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Old 01-07-2013, 11:23 PM   #1
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Another cold heater problem

I've searched and read numerous threads, but all of the information I've found hasn't solved my problem.

I have a 91 Fox body coupe and my heater blows cold air. So far I have checked everything I can think of.

The vacuum lines are intact and I can change from defrost to vent to floor.

The mix door cable is attached and moving, and I believe the door itself is moving because I can hear it and when I put the temperature control all the way to hot there is a slight decrease in the amount of air blowing from the vents. I assume this is because the heater core airflow path is smaller than the A/C condenser path.

The hoses on the heater core both get hot. The inlet hose does have the flow restrictor in place. I flushed the heater core earlier today by removing the hoses and sticking a garden hose in it. It flowed really well from the start, and brown, rusty water came out for just a few seconds before the water went clear. I flushed the core in both directions, backwards first, then the other way, then backwards again.

When the car gets up to normal operating temp., I get cold air no matter what I do. If I turn off the heat for a while (about 10 minutes or more), hot air will blow from the vents at first, but the temperature rapidly decreases until it is cold again.

Thinking I had air in the system, I tried driving with the radiator cap loose in an attempt to burp the system, but I still get cold air from the vents.

I'm resigned to the fact that I may have to do the dreaded heater core swap, but I'm hoping someone may have some advice on something I may have missed. Is there an easier/better way to purge the system of air? Could the flow restrictor in the inlet hose be clogged?

Any help would be greatly appreciated. Thanks in advance.
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Old 01-07-2013, 11:34 PM   #2
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I had a bottle cap stuck in the heater box keeping the hot/cold flap from closing all the way. I was able to see in the box by pulling the blow motor resister off the front of the box. Gives you a perfect view of the flap. I would start there and make sure the flap is closing all the way.
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Old 01-08-2013, 12:27 AM   #3
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I would yank that flow restrictor out of there and chuck it in the trash! I've never ran them in any of my Mustangs. Also, both sides will get hot because for a moment it can fill the heater core with hot water. But as soon as you turn on the heat it will blow all the heat out of it and thus blow cold. With the restrictor clogged or not allowing any flow, the cooled water cannot be replaced with warm/hot water.
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Old 01-08-2013, 12:34 AM   #4
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Thanks guys, I'll try both of these in the morning. I'll try anything to keep from having to change that heater core.
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Old 01-08-2013, 01:48 AM   #5
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My heater core blew up this fall. Not sure if it was from the lack of restrictor but it was the first tube on the inlet side so I would be careful pulling it out.
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Old 01-08-2013, 02:23 AM   #6
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I was thinking I would remove it to inspect it and then hook up the hose to test it without the restrictor in it. I'm going to check the flap first though since it seems to be quicker and easier.

The more I continue to read, the more I am starting to believe the problem isn't the heater core itself. I mean, it doesn't leak, it flows good when flushed, and didn't seem too gunked up for starters. I'm thinking it has to be something else, so changing the core wouldn't really solve anything.
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Old 01-08-2013, 10:50 AM   #7
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That restrictor is there to aid in warming the car up quicker in very cold states. It's not there to stop the "pressure" from getting into the heater core. I've owned probably 12 Mustangs in my lifetime and not one of them had this restrictor on them. If it was designed to protect the heater core from pressure failure then FL cars would definitely have them since our temps get over 100 in the summer time and thus cause much higher operating temps/pressures than northern cars.
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Old 01-08-2013, 11:06 AM   #8
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I would replace it. I had a 93 cadillac that blew cold air, I tried flushing and it barely helped. After awhile that scale just build and builds. I insulates the heat from the coils and lowers flow, just a perfect storm of fail.
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Old 01-08-2013, 08:53 PM   #9
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Quote:
Originally Posted by 99FiveOh View Post
That restrictor is there to aid in warming the car up quicker in very cold states. It's not there to stop the "pressure" from getting into the heater core. I've owned probably 12 Mustangs in my lifetime and not one of them had this restrictor on them. If it was designed to protect the heater core from pressure failure then FL cars would definitely have them since our temps get over 100 in the summer time and thus cause much higher operating temps/pressures than northern cars.
Well I stand corrected then. I read it on Late Models site when I was lookin for heater cores. I guess even official mustang sites have bad info as well

Straight from late models website...

The heater core flow restrictor is an important part in the replacement of your heater supply hose and heater core on your 1979-2004 Mustang. This part helps reduce the risk of your heater core failing during use due to decreasing the pressure that coolant flows into the heater core. This heater hose restrictor is original equipment on 1996-04 4.6L Mustangs but can also be used on 1979-95 Mustangs as well.
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Old 01-08-2013, 09:16 PM   #10
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Went through two heater cores in my 92 fox. It's a pia to replace as you have to pull the dash. I put a restrictor in the second time because the one I replaced blew in the middle. After verifying the doors are clear and working as others suggested I would pick up a real coolant flush instead of using the hose and follow the instructions. You may only have a row or two open in the core so water goes through it but not enough surface area to make heat. I remember driving the whole winter in my first car with no heat. I used flush that spring and there was the heat I was missing. Changing the core is the last resort. Pia.


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Old 01-08-2013, 09:54 PM   #11
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In my way of thinking, a restrictor would slow the flow down inside the core allowing rust and scale to form and block the core. A faster flowing system is a cleaner system.
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Old 01-09-2013, 07:57 PM   #12
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A quick update on what I have discovered.

The hot/cold flap inside the box is working properly.

The flow restrictor isn't clogged. I can push a screwdriver through the hole and I started the car with the hose disconnected and coolant sprayed out really well.

This morning I drained the coolant (no gunk or rust) and put a coolant flush in the radiator. I started the car and watched everything as it heated up. The inlet heater hose became hot first, followed by the heater outlet hose, and finally the upper radiator hose. I had the heater controls set to hot and had the fan blowing on fast the whole time, but still no heat (no real big surprise there).

After about 20 minutes of idling at normal operating temp., I noticed that my radiator wasn't hot anywhere except near the inlet and the cap wasn't hot at all. My fan was removing heat from the radiator though, because I could feel the hot air blowing behind it. I carefully loosened the radiator cap, but nothing sprayed out and I was able to remove it completely. The coolant inside was about an inch below the top of the radiator, but it was barely moving and was only at 130 degrees. When I rev the motor, the coolant level drops and rises again when I stop.

I can't explain how I'm able to completely remove the radiator cap on a running car. The water pump was replaced three years ago. The car doesn't overheat or even have any cooling problems.

Bad water pump? Bad thermostat? Bad radiator cap? I'm at a loss.
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Old 01-09-2013, 08:36 PM   #13
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You may have answered this already but what kind of thermostat do you have in there?
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Old 01-09-2013, 08:50 PM   #14
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The thermostat is a Stant 195 degree. I normally run a 180, but I wanted to keep everything stock with this one. I even run stock timing on it, lol.
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Old 01-09-2013, 09:33 PM   #15
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you possibly still have air in the system.
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Old 01-09-2013, 09:53 PM   #16
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Any suggestions on a good method to remove air from the system?

So far I've driven with the radiator cap loose and idled in the driveway with it completely off. I park uphill so the radiator should be the highest point in the system.

I'd love it to be air in the system, if I could just get it out.
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Old 01-10-2013, 01:33 AM   #17
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Electric fan? What does the temp gauge show during all of this? The water level dropping when reving is normal. And its clear the the cooling system is not getting hot enough to even pressurize. Best way I have found is to jack the front of the car up with a floor jack. My floor jack gets the bottom of the tires about 4" off the ground with the suspension unloaded
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Old 01-10-2013, 03:11 AM   #18
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The fan is a stock setup and was replaced about 3.5 years ago when I replaced the radiator.

I only have the stock temp. gauge, but it goes to the second mark above cold (which reads 130) and stays right there. It very rarely moves up or down, and when it does, it is is just a little.

I thought the water level dropping was normal. It's a good sign that the water pump is working isn't it? I know it sounds dumb, but I've never had this kind of trouble with a cooling system before so I'm not 100% sure.

I'll try jacking the car up and see if that works (don't know why I didn't think of that). It's supposed to rain all day tomorrow, so maybe I can get it done if the weatherman is wrong.
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Old 01-10-2013, 07:38 AM   #19
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FWIW I have NEVER jacked the front end up to "burp" the cooling system. It will work this out all on it's own if your system is complete as designed from Ford. Do you honestly think when these cars were rolling off the assembly line by the thousands that Ford had the time to burp each and every one of them? Hell no! The system is designed to get rid of air on it's own.

If you let the car sit and run through a couple thermostat cycles you will see that the air does escape through the filler neck. The only issues is the air that will be trapped in the upper radiator hose due to it's design. Since it's higher than the rest of the system I don't think it's possible or necessary to remove the air from it.
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Old 01-10-2013, 06:07 PM   #20
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I agree 99FiveOh. I've never had to worry about "burping" a system after replacing a water pump or radiator or thermostat or anything. I'm just so stumped at this point that I'm grasping at straws and willing to try anything.

I tried a new radiator cap this morning. No change in anything.

Guess I'll try the thermostat next.
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Old 01-16-2013, 09:58 PM   #21
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Thanks to everyone who took the time to read this thread, and especially to those who replied and tried to help me.

I changed the thermostat today and I now have heat. I was skeptical that it would change anything, but you learn something new every day.
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Old 01-17-2013, 01:04 AM   #22
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Good to hear! And I'm not saying you have to jack the car up to burp the system, but do you really have to pat a babies back or will they burp sooner or later? It just speeds the process I guess...
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Old 01-17-2013, 08:58 AM   #23
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Good, yeah it should burp out eventually. I've run some pretty convoluted systems that eventually burb out.
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