1.9 TDI Starting Problems: Troubleshooting Guide

Hello,
I'm new here. My name is Maurice, and I'm 18 years old. I've been reading for a while, and now I have a question. I installed a 1.9 TDI AFN engine in my Golf 2, but I've never heard that engine running before.
Now, let's get to the problem:
"When starting, it sounds like it tries to start once, then cuts out briefly, and then starts up again immediately. I've attached an audio file that describes it much better. When it's running, it works really well and has good power, but I don't know if it could be better. I'm just annoyed by this strange starting process. Do you have any ideas what it could be?"

Regards,
Maurice.

Hi,

My suspicion is that the fuel injection amount (initial amount) is too high due to a faulty coolant temperature sensor. This then causes the engine speed to increase, and the engine control unit subsequently reduces the throttle.
The coolant temperature can be displayed in the measurement blocks of the engine control unit using VCDS.
You can also see them in the OBD2 diagnostic data, for example, using software like KOBD2Check.

A new coolant temperature sensor for the engine costs less than 10 EUR. Also, check the cables and connectors.

Best regards, Rainer.

Hi Rainer,

Thank you very much for the super-fast response. I will get the sensor right away. I just unplugged the sensor connector, and the starting behavior is a little better, even at a higher RPM, although the up-and-down fluctuation during starting is not completely gone. Is that normal with the connector unplugged?

Just for fun, I unplugged the connector from the throttle potentiometer, and it started perfectly the first two times, but then the error code reappeared.

Best regards, Maurice.

With the coolant temperature sensor disconnected, the behavior should be strange; the engine control unit will then use the maximum amount of fuel for starting.

It's possible that the fuel injection pump is slightly miscalibrated, or that a tuning box has been installed. The same applies to a potentially faulty throttle position sensor. Without a proper diagnosis, you won't be able to determine the cause.

Since the coolant temperature sensor is very inexpensive and often fails, I would start by testing that.

Best regards, Rainer.

Okay, I'll get the temperature sensor tomorrow and install it. Hopefully, that will fix the problem.
Could it possibly be the flywheel? The engine comes from a car that had a minor front-end collision. I also have a strong rumbling/humming sound when the engine is idling, but I haven't noticed any rumbling while driving. It's not very audible in the audio file, but when you turn the key, the engine immediately shuts off. That doesn't sound quite right...

Maurice.

Why do you think the flywheel could significantly affect the engine speed during startup?

I could now imagine that the flywheel, due to the accident, has a significant imbalance, and that this imbalance could become noticeable during the starting process, in the way that I am experiencing it.

mauze wrote:

I could now imagine that the flywheel, due to the accident, has a significant imbalance, and that this could become noticeable during the starting process, in the way that I have experienced.

And after starting, you no longer notice the imbalance , that's quite a thought process...

I already wrote that it makes a very loud rumbling noise when idling. It also makes a noise at higher RPMs, but not as loud.

mauze wrote:

I had written that it makes a very loud humming noise when idling.

At higher RPMs, it also makes the noise, but not as strongly...}
The engine sounds normal when running on the MP3.

Technical articles on TDI starting problems:
Okay, I'm ready. Please provide the German text you want me to translate.

Okay, I finally got around to installing the water temperature sensor today, but it didn't lead to any improvement. Then, I quickly turned the key, and the engine started up as it should.
I then took a second ignition switch from the stock and now the problem is gone.
But thank you very much for your help!!!

Best regards, Maurice.

Ah... and how do you explain the effect? Did you wake up in the morning and think: "Ah, today I'm going to install a new ignition switch" ?

Okay, so I'm revising my previous answer. I replaced the temperature sensor, but the problem was still there. I even ran the engine and moved it around the yard, so there definitely should have been water flowing to the sensor. Then, out of frustration, I just turned the key all the way and it started. I simply unplugged the connector from the old sensor and plugged it into the new one, and it immediately started working. Then I reconnected the old ignition switch, and the problem reappeared.
I think the ignition switch in the old ignition system is faulty. When you turn it to position 1 (ignition on), then to position 2 (start), the switch briefly interrupts the ignition as you return from the start position, and then when you're back in position 1, the ignition is on again.

Thank you for your feedback!