I recently ripped apart my dash to install a keyless entry system, and through a series of unrelated circumstances the cable running from the temperature control dial on the dash to a valve inside a tube under the engine became jammed. Instead of fixing the jam, I took the easy route and popped the hood and opened the valve manually, which gave me heat. However, after a few days of this, I started getting no heat at all. Sometimes when I first turn up the fan I get hot air that quickly turns back to cold. I double checked the valve, which is still wide open. What I need to know is if I fried something like my heater core or my thermostat, and if there's a way to fix it. I would really appreciate any help with this.

 

i suspect you didnt open your cooling system to allow air in, or your not losing coolant in any way....so give this a shot...

there are 2 heater hoses under your hood that route to the heater core...one flows in (this one has the valve inline) and one flows out...
with the car completely warm feel both hoses...they both should be hot (with the valve open of course). if the return hose (one without the valve) is not very hot then suspect your heater core is plugged....
if your heater core is plugged then replace it...or sometimes you can get away with flushing it...i usually use a hose and compressed air - this can get messy but its effective
this is a good starting point

 

I checked what I'm pretty sure is the input/output hoses and they're both hot after running the car until it's heated up, so I don't think it's the core. Also, I forgot to mention this before, when I shut off the engine the radiator fan kicks on and off sporadically, I'm not sure if this is normal or not, but since it's part of the heating/cooling system I thought I should say something about it.

 

Have you tried actually fixing the jam yet ?

 

No, because it wouldn't solve anything. The jam is just a cable that opens and closes a valve, there's nothing about it that could be causing my problem. Once I get the heater actually working again I'll probably bore out that grommet to a larger diameter to fit all my cords.