ok i got an 89 4.0, i was just capping off my leaky heater core earlier and was looking at what at first looked like a filter in the heater hose right before going into the heater core. when i looked at it again noticed theres a little lever on the bottom of it and a small plastic line coming off the end going through the passenger side firewall. can anyone tell me what that is? i dont have a picture at the moment but i might be able to post one tomorrow.

 

That's the heater valve to control temperature. The little hose is a vacume line connected to the controls inside