On my 1993 VW Mk1 Cabriolet I've been trying to find an issue that causes the idle to rise and dip (to near stalling levels) when power is being drawn (in this case, when the headlights are on - there's no air con).
At first I thought this would be the Idle Stabiliser Solenoid/Valve and so replaced it, however the problem persists. The solenoid is getting power as I can feel it clicking when I rest my fingers against it and turn the lights on, Also, the engine idles stably but very low if I unplug it.
There appears to be no leaks in the vacuum hoses.
It's almost like the stabiliser solenoid increases the idle too much and so clicks off, only for the idle to drop too low and it to click on again, and so on and so on.
Would anyone know what else I should be looking for here?
The engine is a JH - 1.8lt injected (same as Mk1 GTi)