dear Frans!!!

in your case.... it could be that something happen with the throttle cable between the pedal and the throttle body(cables) or.... you have problem with a worn or leaked Gasket from the injection manifold.
To check vacum leaks in the injection system there is a easy thing to do
First of all: Use a Brake-cleaner can and spray the area near the injection intake manifold if there is a leak the Engine will go rotate high as quick OK?
If not...... spray all around the system an tried to find out if the Engin will rotate after this higher or not.

Note: Be carefully with Brake-cleaner to the exhaust manifold don t tried this with an completly hot Engine OK?

Good luck !!!!
cincerly : Markus