After a cold start: turbo not building boost?

The engine is a 1T (Audi 100, type 44, year 1990, 5-cylinder TDI, 120 hp), so there's no mass airflow sensor (before anyone suggests it), no AGR valve, and no catalytic converter. When starting the engine cold, everything works perfectly. I can drive any distance (500 km was fine), with full power and a top speed of 210 km/h. However, if I turn the car off and then start it again while it's WARM, it initially roars briefly during startup, reaching 2500 rpm (which it doesn't normally do), and then it doesn't drive properly. It feels like it only has 80 hp: a maximum speed of 170 km/h and terrible acceleration. I have the feeling that the turbo isn't building proper boost pressure. The car doesn't smoke, and the fuel consumption is normal, so otherwise, it seems normal. If I let it cool down, everything works perfectly again, until the next warm start. I have a feeling that if it doesn't preheat when starting, then something is wrong with the electronics.

Does anyone have any idea what this could be?

Best regards, Björn.

Hello again,

As I suspected: when I disconnect the connector from the coolant temperature sensor while the engine is warm, everything is normal. There's no loud noise during startup, and the engine has full power in all situations. However, if I turn off the engine, reconnect the connector, and then start the engine again, the problem reappears... Maybe I should try plugging the connector back in while the engine is running, but I'm too scared to (sob).

Bjoern

PS: please, please answer!

Hi.

After the initial start-up, the lack of boost pressure reminds me of a stuck VTG (Variable Turbine Geometry) valve in the resting position, something I've experienced myself many times.

But that extra noise after the initial start-up, which can be resolved by disconnecting the temperature sensor connector, what about that? And then there's the normal performance – which shows that the problem apparently isn't related to the VTG (variable transmission gear).

I suspect the possible causes are a faulty sensor and/or a damaged cable connection in that area (e.g., short circuits between different wires).
It would be very appropriate to have the error memory read out in this case. Okay, hier ist die Übersetzung:

"Please provide the text you would like me to translate from German to English."

VTG stands for variable turbine geometry, right? It's unlikely that's what it is, since the 1T didn't have that technology. Keep in mind, we're talking about the very first modern TDI passenger car from 1990. It had a lot (too much) electronics for its time, but it was still missing some things. I'll have the error codes read and get back to you.

Best regards, Björn.

... like your car, is correct (at least as a mass-produced application).

I missed the "BJ 90" part, sorry.

The previous owner had informed me that the coating on the distributor shaft of the fuel injection pump had previously dissolved, and it was replaced at that time.

This happened again: this time, it wasn't the coating on the central rotor that had dissolved, but the coating on the runner wheels that run over the central rotor. As a result, the central rotor no longer looked as clean. The resulting debris had accumulated in the deepest part of the distributor injection pump: in the injection control valve. This valve had become jammed, and the debris had created fine grooves, which caused the valve to 1) jam and 2) leak. Since the valve is made up of a piston that slides within the ESP housing, the problem was a housing failure.
I then made the piston functional again using 1000-grit sandpaper because I couldn't find a used pump. As a result, it no longer seized, but it was still leakier than before. This means that the injection timing is too late at high RPMs, which negatively affects performance. Interestingly, the loud noise during warm starts disappeared, so the engine control unit was no longer switching to emergency mode. I then drove another 3000 km, at speeds of around 130-140 km/h, before installing a used pump. Everything is okay again now.

Best regards, Björn.