Attention: TDR Forum Junkies 
RDMueller
TDR MEMBER
Last winter I began to notice the telltale signs of a heater core leak - smell of antifreeze in the cab and ineffective defrost. I finally got around to replacing it last week and wanted to recount my experience here for the benefit of those who may be about to undertake this job. FYI, I have a 2001 2500 with the 5 speed manual, 2WD. There are some minor differences with the automatic, but the majority of this should apply to any second generation truck.
First off I want to say it was not as bad as I was expecting it to be. You don't need any special tools and there is nothing particularly challenging about it. Still, it is a big job and can be tedious at times. Although I think many complete the job in maybe 3 hours or even less, I'd budget a full day if you've never done it before. It took me about 8 hours start to finish, but I really went slowly and methodically, plus I took several breaks and stopped several times to review some of the instructions and re-watch some parts of a few YouTube videos. I would estimate that my actual time working on the project was roughly 4-5 hours at most.
Here's another thing to know - if you follow the Factory Service Manual, you must recover the R134a and disconnect the a/c lines thru the firewall, completely remove the dash from the vehicle (which also involves removing the steering wheel), and you will completely remove the plenum/air box from the firewall. While this may be the proper/best/correct way to do this job, I decided not to go this route. Having to "open up" the a/c system alone was a non-starter for me personally. If you are having other issues or want to replace your evaporator while you are in there, it might be worth it to follow the FSM. However, what I can tell you is that if all you want to do is replace the heater core, none of that is necessary.
I definitely would spend some time reading some of the write-ups here and elsewhere on the web. Also, watch a couple of the YouTube videos and read the comments. Issue 59 of TDR has a good article. The more you learn ahead of time, the easier it will be.
Anyway, I'll try to recount the experience with as much detail as I can remember. Some of these things don't necessarily have to be done in exactly this order, but this is how I did it.
1 Very important! Remove both battery negative cables and secure them so they cannot come in contact with the neg post (to prevent possible air bag deployment).
2 Partially remove both the left and right bottom door trim/threshold pieces. That gives you access to a single screw that secures each side cowl/trim panel in place. Remove both the left and right trim panels (they each have only the one screw, after removing the screw, just tug out and away on the panel). Removing these trim panels gives you access to the lower bolts that secure the dash to the truck.
3 Remove the glove box, the instrument panel trim bezel (by carefully prying it away from the panel), the little storage cubby below the radio, and the trim panel below the steering column.
4 Remove the 2 screws that secure the hood release to the dash and let it drop to the floor.
5 Remove the parking brake release handle linkage rod from the parking brake mechanism.
6 (Applies to 5 speed manual) Remove the gear shift lever and the entire cup holder assembly that surrounds the gear shift.
7 After removing the cup holder assembly, you will see the airbag module at the base of the dash. Remove the 4 bolts that secure the airbag module to the floor, remove the wiring harness and set aside, and remove the 2 nuts that secure the airbag module to the dash. It may be necessary to loosen the 2 bolts on the side bracket as well. Remove the airbag module.
8 There are several wiring harness connectors that need to be disconnected on the driver's side. One is very large and has a single bolt in the middle that holds it in place. There is a 2nd small plug adjacent to the big one. Then I think there are 2-3 other wiring connectors that need to be disconnected, all on the drivers side. Just place your head on the floor near the pedals and shine up with a flashlight looking for connectors to disconnect. While you are there, look for the a/c control vacuum harness connector (it is in the vicinity of the a/c controls and is a round connector with several small rubber tubes going into it). Disconnect it as well.
9 Remove the 2 nuts at the bottom of the steering column that secure the steering column to the dash. Lower the steering column and let the steering wheel rest on the driver's seat.
10 Loosen, but do not remove the left and right instrument panel roll down bracket bolts. These are the bolts that are located behind the side cowl/trim panels you removed in step 2.
11 Remove the 5 screws at the top of the dash near the base of the windshield.
12 Carefully pull the dash aft. It will pivot on the bolts you loosened in step 10. It should move freely. If not, recheck everything to make sure you didn't miss removing something that needs to be removed. You should pivot the dash as far back as it will go at this point. Now that you have a clear view behind the dash, look for anything else that needs to be disconnected. I did leave a few wires connected near the headlight switch because they didn't seem to be a problem if you are not removing the entire dash. It is also necessary to disconnect the coaxial connector on the right side that is for the radio antenna.
13 At this point you can easily see the a/c plenum box and you can see the top of the heater core. However, you won't have any room to work and will need to move the dash further aft to continue. With the dash essentially resting on the lowered steering column, you can now lift the panel roll down brackets off of the bolts. I had to further loosen the bolts first. Now the instrument panel is free. I moved the right side of the panel all the way aft so it was nearly up against the seat back. Because of the steering column, the left side of the panel stayed forward and this was not an issue.
One thing I didn't like was the way the dash was resting on the steering column top trim. After I finished the job, I saw some pics where someone used cargo straps to support the dash upwards where it wasn't resting on the steering column. I would highly recommend doing this. The way I did it, the metal frame of the panel scratched up the top of the steering column trim. I could easily see this piece getting cracked as well if you are not careful. Lifting it with straps would prevent this from happening.
14 Now it's time to get under the hood. Remove the entire air filter box. Remove the PCM from the firewall (not necessary to disconnect the wiring harnesses, just set the PCM aside). Remove both heater hoses. I found it easier to just cut them off and replace them rather than trying to pry them off the pipes in this hard to reach area. Remove the metal clips on the a/c lines near the firewall (you are not actually disconnecting the lines, just removing the clips so the pipes can freely move forward thru the firewall. Now find and remove the 4 nuts that secure the plenum box to the firewall. One is behind the PCM, one is basically behind the upper left corner of the engine valve box cover, one is at the base of the a/c accumulator, and there is one more that I think was below the PCM.
15 Back inside there are 2 more nuts or bolts securing the plenum to the firewall that have to come off. One is at the upper right hand side of the plenum box and a second that was near the heater core and also has a grounding strap attached. Also remove the 2 hold down clips that secure the heater core to the plenum.
16 At this point, the plenum box should be free to pull away from the firewall. You will need about 4" or so in order for the heater core pipes to clear the hole in the firewall. Be careful since the a/c lines are still connected. They are the only thing keeping you from just removing the plenum entirely. My concern was that if I pulled too hard I might damage the a/c lines or connections. I alternated between pulling on the plenum and then going back under the hood and pushing gently on the a/c lines.
17 Once the heater core pipes are clear of the firewall, lift it straight up and out of the plenum. I also used a box cutter to cut away about a 4" section of the foam insulation directly above where the pipes pass thru the firewall. This saved me from having to pull the plenum even further back from the firewall, which may have been impossible with the a/c lines still connected. I used some adhesive to replace the insulation when I was done.
18 I used rags to clean up the antifreeze out of the plenum. Then install the new heater core and fasten it down with the clips. Carefully push the plenum back up against the firewall, lining up the heater core pipes with the hole and then the mounting studs thru the holes in the firewall.
19 Now you can start putting everything back together pretty much in the exact opposite order of how you took it apart.
First off I want to say it was not as bad as I was expecting it to be. You don't need any special tools and there is nothing particularly challenging about it. Still, it is a big job and can be tedious at times. Although I think many complete the job in maybe 3 hours or even less, I'd budget a full day if you've never done it before. It took me about 8 hours start to finish, but I really went slowly and methodically, plus I took several breaks and stopped several times to review some of the instructions and re-watch some parts of a few YouTube videos. I would estimate that my actual time working on the project was roughly 4-5 hours at most.
Here's another thing to know - if you follow the Factory Service Manual, you must recover the R134a and disconnect the a/c lines thru the firewall, completely remove the dash from the vehicle (which also involves removing the steering wheel), and you will completely remove the plenum/air box from the firewall. While this may be the proper/best/correct way to do this job, I decided not to go this route. Having to "open up" the a/c system alone was a non-starter for me personally. If you are having other issues or want to replace your evaporator while you are in there, it might be worth it to follow the FSM. However, what I can tell you is that if all you want to do is replace the heater core, none of that is necessary.
I definitely would spend some time reading some of the write-ups here and elsewhere on the web. Also, watch a couple of the YouTube videos and read the comments. Issue 59 of TDR has a good article. The more you learn ahead of time, the easier it will be.
Anyway, I'll try to recount the experience with as much detail as I can remember. Some of these things don't necessarily have to be done in exactly this order, but this is how I did it.
1 Very important! Remove both battery negative cables and secure them so they cannot come in contact with the neg post (to prevent possible air bag deployment).
2 Partially remove both the left and right bottom door trim/threshold pieces. That gives you access to a single screw that secures each side cowl/trim panel in place. Remove both the left and right trim panels (they each have only the one screw, after removing the screw, just tug out and away on the panel). Removing these trim panels gives you access to the lower bolts that secure the dash to the truck.
3 Remove the glove box, the instrument panel trim bezel (by carefully prying it away from the panel), the little storage cubby below the radio, and the trim panel below the steering column.
4 Remove the 2 screws that secure the hood release to the dash and let it drop to the floor.
5 Remove the parking brake release handle linkage rod from the parking brake mechanism.
6 (Applies to 5 speed manual) Remove the gear shift lever and the entire cup holder assembly that surrounds the gear shift.
7 After removing the cup holder assembly, you will see the airbag module at the base of the dash. Remove the 4 bolts that secure the airbag module to the floor, remove the wiring harness and set aside, and remove the 2 nuts that secure the airbag module to the dash. It may be necessary to loosen the 2 bolts on the side bracket as well. Remove the airbag module.
8 There are several wiring harness connectors that need to be disconnected on the driver's side. One is very large and has a single bolt in the middle that holds it in place. There is a 2nd small plug adjacent to the big one. Then I think there are 2-3 other wiring connectors that need to be disconnected, all on the drivers side. Just place your head on the floor near the pedals and shine up with a flashlight looking for connectors to disconnect. While you are there, look for the a/c control vacuum harness connector (it is in the vicinity of the a/c controls and is a round connector with several small rubber tubes going into it). Disconnect it as well.
9 Remove the 2 nuts at the bottom of the steering column that secure the steering column to the dash. Lower the steering column and let the steering wheel rest on the driver's seat.
10 Loosen, but do not remove the left and right instrument panel roll down bracket bolts. These are the bolts that are located behind the side cowl/trim panels you removed in step 2.
11 Remove the 5 screws at the top of the dash near the base of the windshield.
12 Carefully pull the dash aft. It will pivot on the bolts you loosened in step 10. It should move freely. If not, recheck everything to make sure you didn't miss removing something that needs to be removed. You should pivot the dash as far back as it will go at this point. Now that you have a clear view behind the dash, look for anything else that needs to be disconnected. I did leave a few wires connected near the headlight switch because they didn't seem to be a problem if you are not removing the entire dash. It is also necessary to disconnect the coaxial connector on the right side that is for the radio antenna.
13 At this point you can easily see the a/c plenum box and you can see the top of the heater core. However, you won't have any room to work and will need to move the dash further aft to continue. With the dash essentially resting on the lowered steering column, you can now lift the panel roll down brackets off of the bolts. I had to further loosen the bolts first. Now the instrument panel is free. I moved the right side of the panel all the way aft so it was nearly up against the seat back. Because of the steering column, the left side of the panel stayed forward and this was not an issue.
One thing I didn't like was the way the dash was resting on the steering column top trim. After I finished the job, I saw some pics where someone used cargo straps to support the dash upwards where it wasn't resting on the steering column. I would highly recommend doing this. The way I did it, the metal frame of the panel scratched up the top of the steering column trim. I could easily see this piece getting cracked as well if you are not careful. Lifting it with straps would prevent this from happening.
14 Now it's time to get under the hood. Remove the entire air filter box. Remove the PCM from the firewall (not necessary to disconnect the wiring harnesses, just set the PCM aside). Remove both heater hoses. I found it easier to just cut them off and replace them rather than trying to pry them off the pipes in this hard to reach area. Remove the metal clips on the a/c lines near the firewall (you are not actually disconnecting the lines, just removing the clips so the pipes can freely move forward thru the firewall. Now find and remove the 4 nuts that secure the plenum box to the firewall. One is behind the PCM, one is basically behind the upper left corner of the engine valve box cover, one is at the base of the a/c accumulator, and there is one more that I think was below the PCM.
15 Back inside there are 2 more nuts or bolts securing the plenum to the firewall that have to come off. One is at the upper right hand side of the plenum box and a second that was near the heater core and also has a grounding strap attached. Also remove the 2 hold down clips that secure the heater core to the plenum.
16 At this point, the plenum box should be free to pull away from the firewall. You will need about 4" or so in order for the heater core pipes to clear the hole in the firewall. Be careful since the a/c lines are still connected. They are the only thing keeping you from just removing the plenum entirely. My concern was that if I pulled too hard I might damage the a/c lines or connections. I alternated between pulling on the plenum and then going back under the hood and pushing gently on the a/c lines.
17 Once the heater core pipes are clear of the firewall, lift it straight up and out of the plenum. I also used a box cutter to cut away about a 4" section of the foam insulation directly above where the pipes pass thru the firewall. This saved me from having to pull the plenum even further back from the firewall, which may have been impossible with the a/c lines still connected. I used some adhesive to replace the insulation when I was done.
18 I used rags to clean up the antifreeze out of the plenum. Then install the new heater core and fasten it down with the clips. Carefully push the plenum back up against the firewall, lining up the heater core pipes with the hole and then the mounting studs thru the holes in the firewall.
19 Now you can start putting everything back together pretty much in the exact opposite order of how you took it apart.
Thanks!
