i just got an 89 camaro with a 5.7 from a 89 blazer and when i looked under the hood the connector was off from the temp sensor so i reconnected it and the car doesn't idle right after driving for a minute or so because when i stop at a light or or just stopped in drive the idle goes up and down slightly and then wants to die but when i take the connector off it idles at about 900 to 1000 and is finebut check engine lights comes on when disconnected and turns off when reconnected my question is what could it be?

 

Huh. That is wierd.

 

Sounds like that connector is the coolant temperature sensor for the ecm; it uses the signal to calculate the amount of fuel to deliver. I would replace it with a new one; they are not expensive.

Hope this helps.