no heat

2. Make sure that the cable for closing off the door/flap for outside air is operational, or you'll be constantly adding/mixing cold outside air into your vent air.

1. Make sure that the cable for opening/closing the door/flap for directing the air flow over the heater core is operational.

Look up and back by lying on the floor (can't remember which side of the hump) and have helper move the controls from hot to cold, and the other from Vent to Recirculate, as you make sure that the cable is actually moving, and actually connected to a lever which moves that door/flap.

And do #1 before #2.

and if you really need to get in to the heater core, here's the best how-to that I've seen

It seems like there are a lot more write ups on mazda trucking. Im going to subscribe there also. I will start tracking this down in about 2 hours as it will take me that long to get home but Ill get back on here and update.

so I back flushed the heater core and it worked momentarily. I'll deal with it until this weekend when I can run water thru it until I'm happy. I got Luke warm air either way which is a step up from cold. At least my Windows will defrost with that. thanks for all the help. I'll get a good sealing air line on it and run air thru it for about 5 min then get water and flood the thing in reverse until I'm happy.

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I had a similar problem with mine, that I fixed by using a zip tie to the little control lever. Since I have no A/C, it works great

If you can get water to flow through the heater core, then try running/sitting vinegar inside it, to dissolve hard water deposits. OK to try to flush in reverse too.

Vinegar is awful for removing scale i think. A diluted solution of phosphoric acid flushing through a system with moving water works much better if you can get the resources together to do so.

Spent a summer at a company which does this all the time. Fond memories of falling asleep in a chair whilst "minding" a pump for 6 hours straight. Phosphoric is generally pretty good about not dissolving metals, though I wouldn't run it through rubber hoses you care about, or your engine block. Provided you have the core separated from the truck, it should be just fine.

Even with vinegar, the following instructions should work. (For vinegar, skip dillution)

Get a 5 gal paint bucket, and a pump. (the cheaper, the better. Like this one
)

Hook up the pump so that it pushes into the heater core, then the heater core returns to the bucket.

Add the acid in. For phosphoric, (likely available as 75 or 85 % concentration) dillute to about 1/4 or 1/5 ratio. For vinegar, I would just run it straight.

Leave the pump whizzing away for a good 5 or 6 hours.

When finished, hook the core up to a clean water supply, and flush the acid out. Check to make sure the flow through it is good. If it is, congrats. If it isn't, start with fresh water, use more acid, give more time.

pKa for diluted phosphoric acid and vinegar are not that different. I use both to remove scale, just figured vinegar might be easier and cheaper to obtain.

You can also dissolve malic or citric acid in water and use that, that's what I use on my ice machine.

My understanding of chemistry (which is... somewhat decent) tells me that the reason Phosphoric acid works better is not PkA but because of the size of the molecules themselves.

That is: CH3COOH is less awesome than H3PO4 (in the sense that when the lil' buggers get to bouncing around and doing science stuff, the smaller, the better)

But yeah. Phosphoric isn't as easy to get as vinegar, though its not that hard to find. Hardware and paint supply stores will have it by various names, so will Lowes and Home Depot.

For example @ HomeDepot, look for "Behr Concrete Rust Stain Remover"
at other places it might just be called "Metal Prep Solution" etc etc.
Wouldn't use the jelly kind. "Naval Jelly" has other stuff in it that will make the flush harder.

By the way. Phosphoric Acid IS sewer-safe.

But yeah. Vinegar should work reasonably well, at least well enough to get flow through the heater core again, restoring most functionality. The difference is that Vinegar solutions often will get rid of the "Big chunks" and most of the blockage, but the scale that is caked on the walls of the tubes (which WILL act much as an insulating medium) is much harder to dissolve away, and very hard to tell if its around. Additionally, the little patches of scale that might linger around cause another problem...

Scale is like cholesterol. Once you get a little, more comes. Its a surface-based reaction after all. A pipe with no scale won't clog up very fast. A pipe with 1% blockage will begin to scale up at the surface. Then, bam! All heck breaks loose. Scale builds up on scale, which begins to clog up the pipe. When the pipe gets clogged, flow slows down, which lets more scale build up ontop of scale... and when it is all over, heart attack.

So, the more clean you can get the pipes inside the heater core, the longer it will last, and the more heating it will do.

For all the effort that could be put into this to restore the heater core, I'd spend the extra few dollars, and the little bit of hardware-store hopping to make the inside of the core good as new. That way, you won't need to worry about this... for oh, another 20 years?

No offense, mister Cusser. Believe me. If $1.35/gallon vinegar worked as well as phosphoric acid, I would have been flushing gallons of that through copper pipes. The truth is though, that PkA isn't everything, and the pros use Phosphoric for good reasons.

Though, in a pinch, vinegar will work. Maybe try and use a bendy brush (like the kind they use to scrub trumpets and such out) through the core halfway through the flush to try and break up more of that nasty scale. (The glassy kind is the WORST.) Stay vigilant.

(This is why I use cheap walmart-bought spring water in my coolant system instead of tap. Funny thing, that. Its the only bottled water I buy)

I'm a little late to this party but if anyone is still hanging around... I'm having the same issues just like warm air coming out of the vents. I've back flush the core in the past (twice) it worked back then but I don't feel that it work this last time. The other day my vent control lever popped and went limp on me. I always forget that the heater Controls are out in the open duh. So I guess after reading this I'll check the doors the the flow. Any thoughts are welcome. Is it a b*tch pulling the dash to change the heater core? I want my good heat back!

Crewcab Steve

Don't jump on thinking that it's a bad heater core. Second thing to try would be to pull off the inlet and outlet hoses and determine whether there is a good flow through the heater core (like with a garden hose) or if it's plugged.

First would be to reconnect the cable and check the flaps. And make sure that ALL the air is bled out of the cooling system. Did you notice that I capitalized and bolded "ALL"? I jack up the front end and run with cap off (heater on full), topping off until it's full, takes like 20 minutes.

Third would be to make sure you have a good Stant or Mazda 192/195F thermostat, like I run year round even in Arizona. This will NOT cause overheating, but the heat delivery will be way better than with a 180F thermostat. If you need to install one, make sure it's oriented the correct way, and see above about bleeding out all the air.