Hey guys. I replaced the heater core on Sunday. took probably 5 1/2 hours over an 8 hour period. It really isnt all that bad if you aren't afraid to dive in and just start taking stuff apart.
I started unscrewing everything and unplugging all the electrical connectors, pulling off that first piece of trim that mostly just snaps in. Pulled the radio and temp. control. When you pull the temp. control, it is probably best to actually take a picture of the orientation of the vaccuum hoses to the control plug. I tried taking a mental picture but I still had to fiddle with getting them to line back up since it was hours later.
I started using the Hanes manual once I had pulled all the easy stuff. I even took out the center console tray and removed the gear shift lever (Just to make more room, which I think was convenient). Once I got the dash tilted forward I got a family member to help lift it out. One thing I didn't do was drop the steering column like it tells you in the manual. I just adjusted the wheel down all the way. It does make for a tight squeeze getting in and out and it scratched up the wheel and other bits of interior along the way, so if you care about that you should probably drop the column.
The part that hung me up the most was the firewall screws. I think there were 5 total and the manual only called out 1. My sonoma is a 99 4cyl, 2wd, ext cab, 5 speed. I had 3 nuts that were screwed onto studs that came from the housing inside the cab. One was real easy being next to the hoses that run into the core, the other two my family member got from underneath the truck I believe. Easier that way, or you can remove the front passenger tire and go through the wheel well (a previous owner did this as the flexible cover that comes down off the sheet metal was obviously cut in a hurry. But very convenient). Then there were two bolts screwed through the firewall and into the core housing. One is kind of below the core hoses, but RIGHT behind the top of the motor. For that one it is best to have a ratcheting box wrench.
The other one is the only one my manual called out. It is inside the housing on the firewall that the A/C condensor is infront of. Where the blower moter is. It tells you to remove the blower motor resistor, which is the electrical connector that is on top of the housing with three 7mm(I think) screws holding it in place. Take those out and you can see a metal mesh on the left side of the hole in the firewall that air passes through into the core housing. The screw was hard to see at first because the mesh was half covering it and the opening from removing the resistor is pretty small so it is hard to get light (or tools) in there. Luckily that bolt doesn't come all the way out, or it would never go back in, lol.
After the 5 bolts are done it is pretty self explainitory how to swap the old core with the new. I should have mentioned this earlier, but you need a 5.5mm socket for the screws on the core housing cover. There's only 2 to remove to get at the core so if you got that far and don't have one, use an adjustable wrench. Once that is done, put it all back together in reverse order. Can be tricky so take your time and do it right. Make sure you have routed all wires to thier locations in a manner that doesn't pinch them when you put the dash back in and just make sure you put all the screws back where they reasonably belong.
If I remember anything else I will edit the post so that the next guy that has to go through this has some pointers to go on. Let me know if this info helped you. I hope it has and let me know if you have a questions. I'd be happy to help if I can.
