DIY: Heater-Core Bypass Mod
 

This mod is as easy as it gets. But the big question of course is, why?

Simple: my stupid HVAC control panel is erratic as hell and I am too lazy to take it in, plus I don't want those cheese-tards at the dealer messing with my center console because I have a ton of computer related wiring and lights mounted there. I doubt they would let me change it my self in the parking lot.

Even when the thing works halfway normal, I still am constantly getting hot fricking air in my cabin no matter what vent/temp settings. I have even tried removing the arm off the cabin vent actuator, but hot air still leaks in. Even if it just trickles in with the fan off, it is annoying to be driving around in beautiful 85-90 deg weather at night-time with the windows down, and get 100+ deg heat trickling in through the vents! I don't use the heater at all this time of year (and we don't get fogged up windows here in AZ much), and when I do need it, I want just floor heat, but the stupid AC comes on by default with it, so the Mazda engineers really pissed me off with that one. So I said F^$% it and disconnected the heater core from the coolant stream.

There are two large rubbery hoses coming off the HVAC interface on the firewall. The HVAC interface is closer to the car's centerline relative to the window washer fluid reservoir (universal terminology for the right hand drive fellows out there). The other two hard lines are AC, don't mess with those.


Do this mod when your car is full cold to avoid coolant burns!
Simply unbolt your washer fluid reservoir (two 10mm bolts) and put it aside to gain easy access to the interface panel. Now stuff some rags under the interface area to catch coolant.

There are two coolant line attachments on the interface panel, and one of those leads to a third connection (via a short piece of hose) to a hard line which crosses over to the other side of the car.

Take a set of large pliers and remove the hose clamp on the rubber coolant line that is on the left side of the interface (as you are facing it).

Next, remove the clamp on the hose end that is connected to that crossing hard line.

Un plug these two hose ends, try not to spill much coolant (not much will come out if you are careful).

Replug the one that came from the left side of the interface panel straight to the crossover hard line. Re clamp.

Now the only one remaining is the short line that is still connected to the right side of the interface panel. Simply bend that one around and loop it back into the interface panel's left side coolant connection, effectively sealing off the heater core, keeping the remaining coolant inside for whenever you need to undo this mod.

It should take you half as much time to do this mod as it did to read it. It is a pretty useless mod, but I am very happy now that I don't get any hot air mixed in (EVER) and even my AC seems cooler too.

I would imagine overall water pressure may have increased slightly as well, without that heater core in play. Interested to hear your thoughts on that.

-C

P.s. if you use your interior heating system for emergency auxiliary engine cooling, this mod is not for you! But in your case, see my Flex-A-Lite fan controller mod instead!

A bettter mod would be to get a manual shutoff valve mounted inline to the supply hose, that way it can be turned on and off at will.
I might just try that mod myself to see if the A/C works any better.

Oh yea, definitely...matter of fact I was looking around at work for a suitable ball valve and some fittings, but I figured I just manually bypass it and see what happens... here is the first update.... kinda disappointing.

Today I got to see something very interesting with this mod.... the perfect conditions happened today that really gave me an insight on the RX-8 HVAC and the RX-8 in general. My car was parked in direct sunlight for approx 4 hours, ambient temps were 104-107 roughly. Storm was coming, was going to try to beat it home (that didn't happen). As most of you know by now, the RX-8 is a frickin heat sponge, and it the interior was sweltering. Windows were up cause of the rain, but I decided to use the vent/fan for cooling instead of A/C since the temperature was dropping outside. And boy did it drop... in 5 minutes of driving, the ambient went from 104 to 71. BUT, the air temp coming out of the vents was no where NEAR the outside temp.... even at a standstill, with hand out the window (i.e. no false windchill factor) it was obvious the vent temps were still running much higher than they should be. Obviously, it is the heat soaked dash board heating the air, and it takes a LONG time for the temps to come down without the aide of the A/C. Good news is, I don't get the random blasts of OVEN air from the heater core anymore, but it is obvious there is some serious heatsoaked ductwork in the RX-8 dash that stifles and heats the flow of fresh incoming air. Eventually the temps did come down though after about 25-30 min of driving in 71 degree ambient temps.

:uhh: The flip-side of this would be a summer time "disable" for the a/c, stop it turning on whenever it feels like.......

The compressor clutch has it's own fuse, a 10 amp near the front of the under-hood box. If the fuse is pulled, the ECU still thinks it's running the show, so it acts normal. (....pssst:uhh: don't let it know what you did!)

I'm rigging a switch next to my fan-on-full switch for this purpose. I don't want to loose my winter defrost capability (just in case I find myself in that kind of situation ever again).

That's a good one Stealth, I will actually use that idea in the winter time so I can use the floor heat without it robbing horsepower when it turns the A/C on it that mode (I can understand A/C in defrost mode, but just floor heat mode??? C'mon!)
'Course I will have to have re-hooked up my heater core again by then!

Yea there you go MM, switch it instead....a Master Override A/C switch...

Just as a question, do you know the size of the fittings on the heater system? I get valves from O'reilly Auto Parts or the local big commercial A/C shop but they are 5/8 or larger for the Const. Equipment I deal with.

does by passing heater core do any lasting damage or any damage at all to the engine ?

No.

Just Did it today, definitely took a lot more time doing it than reading :P
Few advice, get pliers. One for unlocking the clips. The other for taking the rubber pipes off.
Years of heat and rust has made them stuck to the inner pipe.
With my fat hands, little room to maneuver and lack of pliers. I unded up getting plenty of scruffs.