I've got an CRX with ZC1 engine and when the engine begins starting to warm up (and in total warm pahse) the idle varies between 1000 and 2000 rpm's to and fro, approx. every 2 seconds.

When I pull off one small vaccum hose from the back of the intake manifold and close the hole the idle keep its rpm. This hose connects to one of the solenoid valved in a huge black box together with 10-20 other vacuum hoses.

Has anyone an idea how this can be fixed and what kind of checks I can do by myself without bringing the car to a dealership?

I unmounted & checked the idle controller on the back of the intake manifold (where two cooland tubes are connected) but I think this one seems to work allright.