I literally just finished up removing the heater core from my 1990 Iroc-Z 305. I removed both hoses going in through the firewall and plugged the hose from the radiator directly into the coolant hose that goes around the air cleaner and goes down into the engine by the distributor. After I patched everything back up, I started the engine and it was a very rough experience. After 3-4 seconds of roughness, it just kind of slipped into a smooth run. I let it idle for 30 seconds then went on a 5 min drive to bring it up to temperature and burn off any coolant that might have dripped on anything significant. Pulled in, turned it off for 10 min then started the engine again with the same amount of roughness. After lookin around and re tracing the removal, I did notice I unplugged a plastic hub the heater hoses used to run through with a small vacuum hose going into the top of it. Am I having a problem with the vacuum line? I'm kind of stumped. Any suggestions are more than appreciated.

You may have unhooked the heater control valve. If you bypassed the valve, it may be affecting the car. It may need that valve at least plugged into the system to run properly. The vacuum line on the valve may have something to do with it. Are you sure that you didn't disturb anything else along the way? Maybe knocked a plug wire loose or something? Plug end or disributor end?

Which side of the firewall is the heater control valve? Is it the one thats attached to the coolant hose setup under the hood that connects all 3 together? I was very careful when pulling the stuff out because I've accidentally knocked stuff around before by trying to move quick and I've learned my lesson in that aspect. Also, I can't seem to pull out the metal hose that connects the heater hoses and sits right next to the passanger wheel well. I'm to the point of having to cut it out if I can't maneuver it but cutting it will be a pain.