Today I replaced the heater core. Why would somebody do that? If it were leaking or clogged! Clogging happens due to wrong, or mixed or unsuitable coolant or from lime scale from topping up with hard tap water instead of demineralized water. The deposits from very little hard water can be enough. A clogged core means not much heat, usually the passenger side is affected first and more heavily than the other side. There are at least two different makes of aftermarket cores to choose from. I bought mine from Thermotec, but had an issue with ill-fitting pipes, and serious difficulties getting the pipes attached. If I were to do it again, I would try a different one instead, for example from maker Nissens.
No need to remove glove box, centre console, radio, centre vents, or gas pedal!
On models with autotemperature there might be an actuator or something else to take off for access.
Tools needed: Just basic tools, including pliers, screwdrivers, spanners size 10 and 12, a receptable for collecting coolant, a funnel for filling coolant, a little pocket mirror to aid connecting pipes, a good portable light source, and don't forget a yoga mat to lay on for helping to prevent bruises.
Hardware needed: The new heater core, enough new and correct coolant. It is imperative not to mix different coolants, even if both were suitable for the car but from different makers. If it just says "for japanese" or "for Mitsubishi" it is probably the wrong stuff. Beware: The Mirage takes a different coolant than all other Mitsubishi. In Europe Glysantin G30 is good, the same stuff as for non-diesel Toyota. No idea what they sell in the States.
How to do it:
Park the car level, not on a slope. If the core was clogged, the coolant needs replacement! Put a receptable under the left side of the radiator and open the discharge screw. Then do the same under the engine discharge screw. Lift up the little container next to the radiator cap, and empty it out. Put it back. Collect old coolant for proper disposal. Fill radiator to the top with water, and flush it all out by opening both discharge screws. Repeat flushing until it comes out totally clear. Remove the air filter box for access, three bolts size 10. Disconnect the both rubber hoses from the pipes to the core. A pair of suitable pliers helps.
Disconnect the minus pole of the car battery. Spanner size 10 needed.
Push driver seat all the way back. Lay the yoga mat on the floor where the drivers feet would be to protect you from bruises and injuries. Remove the horizontal plastic pieces under the steering wheel, including the piece above the pedals. Small philips screwdriver needed. Looking from the door, you will see the following as in the picture:
The steering ECU on the top of the above picture is next. Mark the four connectors on the steering ECU so their position is is not confused. Remove the four connectors. Remove the cable clip from the bracket of the ECU. With a size 12 spanner, remove the two bolts holding the braket and set the assy aside. The removed assembly looks like this:
Looking from the door, you can now see the heater core. The steering ECU is out of the way and it has to be in order to get the core out. It now looks like this:
Uncrimp the two pipes from the old heater core. A small flat screw driver is helpful. Remove the pipe-clamps holding the pipes onto the plastic with a philips screw driver, and save them for later. Remove the pipes.
With the pipes out of the way, you can now pull the old core straightly all the way out.
Insert the new core all the way into its place. Install both new pipes and connect them to the core. Take care to do it 100% properly! Connecting pipes to core is the most challenging thing to do. You will be laying on your back, struggling with small parts that may fall into your face, bad view, not enough room to breath, etc. You will remember all the curses you ever heared and seemingly forgot. The pocket mirror is of help to view the rear of those wretched connections.
Reinstall the pipe-holding brackets, and check they are not appying any force to the connections.
Fill new coolant until full. Check thoroughly for leaks before doing anything further!
If no leaks are detected, reinstall the steering-ECU and its four connectors.
Reconnect the battery and start the engine. While it is warming up, keep constantly replenishing the coolant so no air pockets remain in the cooling system. When the top radiator hose becomes hot, its time to put the radiator cap on. Keep the engine in idle with the cap on until the fan comes on. There is now pressure in the cooling system so don't remove the cap. Turn off engine and recheck inside for leaks.
If there are no leaks, let the car cool off. The other parts taken off can now be put back in place.
When the car has cooled for at least half an hour, remove the radiator cap to recheck coolant level and refill the external coolant canister. Observe the fill level marks on the canister.
Good luck! Of course, all at you very own risk.