Once more to all, thanks for the excellent advice.
As Chris advised, I swapped the Turn Signal module. No result. Checked the "bomb tail", as Richard advised. No result. Sprayed all connectors under the dash with contact spray. Reconnected and cleaned all grounds. GLLOQ suggested spraying the intake manifold, but the idle goes back to normal as soon you are at a standstill.
During my test ride today, it happened that when driving downhill, I had a high idle and needed to brake all the time as the engine wouldn't brake as normal. Suddenly out of the blue, I felt the engine braking again, and the high idle was gone.
The other moment drove at 50 mph, put the engine in neutral, and there was high idling again. When in neutral and still rolling, I did rev up the engine to 5000 several times. High idle remained. Came to a halt. Idle dropped to normal. Start rolling again, and high idle started as well. Suddenly out of the blue, idling went back to normal.
During all my test rides, I switched the gear many times to neutral. 70% of that gives normal idling. 30% high idling. When at a standstill 100% normal idling.
I have the feeling it has something to do with the Idle Valve Control. But I swapped IVC already with the other car and swapped as well the complete manifold unit, including IVC, with the other car. Both had no results. Perhaps the wiring from the IVC? But that goes directly to the ECU plug. Secondly, how to check this wiring when it is intermitted a problem? And what is the relation with the speed sensor, as all is normal at a standstill.
