Poor or no heat issue with 1992 Mazda 626

[Digger89L]

Here’s my long, sad tale of misery ….hope someone has the answers: My 1992 626LX sedan is only blowing warm air ..and I’m getting worried now as the minus 30 stuff is well on its way!! (Western Canada)

This past weekend I drained the old coolant, took the old thermostat out, added a can of coolant system flush, re-filled the system with clean water, ran it for 20 minutes, drained out the water & flush stuff, and then also disconnected the heater core hose (top one), pinched the bottom hose closed and poured more coolant flush & water mix into the heater core, let it sit for 30 minutes, and then drained. Took off the bottom core hose, and blew air through the core, put the hoses back on, put a new thermostat in, filled with new 50-50 coolant distilled water mix. And now I see no improvement in the heat coming from the heater. The temp gauge goes up to above half-way when the car has run for about 5 minutes on the highway, but there’s very little warmth being kicked out of the heater. Did I get a bad thermostat? Do I need a new rad cap?

The blower motor is kicking out slightly warm air only. All four speeds work on the blower as well. When I set the air selector to “recirculate” (meaning I’m re-using interior air) it doesn’t get any warmer either. I’m thinking maybe the flap in the heater system is not aligning correctly, and letting in too much cold air from the outside, and thereby cooling the heated air from the heater core before it gets into the car.

I took the car for a spin down the highway a couple of miles. Starting off (because the engine was warmed up) I had decent warm air coming out of the heater, but as soon as I got to highway speed (100 kmh) the airflow cooled off considerably. I switched the heater control from HOT to COLD …and the airflow went completely cold. The back to HOT, and the airflow warmed a bit. That tells me the valve in the heater core that controls coolant flow is working properly (right?). Got back from my test drive, and checked the hoses again: top rad hose fairly warm …bottom rad hose still just warmish. What the H**L does that mean? Blocked rad core?? I just flushed it out a week or so back …no indication that it was blocked. ????

Yesterday was a frustrating afternoon. 1. I drained the coolant and took out the new thermostat that I had installed earlier, and did the boiling water trick. It opened fully at 193 degrees. (I put the thermostat in a 1 litre pyrex container of water and microwaved it until the water was boiling. Then I used a digital candy thermometer to measure the temperature.) So, I was able to confirm that the thermostat was working properly. (tested the one I took out …the OEM two-stage type …it was working okay too. $21 wasted there).

2. I put the thermostat back in, and then put the coolant back into the system. Warmed the car up until the cooling fan came on….temp gauge was on the high side of the midrange on the gauge. At that point, the heater was putting out a decent amount of heat (but it was +4C outside!).

3. This time, as I had warmed up the car until the cooling fans came on, the bottom rad hose was hot as well: so, I know the rad is not blocked; the thermostat was functioning properly.

4. I took the car for a spin down the highway again ….and still not very much heat at highway speed. I realize this could partially be a result of the coolant cooling down a bit, but the temp gauge did not move appreciably. But still very weird.

5. As a last resort, I tried the old red-neck winter fix: cardboard in front of the rad. I cut a piece of corrugated cardboard and covered about 90% of the radiator ..cardboard right against the rad. Did the road test again, and surprisingly the output from the heater remained considerably warmer. I know that it was +4C (+39F) yesterday, and that would clearly impact how well the heater worked …so I’m anxious to see if this fix will hold up when it gets down to -35C again.

So …after all this, I still haven’t nailed down the exact cause of this problem …… input please???

Topic

[Digger89L]

Quoted From dreamer2355:

One of your radiator hoses will be cooler than the other depending on the coolant circulation flow. BOTTOM RAD HOSE WAS ALMOST STONE COLD …WHILE TOP HOSE WAS ALMOST TOO HOT TO TOUCH. NOT NORMAL IN MY EXPERIENCE.

(I am going to go on the assumption you understand how the cooling system works and how the radiator acts like a heat exchanger?) I REFUSE TO COMMENT ON THIS “COMMENT” …I AM A 61 YEAR OLD MAN WITH MANY YEARS OF AUTO CARE AND MAINTENANCE EXPERIENCE …..

Out of curiosity, what ratio of antifreeze and water do you have? 50-50 AS RECOMMENDED

There should be some change however in the level of coolant over flow in the reservoir. Obviously fluids will expand when heated. AGAIN, NOT TO OVERSTATE THE OBVIOUS…THIS GOES WITHOUT SAYING.

Does anyone have any helpful input??

[djdevon3]

When you flushed the heater core everything flowed out nicely from both inlet/outlet I assume. You flushed in both directions through the core right? Why it’s now acting like it has an obstruction is weird. It’s possible the obstruction isn’t in the heater core but in the hose leading to the heater core from the engine. Sounds like it’s time to do another drain and check out the flow through the heater core as well as the hoses. Obstruction has to be there somewhere you just haven’t found it yet.

[dreamer2355]

My post was not meant to offend you at all regarding how the cooling system works.

I am trying to give you the best advice as i can with the conflicting information your posting seeing you are still struggling with the same issue.

Does the lower hose feel pressurized? Did you check for cross flow in the radiator?

I have a feeling you have a combustion leak that is enabling air into the system.

I think that ‘possibility’ has already been stated before in this thread but we never got an answer. Do you have the tools at home to check for this?

[dreamer2355]

Quoted From djdevon3:

Might want to let the car cool down before adding coolant after a spirited drive. Even Eric isn’t immune to getting a face full of coolant but steaming hot coolant is something that can change your life. I had a neighbor who burned his face really bad taking a tractor rad cap off while he was working it in the field. Some mistakes follow you for the rest of your life. Let the car cool first. In -30 weather that shouldn’t take too long.

I have no idea how i missed this post. On this vintage of vehicle, you would add the coolant into the over flow tank which is not pressurized, if in fact the vehicle actually needed it.

[Digger89L]

When I initially started dealing with this problem, I checked for crossflow through the rad when I did the two-times rad flush and system clean – lots of flow.
And, of course, I know that is is not a pressurized system, and that I have to add coolant into the overflow / expansion reservoir, and not directly into the rad – hot, or cold.

I don’t feel that I am posting “conflicting information” …I just trying to clearly explain the problems I’m experiencing.

If I have a combustion leak into the coolant jacket (forcing air into the coolant system) would I not also have a coolant leak into the oil system, or at least be burning off coolant, or have evidence of coolant contamination in the oil filler cap, or in the crankcase? I have none of these symptoms. As well, I’m advised that air leaking into the coolant system would almost always cause overheating, as opposed to underheating. Since the coolant system in the 92 Mazda 626 is not a closed, pressurized system, it makes sense to me that any air that might be in the coolant system would be automatically purged into the coolant reservoir during the normal heating up and cooling down process ..not quite sure why it would get “trapped” somewhere in the cooling system.

I’m going to do another full system burp, using the special overflow funnel system, and see if I can purge any more trapped air. As I have already replaced the top and bottom rad hoses, I will next look at replacing the heater core hoses, as it has been noted that deteriorated hoses may allow air into system without the hoses themselves showing evidence of coolant leakage.

[380]

If the problem is truly within the heater core i think the only way your going to get it fixed is to replace it. i would go through and eliminate all other possibilities and then just replace the core.

[EricTheCarGuy]

I’d first like to say that system IS pressurized as the radiator cap is there to perform that exact task (and it is an important one), I talk about this in detail in the Overheat video that I posted for you. To bleed the system please follow the instructions in the Bleeding a cooling system video that I posted for you, the MOST COMMON CAUSE OF HEATING AND OVERHEAT PROBLEMS IS AIR IN THE COOLING SYSTEM so this is always the first place to start, once again you don’t need a bleeder valve as they just put those there to help fill the system better.

Second a combustion leak does NOT mean that you will have coolant in the oil or vice versa, it depends on where the leak is, if it’s directly in the combustion chamber you will NOT see coolant in the oil but the coolant will be consumed in the engine and eventually cause an overheat issue when the coolant gets low enough, once again the information in the Overheat video I posted shows how to check for a coolant leak into the combustion chamber.

Please follow these instructions and watch the suggested videos as they come from years of experience dealing with issues like this.

[djdevon3]

I don’t think he ever mentioned missing coolant. Far as I can tell from the topic it’s just not circulating correctly. Combustion gas in the coolant would be a good thing to check for though. Over a long period of time would combustion gas turn the coolant a different color? Say from lime green what?

[dreamer2355]

Quoted From djdevon3:

I don’t think he ever mentioned missing coolant. Far as I can tell from the topic it’s just not circulating correctly. Combustion gas in the coolant would be a good thing to check for though. Over a long period of time would combustion gas turn the coolant a different color? Say from lime green what?

It would dependent on the severity of the issue.

The combustion leak and the cooling system pressure test was noted to be checked on the first page of the thread i believe.

To the original poster – The block tester and cooling system pressure tester can be loaned out at any good auto parts store. I would do those tests just for a piece of mind including the radiator cap.

Good luck and keep us posted.

[Digger89L]

Just to re-cap: new rad cap, new upper and lower rad hoses, using OEM original thermostat (testing and working okay), complete coolant system flush X 2 , quadruple flush of heater core (twice with coolant system flush, twice with CLR), existing heater core hoses trimmed back and reinstalled with new screw clamps, new 50-50 coolant installed.

This past weekend, I did another complete “system burp” using the special funnel, and with the front of the car elevated. (noticed minimal amount of air bubbles being expelled).

Did a 30 km test run: engine heats up to normal within first 3 kms (ambient outside temp at -5C). Generous amount of heat being produced from heater, and all positions of heater controls working normally …however, lower rad hose still quite cool to the touch.

Did extensive internet search for “lower rad hose running cool” and the consensus seems to be that this is “normal” in a properly functioning cooling system.

Fortunately, the weather has been SO mild here lately …and with spring on the way …I may not get a chance to test the heater’s heating ability until next winter!! here’s hoping! …..

[djdevon3]

In case anyone wants to check out the factory manual… the heater section is from pages 15-126 to 15-138
http://www.pmx626.info/index.php?dir=Ma … 988+WSM%2F

[EricTheCarGuy]

That is correct that the lower radiator hose should be much cooler than the top hose as it is the return to the engine after the coolant has been run through the radiator, if the radiator is working properly this is the normal condition of that hose.

[Digger89L]

Quoted From EricTheCarGuy:

That is correct that the lower radiator hose should be much cooler than the top hose as it is the return to the engine after the coolant has been run through the radiator, if the radiator is working properly this is the normal condition of that hose.

Well, thanks for confirming that, Eric. After this last (and hopefully final) burping session I haven’t noticed any change in the heat output ..of course, as I said, the weather has been VERY mild here this winter, and its not really the best time to be testing a car’s heater. I’ll chime back in if I have any more to report.

Thanks again for all the input, guys.

[EricTheCarGuy]

It’s my hope the issue is solved but thanks for keeping us up to date and let us know if you notice any changes.

[college man]

sorry to chime in so late.what happens with the air in the system is the air causes the water pump to cavitate.(not move fluid) the air in the pump will create a
steam water effect. fluid will not move with air.I had this same problem on one of my nissan. it took days to rid the air, when we drove the car the heat
would be ok.but when we stopped and idled the heat went cold. what I did was kept taking off the radiator cap and squeezing the upper and lower
heater hoses.I even squeezed the heater hoses.I could watch small bubbles come out of the top of the radiator.if you bought the spill proof funnel
try snapping the throttle 2-3 times. also try the hose massage with the engine off.keep the over flow full and keep at it. I hope you post back with
good news. good luck and keep us posted. C8-)

[Author]

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