VW V6 TDI: Glow Plug Issues & Solutions

"Hi, I've been suspecting for a while that my V6 TDI isn't preheating long enough because it only starts reluctantly in the morning. I then adjusted the preheating time to the maximum possible value in the settings, but that didn't help. Then, with the engine cold, I checked the temperature sensors and they all showed a reading of 14°C (+/- 1°C), so I think I can rule out the temperature sensors."
This morning, the temperature sensor showed 3°C, and the glow plugs fired for just under 2 seconds (which I definitely think is too short). Consequently, the engine started poorly and with a lot of smoke. Because I have a voltmeter in my car, I can be sure that the glow plug system isn't heating up properly, as the voltage remains above 12V (normally it drops below 12V when the glow plugs are firing). I then started troubleshooting by turning the glow plug system on and off, and it worked perfectly fine after that.

I'm quite at a loss now. Without the VAG-xxx code, I would suspect the temperature sensor, but since I've measured the temperatures and considered them to be okay, can I rule out the sensor then?
I also tested each of the spark plugs individually, and they are in perfect condition since they were only replaced 50,000 km ago.

I would be very grateful for some suggestions.

Thank you very much.

Hmm.

Is the preheating really that important, or could it be starting poorly due to other defects?

I actually saw a VW Polo (1.7 SDI) start without any problems after a night when the temperature dropped to -15 degrees Celsius, and it didn't need any preheating. My then-girlfriend just said, after I almost fell asleep: "You said that a diesel engine doesn't glow anymore."

Okay, an SDI has a higher compression ratio than a TDI.

Maybe you could try jump-starting the car tomorrow morning with jumper cables and see if it starts better then.

Is it very cold in that little area?

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

"No, there's definitely a problem here. Usually, it would glow and start up perfectly. Now, it doesn't glow (or not enough) anymore, and as a result, it starts very poorly. When it's hot, there's no issue; it wasn't a problem in the summer either."


-- Hey, it's a small village near the border.

As I said, let's test it out.

- Disconnect the relevant temperature sensor and check if it glows (it should glow for a long time).
- Use thick cables for "hot starting" (should start perfectly).

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

dieselmartin wrote:

As I said, let's test it out: - Disconnect the relevant temperature sensor and check if it glows (it should glow for a long time). - Use thick cables for "hot starting" (should start perfectly). m;

Regarding point 1), okay, I will do that.
Point 2: I have no idea how to do that right now, but I will first proceed with point 1 and then come back to point 2 later.

So, if point 1) were working correctly, then wouldn't it be the temperature sensor that's malfunctioning?

If the actual temperature is indeed 14°C, it shouldn't require preheating at all. However, you can check with VAG-COM to see how long it actually preheated or should have preheated.
As I said, he doesn't need or use that feature right now, so the problem is likely elsewhere. For example, it could be related to air pressure.

mserge wrote:

Then, I checked the temperature sensors while the system was cold, and they all showed a value of 14°C (+/- 1°C), so I can rule out the temperature sensors. This morning, the temperature sensor showed 3 °C, and the glow plugs fired for just under 2 seconds (which I definitely think is too short). Consequently, the engine started poorly and with a lot of smoke. Because I have a voltmeter in my car, I can be sure that the glow plug system isn't heating up properly, as the voltage remains above 12V (normally it drops below during preheating). Then, I started troubleshooting by turning the glow plug system on and off, and it worked perfectly then.

Hi,

According to the actuator diagnosis, the glow plug is working correctly, but it doesn't activate during normal operation before a "cold start"?

Then either the software is not properly calibrated to the engine's heating requirements, or the sensor is reporting an excessively high temperature.
-> Read the sensor value before the next cold start and compare it to the actual outside temperature to check for plausibility.

christians wrote:

If the actual temperature is indeed 14°C, it shouldn't need to preheat at all, but you can check with VAG-COM how long it preheated or should have preheated. Just as I said, he doesn't need or use that feature right now, so the error is likely somewhere else. For example, it could be related to air pressure.

"Sorry, the 14°C I measured were from a few weeks ago, not today. At that time, the outside temperature was indeed 14°C. I didn't explicitly mention that."

ulf wrote:

-> Read the sensor value before the next cold start and compare it with the actual outside temperature to check for plausibility.

Okay, I'll try to do that tomorrow and let you know how it goes.

Thank you.

Perhaps there is a loose connection, and the...
The temperature is only sometimes accurate.

Okay, so unplugging it caused it to glow continuously.
Insert new antennae, no glowing.

This morning, the temperature was as low as 3°C, but it still glowed for about 4 seconds and started up quite well.

Okay, I'm going to check the idle speed control now, and as Christians mentioned, I'll also check the fuel injection pump to see if it has become misaligned.
Perhaps the poor starting performance is due to something else entirely, and only the glow could improve it.

So, if the start time of the injection isn't set correctly, the engine will also run poorly. A longer pre-glow period can help, but it doesn't solve the underlying problem.
It's best to use a VAGCOM diagnostic tool and go into channel 04 to check the basic settings and verify the injection timing. It should be 2 degrees before top dead center (BTDC), plus or minus 2 degrees.

"Something must be wrong with the control unit or some kind of connection. This morning, at 0°C, the preheating light was on for about 2 seconds, but when I checked the built-in voltmeter, I noticed that it didn't actually preheat (no voltage drop). It made a loud organ-like noise for about 5 seconds before it reluctantly started, although it ran smoothly on all 6 cylinders."
I read the data using VCDS, and the coolant temperatures were all between 1 and 3.4°C. The preheating time was set to 0!
I then tried to enter the largest possible value, but it didn't have any effect.
Could someone who also drives an AFB, AKN, or AKE please pay attention to whether and for how long the preheating light illuminates when the temperature is around 0°C?

Thank you very much.

mserge wrote:

There must be something wrong with the control unit or some kind of connector. I read it out with Vag-Com, and the temperatures were all between 1 and 3.4°C. The preheating time was set to 0!

After the engine has already been running and the water has warmed up a few degrees?
Then, according to my knowledge, the "new" preheating time will be displayed, and it might have already been zero.

If, despite a clear water temperature reading of 0°C, the preheating time is set to 0, then, in my opinion, there must be something wrong with the software .

Okay, so about 4-5 seconds at 0°C. However, mine also starts "well" even if I don't use the glow plug preheating. I don't have a display, but you can tell by the interior light.

The preheating behavior of my car is a bit strange. Under identical conditions and with an outside temperature of 5-10°C, I sometimes notice different preheating times. However, it doesn't seem to affect the starting performance.

So, I think it should start immediately at 0°C without needing to be preheated.
With my car, I only noticed that 3 out of the 4 glow plugs were defective when the temperature dropped below -10°C.
I also suspect it's due to an incorrect injection start point.