AAT startup behavior is puzzling

Hi everyone,

Today, I'd like to take a chance and post my first question in this forum.
My name is Holger, and I own an Audi 100 C4 from 1991 with the 2.5 AAT engine.
The vehicle recently experienced a timing belt failure.
As a result, a new cylinder head and new pistons were installed.

Unfortunately, the starting behavior raises some questions after further analysis.

"When the vehicle is preheating, the starting behavior is 'normal,' meaning..." The vehicle starts immediately.
In this case, the shelf life is insignificant, even after many weeks, it's not a problem.
(Vehicle is not for daily use.)

If the engine doesn't glow before starting, the starter motor may take several seconds to crank the engine during a cold start. The engine runs smoothly after that.
If the engine is turned off shortly after, it will start smoothly again.

Generally, the engine runs smoothly, which suggests a good initial setting.
The vehicle reaches its top speed and is very fuel-efficient.
The battery is new.

Does anyone know about this phenomenon?

Thank you for your help.

Greetings from the sunny Lower Rhine region.
Holger.

Hello,

Check the coolant temperature readings from the engine control unit; often, the sensor is defective.

The initial speed (release) may depend on this, but this is speculation as it depends on the specific ECU.

The initial amount certainly depends on this, and it increases over time with what is called "organizing."

Best regards, Rainer.

Hello Rainer,

Thank you for the quick response.
I haven't been able to read the control unit using VCDS so far; perhaps the vehicle is too old.

You can replace a sensor if you suspect it's faulty; it's usually a straightforward process.
The idle speed has been considered before, but:
Even after prolonged idling and with the starter motor running at its maximum speed, it started immediately after pre-glow, so I would rule out the starter motor/speed sensor as the cause.

Greetings from kilometer 815 of the Rhine River.
Holger.

Fillmaster wrote:

Even after prolonged idling and with the starter motor running at its maximum speed, it started immediately after pre-glow, so I would rule out the starter motor/speed sensor as the cause.

Incorrect. If the coolant temperature is low, the cranking speed is irrelevant for the ECU's start authorization. And vice versa.
However, this is not the case for every ECU, so it should still be checked.

Hello Holger,

Were the injector nozzle leak hoses replaced during the cylinder head installation?
The tires are old and worn out, especially at the edges, likely due to diesel fuel exposure.
also auch für längeres "orgeln" beim Starten von AAT usw. AEL.
It was at least like that for my AAT. After installing new hoses and...
The additional securing with hose clamps changed the...
Significantly improved willingness to start.

There's a very interesting article on this topic in the "Technical Articles" section.
useful report titled
"Starting problems and glow plug issues in TDI engines with VP37/VP44 injection pumps."

It also helped me personally in resolving various issues.
"Helpful information about the quirks of my A6 C4 TDI."

Hi KHMenzel,

Yes, the drip tubes are new.
For my understanding, would these hoses then cause poor starting performance at all temperatures?
However, there are no problems if it has been preheated.

I'll take a look at the recommended report; hopefully, it will shed some light on the matter.

Thank you for the information.

Do these vehicles (Audi 100 C4) already have a pre-glow function that activates when unlocking/opening the driver's door?

Thank you and best regards.
Holger.

Hi,
Which VCDS interface are you using? HEX-V2 or older (HEX-USB)?
Herbert.

Fillmaster wrote:

Do these vehicles (Audi 100 C4) actually have a pre-glow function that activates when unlocking/opening the driver's door?

To the best of my knowledge, no. At the current temperatures, the vehicle should be able to start easily without the glow plugs being connected.

It could be due to air in the fuel line, poor engine compression, or incorrect fuel injection timing.

Best regards, Rainer.

Hello Rainer,

I was able to rule out air by using a transparent intermediate connection.
Furthermore, a leak would also cause the willingness to start to decrease with increasing standing time, but this is not the case.
Starts immediately after the warm-up phase, even after a long period of inactivity.
Would the vehicle reach its maximum speed given the suspected misconfigurations?
The fuel consumption and driving characteristics are excellent.

Regards,
Holger.

Herbert wrote:

Hi, Which VCDS interface are you using? HEX-V2 or older (HEX-USB)? Herbert

Hi Herbert,

I am using Hex-USB.

Regards,
Holger.

"With Hex-USB, it should be possible to read the data from the engine control unit (ECU). This refers to the lower bit rates, as Ross-Tech indicates that Hex-V2 has a problem with those."
hg
Herbert.

Hello Holger,
The AAT likely doesn't have an OBD connector but instead uses the smaller individual and double connectors in the engine compartment. You'll need the right adapter for that.
Simple test for your problem:
Disconnect the temperature sensor plug. Then, the control unit assumes that it is cold, initiates the preheating process, and does not reduce the initial amount [of fuel/air mixture].

Best regards,
Micha

Hi Micha,

I have the corresponding adapter, and I have also found access to the control unit .
The option involving unplugging the device is something I'm going to try out in the coming days.

Thank you and best regards.
Holger.