Heater matrix removal

 I have to replace my heater matrix because of overheating problems (they go away if I bypass the matrix) tried several times to descale it with no success, so it has to come out, now I know it can be done without removing the dash, but before I make the decision as to which way to replace it, can anybody tell me which way it comes out, i.e pedal side or glove box side(RHD car)

Thanks in advance.


Yes this can be done from the drivers side easier if you remove the seat.

This can only be done on the mk 1 cars as the mk 2 has solid pipes no joining rubber hoses.

Look  up into the middle and you should see a white plastic rectangle cover which hides the heater matrix held in place with 4 screws.

Remove/unclip one of the heater cables as its in the way then move the heater control so it moves out of the way.

Undue 4 cross head screws then the matrix is in sight you have to undue 2 jublie clips then be careful as the rubber pipes have been on for a while, use a small headed screwdriver to tease them off with breaking the seal, but before doing this blow the pipes through to get any access water out easy to do this from the engine bay.

Then the matrix just slides out, pull out and up motion towards the steering column.

Place some paper down incase of any water left in the pipes.

I used so descaler that i put in the kettle and it works or acid if you have any.

Hope this helps.

cheers ian

 Right, I made a start tonight, and this is what I have done so far:-

Part 1.

  1. Lower hood.

  2. Remove drivers seat (this makes access far easier)

2.5 Pick up money from under seat, I found £3.05p.

  1. Remove panel from under steering wheel, two screws.

  2. Install strong/bright light safely under the dash, I used a 60W lead lamp with protective cover.

5 Now lay on your back with your head under steering wheel, look up to the right, you will see a white plastic cover with a thin metal rod diagonally across it, this rod needs to be removed, you will need a small screwdriver to carefully lift an internal tab to remove the rod (complete with white plastic connector) from the metal cam, this will become clear when you are looking at it.

  1. Now remove the plastic cover mentioned earlier, mine has two screws diagonally opposite each other (top right and bottom left of cover and the are a bright gold finish).The cover is now being held in place by two plastic pegs, opposite corners to the screws,  four screws were mentioned further up this thread, but mine only has two screws and two pegs, so this may change due to model year, mine is May 1991.

  2. Carefully pull the cover towards you (towards the steering column), this will now expose the heater matrix and asssociated pipework, mine look to have been weeping near the Jubilee clips.

8.Lift bonnet and drain or partially drain cooling system, I’m completely draining mine and refilling with fresh anti-freeze.

That is as far as I have got tonight, about an hours work,and so far all has gone well, I intend to finish this on Sunday, so part 2 will be posted up asap, I was going to take some photo’s but you honestly won’t need any, it really is straight forward.  I am also going out shopping to buy some new hose to join the matrix to the pipes. I will be replacing the Matrix with a used one I bought from Autolink for about £30, it is very clean and I’ve descaled it although it did not need doing, only clean water came out, no hard water scale or corrosion, I know that it is a slight gamble, but the job is so easy, that if the matrix does play up, I’ll buy a new one from MX5 parts and fit that, but hopefully it will be ok and I’ve saved more than £200.

Watch this space!!!


vspeclazza. you ar doing a good job so far, am lookoing forward to the the last bit you take out. When you say you descaled the matrix you bought, what exactly did you use to clean the interior of the matrix?  If you do fit this second hand matrix will you consider cutting the old one up into four pieces so we can see what is inside and also to get some idea of what it is that clogs the matrix up?  Photo’s of the cut up matrix would be good too. 

 Pollyanna, The de-scaler I’m using is an industrial type I got/(borrowed) from work, it’s more aggressive than what you can buy over the counter at your local diy store, 1 part descaler to 10 parts warm water, I’ll let you know the make when I go out to the garage again. As a result of de-scaling, I now know that the replacement matrix does not leak and has a uniform heat across it, i.e no cold spots.

Not to sure what I’m going to do with the old matrix yet, it all depends on it’s outside appearance, if it looks solid i’m thinking of getting it reconditioned, but what I will do, is unsolder the larger tank and have a look inside, will also post photo’s of that.


 Part 2

Didn’t go out tonight so spent some more time on the matrix removal.

  1. Lift bonnet and drain cooling system through radiator drain plug, be careful to use correct size screwdriver as the plug can be quite tight and is made of relatively soft plastic.

  2. Remove heater pipes at entrance to bulkhead, be gentle with this and do not rush it as the metel pipes (not sure what they are made of,  look like a mix of copper and brass) are very very soft and easily damaged.

  3. connect a length of heater hose (I had some in the garage) to the left hand pipe,  nearest the engine,you only need to push this on, no need for a jubilee clip, then put a jar under the right hand pipe, then blow into the hose to force the water out of the matrix this is easy to do and does not require to much puff, you will be able to remove enough fluid to clear the pipes and lower the level of fluid in the matrix.

  4. Moving back inside the car, lay some plastic sheet on the floor of the car, I used a plastic dust sheet from B&Q and covered the entire drivers side floor, tucking it up behind the pedals and under the heater box, then I laid some towels on top of the plastic sheet.

  5. Laying on your back, look up under the dash to the heater pipes and undo the two wire type hose clips, I used a small 1/4 socket set with a 10mm socket attatched, there is plenty of space to use this, when the clips are loose enough, push them up so that they are clear of the rubber hose.

  6. Now for the hard bit, you need to break the seal between the hose and the metal pipes on both ends, to do this I used a small terminal screwdriver which I bent to 90 degrees, I then forced this between the rubber hose and the pipes, working as far round as I could reach, take great care with the top pipes, the ones from the bulkhead, you DO NOT want to damage them. I should of run a stanley blade down the hoses as I’m fitting new ones, one of the reasons I didn’t use a knife was in case there was fluid inside, but as it turned out it would have been ok.

  7. Hopefully you should have freed the hoses enough to push them up and away from the matrix, I used a medium BLUNT screwdriver to do this, and took my time so as not to damage anything or slip and damage anything else.

  8. With the hoses carefully removed (I did not get any spillage, obviously the blowing worked) the matrix will pull straight out, I just needed to push a wiring loom to one side, I also kept the matrix as level as possible in case the was any fluid left inside, again no spillage.

  9. As I was leaving the job for the night, I went back to under the bonnet, I replaced the radiator plug and reconnected the two heater hoses at the bulkhead, so apart from refilling with fresh antifreeze I’m finished under there.

I’m going to refit everything on Sunday cos I’m out all day tomorrow, so part three will follow.

General:- I took the old matrix indoors and weighed it, It was 85 grams heavier than the used one I’m replacing it with, I know that’s not conclusive but I reckon that’s down to it being full of hard water scale. I next filled it with boiling water and guess what, the middle third only got lukewarm, it’s currently in the utility room sink filled with de-scaler, I don’t think it will clear though, the tube are very thin and are probably blocked solid. The descaler I’m using is an industrial kettle de-scaler sold by Tower chemical(own brand)

 Part 3.

  1. Time to fit the ‘new’ matrix, first job is to cut two pieces of heater hose to length, same as the ones you removed earlier, now get back under the dash and fit them to the heater pipes from the bulkhead, push them on so that the ends of pipes come through the hose ( they are a tight fit, I lubricated mine with some fairy liquid) then push the wire hose clips over the short length of hose you have just fitted.

  2. This is optional, but I sprayed some Neutradol air freshener into the aperture of heater matrix box.

  3. Now fit your nice new matrix back into the heater box aperture, (lubricate the two spouts with a little more fairy liquid, just a smear} it should go in very easily.

  4. The next job is to slide the short heater hose’s down onto the matrix, this for me was the most difficult task due to them being a tight fit, take your time, they will go. Now get your small 10mm socket and tighten the hose clips.

  5. As a precaution, cover the drivers footwell with some plastic and old towels.

  6. Now it’s time to re-fill the cooling system with the correct mix of fresh antifreeze, I used a 30% antifreeze to 70% water, the trick to minimize air locks is to lift the front of the car as high as possible and re-fill the system very slowly.

  7. Now look back under the dash and check for any leaks.

  8. Place a tall funnel into the radiator filler hole, cover the front of the car with more plastic sheet.

  9. Start the car and let it idle, as it warms up and the water circulates the level of water will rise up the funnel even to the point of it coming over the top, hence the tall funnel and plastic sheet, this is just air being expelled, keep topping up as required, let the engine warm up to operating temperature, keep checking inside the car to make sure there are no leaks, also you should squeeze every hose you can reach to encourage the air out. I normally do this until the cooling fan has completed two full cycles. Finally switch off engine, check water level and refit radiator cap.

  10. Back inside the car, refit the white plastic cover over the matrix and screw the two gold coloured screws into place, the bottom one is a bit fiddly, but ok if you are patient. Next reconnect the white plastic thingy to the heater flap, it just clicks into place.

  11. Now refit the cover that goes under the steering colomn, and fix in place with the two black screws that you removed and kept in a safe place!!!.

  12. Refit the drivers seat (after hoovering the carpet clean) four 14mm bolts and fit bolt covers. Don’t forget to reconnect the seat speaker cables if you have them.

That’s it folks, you should now have a nice hot heater, I’d forgotten how good they are cos mines been useless for ages.

You can now go to the pub and buy yourself a pint with the money you found under the seat.

I removed the tanks from my old matrix and this is what it looke like, not fully blocked, but enough to restrict flow and cause overheating.

I,m reasonbly competent with a spanner, but honestly I reckon anyone could do this job, my biggest regret was putting it off for so long.

If anyone else trying it needs anymore advice, pm me and I’ll do my best to help.

 An excellent write up. As soon as it warms up a bit I am going to get mine done. I think I will try to get an after market matrix to see if they work out cheaper than the one from MX5PARTS.

Did you by any chance take the temperature of the hot air of the old matrix before removing it, and then check the temperature of your replacement matrix to get some idea of the improvement in temperature?

 Thanks Pollyanna, No I didn’t take the temp at the air vents, but my old heater was so poor, there was hardly any heat coming from it, calling it lukewarm would be over generous. The differnce now is remarkable, very hot indeed, and that was with the hood down albeit in the garage, I’d imagine it’ll feel to hot with the hood up. Good luck with yours, like I said I’m avaliable to give any advice if needed and help any one whose local to me.

 Well up for trying again.

Can anyone give me tips on how to remove the rubber hose pipes that join on to the matrix from the metal pies at the bulkhead? In the limited space it is very difficult to ease them off and broke on the the heater plastic arms in the process!! Any help would be so appreciated as I want to get this bloomin’ thing done.


 When I did some work on mine I was advised to very carefully cut the hose along its length where it is fastened to the copper matrix pipes so that you can easily prise off the rubber hoses.You must cut the hose by having a Stanley knife blade positioned with the front angled edge on the copper pipe at an angle so that you don’t score the copper pipe as these pipes are very thin and you could I suppose end up with a leak when you put it all back together again.  All this entails fitting a new set of hoses of course.  This is difficult to comprehend but when my wife is back from the Doctors I will ask her to take a picture of what I have been describing to try to make it a lot clearer and post it on here.

 Sorry it has take so long to take this pictures, that should explain better the words I posted in the above post.  


 Thanks so much Pollyanna for taking the trouble to publish these. Very Useful. Did you replace them with stock generic straight hose or the genuine replacment Mazda ones? I broke the cam holding the rod in front of the cover of the matrix as very brittle. Not sure how I can fix that one as accessing it well enough to be able to effect a repair in such confined space is far from ideal. Work on that one.

Hoping now to find time WITHOUT INTERUPTION to have another go. Even considered taking out the whole dashboard to give better access but looking at the job er maybe not.

Thanks again to the fabuloushelp and tips you guys.

 I replaced the hoses with the correct shaped ones from MX5PARTS, it makes them easier to fit without any kinks in them if you the correct parts. The thought of removing the dashboard was the one reason that I didn’t do anything about changing the Matrix but now it has been shown that it can be done without removing the dashboard I am up for it when the weather warms up a bit more.

Hose no.1 can actually be replaced without removing the CAS (which messes up the timing) or the coil pack, but you need to find a pair of pliers that can get in there.  The worst bit is cutting the old hose off the engine inlet without cutting your fingers or any wires.  Hose no.2 is trivially easy.

I don’t understand how the matrix gets scaled.  My car has had Mazda green coolant in it from new, changed every two years.  I didn’t see scale on the thermostat when I changed it and the inside of the radiator looks clean.  Has someone mixed up their own coolant using hard tap water?  Generally soft tap water is OK for this job, but the stuff round here is just too hard.