Warm Start Issues After MSG Swap 1Z to AFN

Hello.

Here's a brief introduction to get you started:

For about 2-3 years, I've been an active reader of this forum, and so far, I've been able to resolve all my issues thanks to it.
Now I've reached a point where the search function isn't sufficient to solve my problem.

I have decided to start my own topic.
Since this is my first time posting, and I'm not yet allowed to post everywhere (I don't have "full privileges" yet), I hope this is somewhat appropriate for this forum, and that I can get some help.

Regarding the actual problem:

I have a Volkswagen Golf 3, manufactured in July 1994, with a 1Z engine.

Last year, I gradually converted the 1Z engine to AFN technology.
"That is to say, the turbo, vacuum system, and ESD components have been replaced!"

As a final step, the engine control unit was replaced. This took a bit longer because the missing immobilizer system had to be retrofitted.

Oldest MSG: 028 906 021 C (1Z)
New MSG 028 906 021 FQ (AFN)

The old wiring harness was left in place. Also, the electronic stability program (ESP) was not replaced.

Problem: The engine only starts after 3-5 seconds of cranking when it's warm.
That is to say, the colder the engine temperature, the more likely it is to start. With the 5°C temperatures of the past few days, it started normally in the morning.
I've also noticed that the yellow warning light no longer comes on at all (after the MSG replacement), not even briefly.

My theory:
The MSG (or ESP) injects only the amount of fuel necessary for idling, meaning that it injects more fuel when the engine is cold than when it is warm.

Now, let's look at the data I extracted:
No error code.
RWS reports 0.7V when ignition is turned on (does anyone have a precise value for the AFN?).
The RWS is reporting approximately 1.5V when idle.
Fuel injection volume at idle: 2-3mg per injection cycle, with a warm engine.
Fuel injection volume at idle: 5-6 mg/stroke, with a coolant temperature of approximately 7°C.

I know that the ESP system is different on the AFN model, but the engine runs perfectly fine after starting.

The slightly low values for the idle injection volume suggest a shift in the MSW (more injection) (-> which should actually be beneficial for starting), or a RWS (regulation offset) that is set too far to the left (-> which also leads to a "richer" injection).

All other values appear to be within the normal range for now.

Does anyone have any idea what might be causing this?

In my opinion, the problem is likely due to insufficient fuel being injected at startup (if I disconnect the engine temperature sensor, it starts perfectly).

Thank you for reading.
I would appreciate solutions to the problem!

msp18 wrote:

As the final step, the control unit was replaced. Oldest MSG: 028 906 021 C (1Z) New MSG 028 906 021 FQ (AFN) Problem: The engine only starts after 3-5 seconds of cranking when it's warm. "That is to say, the colder the engine temperature, the more likely it is to start. With the 5°C temperatures of the past few days, it started normally in the morning." Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. Does anyone have any idea what might be causing this? In my opinion, it's only because too little fuel is being injected at the start (the connector is disconnected from the engine temperature sensor, and it starts perfectly fine)

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Hi,

You're very close.

Actually, the AFN software is quite unfriendly to warm starts out of the box (I had similar problems).
This can be fixed with chip tuning, or you can trick the engine control unit (ECU) into thinking the engine is only partially warm during startup by using a relay-controlled resistor in series with the coolant temperature sensor. This also works.
You can find a relevant user experience report in this forum by using the search function.