Golf 4 AXR won't start properly

Hi,

Golf 4, manufactured in 2000, with an AXR engine, occasionally fails to start completely.
You start the engine, and it can run for 20-30 seconds, sounding like it has zero compression, completely misfiring, but the engine still turns over, so the starter motor is working correctly.

Turn the ignition off, ignore the car for 5 minutes, then try starting it. It starts in 1.5 seconds, which is great, just like a warm start.

As I said, he does it completely arbitrarily! You drive for an hour, park the car, get out a box of cigarettes, get in, and it won't start anymore.
Ignition off, light a cigarette, get in, start, everything's great.

Relays and mass airflow sensors are new!

Hello,

What does the bypass valve do? Is there no white smoke, or a small brown-black puff, when it starts up after a previous failed start (i.e., not getting any diesel)?

Best regards, Rainer.

Rainer K. wrote:

What does the shut-off valve do?

That would be something new... and typical of high-tech pretensions, because the AXR has a motor-controlled EGR throttle valve, the position of which, as far as I know, cannot be controlled from the outside .

It's possible that the throttle angle sensor is faulty and occasionally reports "throttle open" during startup, even though the throttle is still closed.

Does the MIL (Malfunction Indicator Lamp) light up? (It would be expected to, since it's related to emissions components.)
What does the error memory say?

Hi,

I occasionally experience the same effect, but it has a completely different cause:

'The WFS (Warm Function Start) is not activating for me. You can tell because the preheating light doesn't even turn on for the required second. If I remove the key and restart, everything works as it should. Sometimes I have to repeat this 'ritual' several times, but it's enough to just turn the ignition on and observe the preheating light.' It has nothing to do with the temperature.

It's been a part of my life since day one, and it will continue to be until my very last day.

Regards,
Master craftsman.

Obermeister wrote:

Hi, I occasionally experience the same effect, but it has a completely different cause: The WFS is not being activated for me. ....

For this reason, the G4 AXR has an extra light. If this light doesn't turn off, the engine will only start briefly and then immediately shut off again. If the vehicle also has a Multifunction Display (MFA), the problem will be displayed in plain text.

Regarding the issue of PD engines that are difficult to start, the first thing that comes to mind is the Hall sensor on the camshaft.
Unfortunately, not all faulty Hall sensors seem to leave an error code in the diagnostic memory.

Otherwise, I would start by checking the usual suspects: the pre-charge pump, the tandem pump, air in the system, and the differential pressure sensor.

Regarding the throttle valve on the AXR engine:
In its resting state, it is open. It is only briefly closed when the vehicle is parked, and then it opens again. Therefore, it should be open when starting the engine. As shown in the image below.

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Obermeister wrote:

The WFS is not being activated for me.

However, this symptom certainly doesn't occur in Bertil's usual suspects.

Quote:

It acts as if it has 0 compression, but the engine is turning, so the starter motor is working correctly

.
To explain something like this, especially something that "spontaneously self-heals," I believe you can't avoid mentioning the throttle body...

ulf wrote:

The WFS is not being activated for me. Obermeister wrote: However, this symptom certainly doesn't occur in Bertil's usual suspects. Quote: It acts as if it has 0 compression, but the engine is turning, so the starter motor is working correctly . To explain something like that, especially something that "spontaneously heals" itself, I believe one cannot avoid mentioning the throttle valve.

"When I disconnect the engine wiring harness and then try to start the engine, I get the exact symptoms of zero compression (despite the spark plugs being open)." The motor spins freely and smoothly without noticeable resistance.
Unfortunately, this error message provides no clues as to the cause of the error.

Bertil wrote:

When I disconnect my engine wiring harness and then try to start the engine, I get the exact symptoms of 0 compression (despite the DK being open). The motor spins freely and smoothly without noticeable resistance. Unfortunately, this error description provides no clues whatsoever about the error.

Aha...
Now I'm just wondering how "no power" can steal the physical compression despite the open DK .

I've repeatedly oversped my starter motor by directly connecting it to the positive terminal (B+) and I've always clearly recognized the "uiuiuiui..." sound, which indicates the excessive starter motor speed.

Perhaps vagtuning should try starting an engine without (spark/glow) plugs and listen to the starter motor -> "iiiiiiiiii" instead of "uiuiuiuiui" .

Is this the case for all of them, that the flap is open when there is no power?
Could it possibly be a stuck valve that gets stuck in the 'CLOSED' position after being turned off?

I am certainly aware of the difference between 'uiuiuiui' and 'iiiiiii,' but the engine sounds as if it's running completely without compression. It revs up to starter speed and stays there, with no noticeable fluctuations. Also, there are no unusual jerks or hesitations when starting, as would be normal.

Hi,

Well, the compression definitely can't be gone, otherwise it wouldn't start anymore.

Even if the valve is closed, I can't imagine that the engine would rotate evenly at the starter speed, because I don't believe the valve is so airtight that you could create a vacuum behind it (which would explain the even sound).

What happens if you push a car with the clutch engaged and the ignition on (while taking appropriate precautions in case it starts)?

Best regards,

Jan.

vagtuning wrote:

Could it possibly be a stuck valve that, after being stopped in the "CLOSED" position, becomes stiff?

Yes. For EU3 engines, it can even happen without an error message, because the valve position is not monitored.

With the AXR engine, there should be something in the error memory regarding the monitoring of the valve position, in my opinion. However, this is only the case unless the monitoring system itself is faulty -> see above.

ulf wrote:

In the case of the AXR, there should be something in the error memory, in my opinion, because of the monitoring of the valve position... unless the monitoring system itself has a problem -> see above.

Faulty stepper motor and position sensor?
Well, that would be like hitting the jackpot.


But I have to admit, I'm slowly starting to get used to the idea.
Sure, please provide the text you would like me to translate from German to English.
The connector (4-pin) for the bypass valve is strategically located in a position where it's very susceptible to oil dripping onto it. The connection of the charge pressure pipe above the connector (see photo) is not 100% oil-tight. Some oil droplets always find their way out, and depending on the mileage, a significant amount of oil can seep into the connector. This results in poor contact. This could cause both the monitoring system and the actuator to malfunction.
Every time I cleaned the air filter, I always wiped the oil off in this area. I may have prevented something worse from happening to me.

Check if this is happening to you too!

Apparently, the crankshaft speed sensor is defective.
This afternoon, we will see if the 20 euros were worth it .

That doesn't actually explain the problem, because if you disconnect it completely, it starts just as well or poorly.

Let's see.

To my knowledge, a TDI engine won't start if the crankshaft position sensor is disconnected. The camshaft sensor is a different story. Whatever you did there... it probably wasn't the crankshaft position sensor.

Okay, so the crankshaft position sensor has been replaced, and there are no more errors in the system's memory... It worked fine for a week, but now it's happening again. It took 30 seconds of cranking at the gas station... and people are looking at you like you filled up with premium fuel.

NW sensor check... if it's faulty, the PD (presumably referring to a specific device or system) will start very reluctantly.
It would have been a miracle if it were the crankshaft position sensor.