger, the engine would run lean and produce soot. It appears that the engine is not receiving enough fuel. A common cause for this is the MAF sensor.

Hi Uwe,

You are right about the missing throttle valve.

However, a severely constricted intake manifold boot (e.g., on a Golf 3) and/or an air filter that doesn't allow for optimal airflow can create a certain amount of vacuum all the way to the turbocharger, especially at high engine speeds and boost pressures.
Then, due to (larger) holes between the mass airflow sensor (MAF) and the intake manifold, a significant amount of unmeasured air can enter, which the MAF sensor does not detect. This, in turn, can lead to a reduced fuel injection amount, resulting in a loss of power.

Hi Rainer and Ulf!

You are absolutely right. The air velocity in the intake path also creates a vacuum, which can draw in air. However, I think that the effect is less significant with smaller holes.
My own experience, after extensive troubleshooting in the workshop, including replacing various parts and reverting to the stock chip for analysis, also pointed to a faulty mass airflow sensor (MAF). Several people in this forum had pointed me in that direction about a year ago, but my workshop convinced me to pinpoint the error using measured values. While a visit to the workshop is certainly worthwhile, the simple replacement and high failure rate of this part justify trying it out given these symptoms. It cost me several hundred euros not to listen to this forum.

Best regards,
Uwe

Hello everyone,

After disconnecting the connector at the mass airflow sensor (MAF), the performance, including the top speed, is the same as with the connector plugged in. Now the question is, is it the MAF sensor or perhaps the engine control unit (ECU)?

Does anyone happen to know the relay layout from the fuse box? The one on the right sometimes clicks when I accelerate, if I've located it correctly.

I would appreciate any information you can provide.

Best regards, Diro.