91 metro ideling problem

I have a 91 metro that I haven't had but about 3 months, It ran fine when I first got it and was getting about 42 miles to the gallon. Well noe all of a sudden it want idle when it gets warm or run at all hardly at night when you turn the headlights on. I changed the pcv valve and cleand the hose and changed the air filter and plugs and wires. I am wondering if the light switch could be bad and is causing some king of break down somewhere and making it to run rough and the gas milage to go way down? Anyone have a suggestion?:screwy: :banghead: thanks.....

Have recently adopted a '91 Geo myself, have you checked your alternator?

Well I have had it running and disconnected the ground cable from the battery and it made it idel better (smother) but what it does is it starts ideling rough or dies when I turn the lights on or the cooling fan comes on.:banghead:

I have a 2000 metro and there are differences in the control system but in mine (1.3L) I know that the ECM is designed to increase the idle speed based on the engine conditions such as hot/cold, electrical loading, when a/c is turned on etc. If this is not working correctly then you could get problems with stalling. In the 2000 1.3L there is an IAC valve and the ECM sending PWM pulses to control it based on all these conditions. I presume a similar setup is there for the earlier engines and that may be malfunctioning and causing this.