Passat TDI: Engine Stalling After Cold Start

Hello diesel mechanics,

For the past 3 days, I've been experiencing the following phenomenon:

"When I start my Passat in the morning or after it has been standing for a long time, the engine turns off shortly after starting. To get the engine running again, I have to crank it for about 10 times (until the "STOP" light illuminates on the display). Eventually, the engine starts reluctantly, but if I give it gas, it runs again."

At first, I thought my fuel gauge was malfunctioning and that the tank might be empty. But that's not the case. Also, it only makes those noises when the engine is cold. When the car is running or when it starts warm, it doesn't have any problems. Neither black smoke nor soot is noticeable, whether the engine is cold or warm.

I then went to the VAG workshop to have the error memory read. The result: no errors that could be related to the problem (the heated exterior mirror is not working). The workshop suspects a mechanical fault. Everything the master mechanic said then suggested a cost that would be comparable to buying a new car.

Hat jemand eine Idee, worauf der Fehler zurückzuführen sein könnte? Ich habe in den vorhandenen Foren-Beiträgen leider nichts gefunden, was mir weiterhelfen könnte. VielThank you in advance!

Sure, here is the translation of the text from German to English:

"Note:"
It's a Passat (3B) V6 2.5L TDI/150 hp, automatic transmission, manufactured in November 1999, engine code: AFB, mileage: 186,000 km.

Hello,

I would just replace the coolant temperature sensor as a precaution. It probably costs around 30€, and you can install it yourself in a few minutes.

"This component tends to fail across the entire company, and if it provides even somewhat plausible readings, it won't be detected by the error memory."

p.s. I know that it's not particularly professional to simply replace parts based on suspicion, but at 30€ plus 5 minutes of work, it's "acceptable" in my opinion.

First, thank you for the quick response.

Another mechanic had already given me this tip earlier today, before I went to the VW dealership.

I confronted the mechanic there with my suspicion, but he told me that if the coolant temperature sensor is broken, the system is tricked into thinking it's extremely cold outside. Therefore, he said, the engine should definitely start, even if it's cold. Problems would only occur with a warm start.

However, since I'm experiencing the opposite problem, namely difficulty starting the engine when it's cold, he could rule out the coolant temperature sensor.

I'm a little unsure now. Does the VW master not know what they're talking about, or are you wrong?

Supplementary questions:
"Unfortunately, I don't have a repair manual for my Passat yet, so I don't know where the coolant temperature sensor is located. Can you tell me where it is?"
2. Can I check myself if the coolant temperature sensor is defective (I don't have any workshop equipment, but I do have a multimeter)? If so, how?

Quote:

I confronted the master there with my suspicion, but he told me that if the coolant temperature sensor is broken, the system is made to believe that it is extremely cold outside. A cold engine, therefore, should start in any case, he said. Problems would only occur with a warm start.

hi.
That's correct. If the sensor is completely defective or, for example, the connector is not plugged in, the control unit will use substitute values such as "extremely cold."

"However, the sensors can also provide incorrect values, such as a constant reading of +40°C, or they may fluctuate constantly between -27°C and +38°C (for example)."
Since these values are within the acceptable range, no error is being logged.

My suggestion:

Disconnect the connector from the temperature sensor. The control unit will then use substitute values, as mentioned above, so it's not a big deal.
"If that doesn't work, try replacing the temperature sensor; if it still doesn't start, you'll need to investigate further."

(You will then receive an error code in the diagnostic system, and possibly a warning light will flash on the dashboard. This needs to be cleared (or have the error code reset) after the action.)


You'll find the sensor – after removing the engine cover – roughly in the center between the two cylinder heads, towards the front of the vehicle, approximately near the oil filter housing, inside a (metal) coolant hose.
The giver themselves would probably be green.

How long does the glow plug filament glow during the preheating phase?
Does your Passat have a voltmeter in the instrument cluster? If so, does the voltage drop below 12V when the glow plugs are activated? If it does, then the glow plugs are likely working properly. If not, you probably have a problem with the glow plugs or their wiring.

Is your vehicle parked uphill, with the front facing upwards? If so, try parking it downhill instead. It's possible that diesel fuel could flow backward from the engine into the tank overnight due to a leak in the diesel lines, filter, or pump.

Was ist passiert, wenn ein Austauschmotor wurde auch eine neue (überholte) Einspritzpumpe eingebaut?

I suspect the problem is with the diesel pre-injection line. Does it contain air if there's a fault?

I don't think it's supposed to glow; it doesn't start when it glows, and it immediately turns off. Even without glowing, it should still work.

It can't be the immobilizer, right?

Herr Antje wrote:

It can't be the immobilizer, right?

You'll see this when the immobilizer symbol in the instrument cluster turns on and off after the engine has been turned off.

@ Schraubi-Schlumpf:
It's not the coolant temperature sensor. The error persists.

@ mserge:
Auto steht auf gerader Ebene. No inclination.
The glow plug illuminates for approximately 1-2 seconds. The glow plugs were replaced during the engine overhaul. To my knowledge, even with a faulty glow plug system, such an error should not occur at these temperatures (12-15°C), right?
The fuel injection pump is old and has not been refurbished.

@ Mr. Antje:
Wegfahrsperre ist es nicht. Checked.
Where can I check if there is air in the diesel pre-filter line? The hoses are made of black rubber.


Supplement to my phenomenon:
"In the last few days, I've managed to work around the problem by giving it some gas while starting. The engine wouldn't stall, but even with full throttle, it would only run at 1200 RPM. Then, after about 10 seconds, if I pumped the gas pedal a little, the engine would rev up."
To clarify: the problem only occurs during cold starts and after a longer period of inactivity (approximately 1 hour). There are no issues with warm starts, and the engine starts immediately.
I suspect there's a problem with the fuel supply, but it only occurs after a longer period of inactivity and isn't a constant issue.

I suspect you have air in the fuel lines. There's likely a leak, either in the oil drain lines or somewhere else. It's probably very minimal, but you need to find it, otherwise your pump will fail.
There's a transparent section installed in the fuel line so you can see if there's air in it. Please do the test I wrote out for you.

Fill up the gas tank, then park the car facing downhill. If it starts without any problems, you've found the issue.

And you're right, you don't need a working glow plug system when it's 12-15°C, but since we're already around 0°C in the mornings here, it could have been an issue.

Good luck!

Hello,

I experienced the same phenomenon with a Volvo 850 TDI, model year 1996, with an engine similar to the AEL engine from the Audi A6 C4.
Sprang bei Kälte und kaltem Motor kurz an und ging sofort wieder aus. Danach musste ich orgeln bis er wieder ansprang und auch weiterlief. No problem at all.
He was indeed volatile, but he just didn't want to continue running.

Ultimately, I replaced the pre-filter line for the diesel, which connects the diesel filter to the VE pump.
Specifically, I didn't replace the existing hose because it runs in a very awkward position behind the timing belt of the camshaft, around the engine. Instead, I installed a diesel-resistant hose "straight across" the engine bay. After that, I encountered problems with cold starting.

As part of this project, I also measured the voltage of the glow plugs and connected it to a light bulb inside the vehicle to see what would happen.
Using a test light, I observed that the glow plugs continue to receive current even after the control lamp on the dashboard turns off, and that the current is load-dependent. There is no voltage when the accelerator is pressed, but voltage returns when the accelerator is released.

I haven't been able to determine which temperature sensor is being used, or whether a timer function is being used to activate/control the current.

Greetings.


Andreas.

I double-checked: my diesel line also has a transparent section under the engine cover.
There was some diesel fuel in the tank, but air was behind it. When starting, the diesel fuel was supposed to be pumped to the engine, but no fuel was reaching it. Therefore, it appears that the engine is not receiving any fuel.
Could it be a problem with the diesel pre-injection line if this error only occurs during cold starts? Shouldn't the error then be present all the time?

Normally, this transparent piece is filled with fuel, and it remains that way.
even if the box sits for several weeks.
If there is even a tiny hole in the inlet pipe, air will slowly leak in, and...
This leads to the error you described.
The leak is likely on the side where the air bubble is (towards the filter).
I would start looking there.

Best regards,
Rudi

There are many potential leak points, including the hoses on the filter, the thermostat valve on the filter, the return lines, and the pump itself. I would suggest temporarily clamping the hoses overnight to see if that stops the leak.

...and most importantly, stop messing with it until the problem is fixed. Your VP44 pump might not be able to handle that, and it could break, which will be expensive.

Yesterday, I tried changing the diesel filter, and sure enough: the air bubble in the diesel supply line is gone, the line is completely full, and the car starts without any problems.
Even when it's completely cold (I just tried it after it had been standing for 16 hours).

It seems the cause has been located: the diesel filter was clogged.

Nevertheless, the question arises as to why the vehicle only had problems with cold starting before the diesel filter was replaced.
I always thought that if the diesel filter was clogged, it would cause a constant problem with fuel supply, including issues with starting the engine when it's warm and during driving.

Does anyone have a plausible explanation for this phenomenon?

P.S.: At this point, I would like to express my sincere gratitude to everyone who has taken the time to address my problem and try to help. You are all amazing!

Hello PBBS,

Your conclusion is unfortunately incorrect.

Your diesel filter was not sealed properly; it was leaking!

He had a small leak somewhere (probably under the fuel filter on the return line). Because diesel engines don't have pressure in the lines, diesel doesn't leak out, but only air gets in. This mainly happens when the vehicle is parked for a long time.

If the filter had been properly sealed, you would have experienced problems at high speeds and under heavy load.

Regards,
Guste.