Irregular idle

I just changed my heater core and now my car has an irregular idle. I'll start it up and them it will rev to 3rpm and then die down and cut off. After about 5 tries it will idle out but once I put it in drive it starts again but it doesn't cut off. It's like the car is pulling forward. My engine light is also on could any of this be caused by bad spark plugs? I plan to change them and the wires this weekend.

Is it possable that you knocked a vacuum line of while dissconecting the heater hoses at the firewall ?

Is it possable that you knocked a vacuum line of while dissconecting the heater hoses at the firewall ?

I'll back track and check it out because i had a friend put it on a lift to connect the hoses. What if they are connected though?

Could be plugs and wires, could also be the iac, but you say it started immediatly after changing the heater core ?

Could be plugs and wires, could also be the iac, but you say it started immediatly after changing the heater core ?

Yeah, it did start but the engine light would go off and on but now it won't go off. I had it check a shop and they said not to worry about it I just needed a tune up but this was in the summer. Now it cold and the car is act totally different but the heat works!!!!

Did the idle problem start right after the heater core change ? how long between ?

What size motor ?

Did the idle problem start right after the heater core change ? how long between ?

What size motor ?

No but I can't tell if it has to do with the weather or not. This is the worst its ever been. It would mostly act like that when the engine light came on but like I said before it would never cut off. The engine light cut off this morning now it runs fine but the weather isn't that cold. I bought wires and spark plugs with the intention of changing them tomorrow. It's a V6 3.1L