idle problem

I have a 1994 Mitsubishi Libero/lancer wagon 1.8L N/A . Now my problem all started after coolant was pissing out from the top hose. Anyhow, I replaced that, and then when I put it in gear, take off and come to a stop, my idle drops under 500, rises up a bit and then drops back.. When I first start it, it idles at 1200(down from the usual 1500), seems to idle smoothly, but then as soon as i touch the throttle(even for a second) it drops down to 800. And after its been driven, even like 10metres it drops to around 500. I also rolled it down the driveway, and when i braked, it scraped the rotors, as if the car wasn't on. I Have sprayed the intake pipe with engine start, to see if there is a leak, no luck ): . Although, I loosened the clamp on the intake pipe, and saw on the bottom of the pipe there was a small tear on it just underneath the throttle, but doesn't seem big enough that it will expose a hole to the throttle. It has blown a bit of white smoke from the exhaust, and it also burns a bit of oil. Its had a timing belt/water pump, and thermostat changed about a month ago. It does drive, seems okish in drive, but I have never driven it longer than 100m or higher than 3k rpm since the troubles. Any help?