Can you remove the heater core without removing the dash? Yes, the process is listed below, however also read the comments by John and Paul at the bottom before you decide to take the leap.
First remove the 2 heater hoses from the heater core pipes on the firewall side under the bonnet ..they are located on the drivers side at the rear of the engine.
Remove the rubber grommet situated between the pipes at the firewall
(One thing that will help a lot is a by-pass hose to connect from the engine outlet to the inlet. This will allow you to drive while working on the heater core. This one I picked up at the local auto shop and cut the ends off for length. No reason you couldn't use some flexible hose.)
Remove the panel under the steering column and remove the 2 plastic heater/aircon tubes..remember to disconnect the plugs for the temp sensors etc from the tubes.
(This is not absolutely necessary. It just gives you a little more room to work. I've done mine with it in place.)
You will see a black plastic box that I think controls the heater flaps...remove the plugs and screws (approx 5 chrome looking ones) holding the box in place (including 2 tiny philips head screws on the bracket) and remove box. Some screws are difficult to get to and you need a small screwdriver or a quarter drive socket and I think they are 6 or 8mm heads.
You will now see a black plastic cover over the heater core pipes held on by the same chrome looking screws...remove this.
(The top pic is what you will see when you climb under the dash on the driver's side. There are 4 screws on the controller box, and 4 plugs. The lower pic shows the top screws in the controller bracket. The top screw in the plastic cover in unfortunately in the shadow. Most of these screws you will have to do by feel, 'cause you can't see them from below.)
Now for the tricky bit...from under the bonnet you must get a quarter drive ratchet and appropriate sized extension (approx 1" to 2") with an 8mm socket and place it through the hole and try and locate the small hex headed screw that holds the heater core pipes to the rear of the heater box...this is the hardest thing to do on the whole job!! See tech-tip, bottom of page.
(I agree. This is a PITA. Here's a couple of pictures to give you an idea where the screw is. Itls located above and toward the left of the lower pipe. It also has a phillips head so you can use a small screwdriver. I've even used a small blade screwdriver. The screw I've used for the picture is not the same sort that is fitted.
To make it more interesting, there's a foam rubber piece that may cover the screw. You'll need to push it down and out of the way.
Once removed, go back under the dash and unscrew the 2 heater pipes from the heater core (4 screws for each pipe).
There is also a spring and a cable attached to the brackets that controls the on/off valve and that must be removed....make sure you remember where it goes back as it is quitedifficult to put back on.
9. Look now at the side of heater box itself and you will see a little bracket that holds in the heater core. It is held in place by one screw that swings out of the way when it is loosened and voila!... the heater core will slide out of the heater box.
(This bit is pretty straight forward. The cable clamp clips over the top of the bracket. Pull on the top and it will pop off easily. The bottom of the clamp fits through a hole in the bracket.
IMPORTANT. Do not remove the pipes! The top pipe will lift above the heater core as it slides out and the lower pipe will push forward enough to get it past. Leaving the pipes in will avoid the temptation to disconnect the cable from the tap. Refitting the cable is extremely difficult+++.)
Re-fitting is the opposite of the above except for a few minor points....you must slide the heater core back in WITHOUT THE PIPES ATTACHED. However before doing so read the next section...
The next part is very difficult.... before attaching the pipes you must re-attach the cable through the little hole on the pipe bracket.
(Not necessary if you have left the pipes where they are)
I found this difficult to do as you have turn the pipes and bracket upside down and round and round the get it through the hole and there isn't a lot of room to work in. Also don't forget to re-attach the little spring that helps operate the on/off valve. Remember, you must slide the heater core back in WITHOUT THE PIPES ATTACHED, then apply some good sealer (ultra grey for example) to the faces of the heater core and the pipes and then screw each of the pipes back on with the 4 screws.
Now the hard bit...you must then replace the screw that holds the pipes to the back of the heater box. This can only be done from under the bonnet and is much easier if you have 2 people...one placing the screw through the firewall on the end of a socket and the other under the dash guiding the screw into place. This is not easy (I did it by myself but wish I had help) and it is tempting to not replace it but the heater core will probably move around and vibrate and cause other problems so I recommend that it be done.
(Recommended. But I left mine out cause it just didn't seem like it was worth the effort. No rattles or vibration so far. Actually, by the time you refit the boot in the firewall and connect the hoses, the pipes can't move very much anyway. If you do put it in, I found blue-tack is great for holding the screw onto the end of the screwdriver/socket.)
Once the heater pipes are bolted on it is a simple matter of refitting all the other bits. Make sure you remember to replace the plastic cover that goes over the heater pipes before replacing the control box and make sure to plug in all plugs that were removed.
I couldn't get the rubber firewall grommet back into place without destroying it so I sealed the firewall hole with silicone (bit rough but no choice).
(Use a light oil or grease to help it slip over the pipes and into the firewall.)
It was approx 3 months since I did this job and I did remove the dash, as I didn't realise until I had done it that I didn't have to!! There are some minor things that I have probably forgotten so if you find something I haven't mentioned please let me know. A suggestion is to remove the drivers seat for more room to work under the dash.
Some additional points by John Penlington, from Unique Autosports:
We have done it both ways removing and not removing the dash. We believe removing dash is the surest way with less hassle as the pipes though the firewall can be trouble and more room to get to it, plus you can fix the top of the dash were they lift as it also is much easier to do than in the car as we do.
You don't have to pull the dash but it certainly is easier to get to if you do and yes it is the only way to fix the lifting dash problem so maybe its a good time to kill 2 birds with one stone. Also you don't have to disconnect the aircon either way so make sure that whoever does it doesn't disconnect the a/c as that can cause all sorts of other problems due to idiot a/c regassers (as I have learnt).
There is no way I would consider pulling out the dash. Using this method, you can still drive around when the work is half done and it takes much less time. It took about a day and a half to remove the dash and replace the core. It took about 4 hours to do it this way. Depending on how you go with the screw in the front of the heater box, it shouldn't take you much longer.
Tech-tip by Richard Scullion 2/10/06
Thanks for your tech article on removing the heater matrix without taking
out the dash - saved me a load of hassle. I wanted to suggest an addition
as regards the bolt you have to reach through the bulkhead. The article suggests
you should be able to get a socket on it, but I couldn't get a socket square
on at the angle I had to go through the bulkhead at. After several hours of
trying different ways I went to Halfords and bought an 8 mm spanner with a
180 degree swiveling joint on the end (picture attached). This can be pushed
through the bulkhead straight and then bends to sit square on the bolt. I
also discovered you can reach it with one of these from inside the car but
I think it's still easier to go through the bulk head.