PSA: Slow acceleration at low speeds, but not at high speeds = EGR vacuum leak
Albeit in retrospect this looks obvious and easy, it was actually hard to figure out. So, I'm just writing and leaving this here for others, who might encounter the same or similar symptoms, but just as me, can't figure out (well, at least for a while) what's wrong with the car.
So, if you've an engine with
1. vacuum actuated turbo
2. vacuum actuated EGR cooler valve and/or EGR valve
3. you're experiencing
- loud engine and slow acceleration when the engine is cold, is in low gears and the car is going at low speeds (like when you're launching after a stop at a traffic light)
- but good acceleration and only light engine load at higher gears and higher speeds, and when the engine is warm
- your idle sounds rough, the engine warms up slowly, but when it does, your idle also gets a lot smoother
- you've medium-level turbo lag (ie. you floor the pedal and it only starts to accelerate after a second)
- despite all of these you have no boost (turbo), EGR or any other codes thrown
then your problem might be
A VACUUM LEAK IN THE VACUUM LINES OR AT THE DIAPHRAGM(S) OF THE EGR COOLER VALVE OR EGR VALVE.
You can obviously test for this with a vacuum tester device, by checking first the vacuum lines, then either directly or through the lines the vacuum actuators themselves, whether they all can hold vacuum. If you have a small leak somewhere, they will not hold the vacuum, or if you've a larger leak, you won't be able to create a vacuum in them in the first place.
Now, the explanation for the weird symptoms and no codes (not even intermittent ones) thrown is, that the leak in the vacuum lines of the EGR valves is causing problems in the turbo control through the same vacuum line system. Obviously if air can enter into the vacuum lines (at the EGR valves or somewhere else), then when the ECU tries to actuate the turbo vanes through the vacuum lines, it can't actually actuate it, because of the lack of proper and strong enough vacuum in the vacuum system.
Under normal circumstances such control problems (for whatever reason they're happening) would throw some fault codes (typically an underboost code) after a while. However, because the EGR valve and EGR cooler valve are only actuated occasionally and for short periods of time during normal driving, the problem with their vacuum lines actually unable to hold vacuum is only causing problems for similarly short periods of time - which will not be enough to throw a code, because the ECU will accept them as part of normal working tolerances. Also, the vacuum system itself can compensate for the loss of the vacuum (ie. it can generate more), but only with a slight delay.
This occasional malfunction in / strain on the vacuum system is what's actually causing the symptoms mentioned above. In short, whenever the ECU tries to actuate the EGR valves, because of the vacuum leak, it partially looses the ability to actuate also the turbo vanes properly at the same time - and that's what's causing an increased load on the engine, lack of power, but mostly only when the engine is cold, and when you're trying to accelerate in low gears at low speed.
The solution to the problem is obviously stopping the leak, by replacing the damaged vacuum line(s) or actuator(s). However, as an intermediary countermeasure and when it's not a vacuum line but an actuator diaphragm at a valve that's damaged, the vacuum leak can be stopped by disconnecting the vacuum line from the damaged valve, and plugging it (ie. the vacuum line) with an appropriately thick bolt. This will not make the actuator and the valve work again (which didn't work anyway because of the vacuum leak), but at least will allow proper vacuum through the vacuum system to be preserved, and actuate the turbo vane rod instantly and with appropriate force, at all times.