no heat in 2000 mazda miata
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Joined: Feb 2007
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Things To Check...
There is no shutoff valve for the Miata's heater. Here are some steps to help diagnose your problem:
First, check your coolant reservoir. If your coolant level is very low, your heater won't get hot. You may have enough coolant to keep the engine from overheating (especially in the winter while you're driving), and still not have enough coolant to keep the heater hot. If you have a sweet smell inside the car, or you find the carpet is wet, you could be leaking coolant from the heater core. Also, older NB's are notorious for developing cracks in the plastic header tank of the radiator. Look around the top of the radiator and see if there is any buildup of greenish, powdery residue around the top seam of the radiator header. Pull the foam "weatherstriping" up on the front top of the radiator, and check the seam under that for the residue. If you are low on fluid, you could have a small leak that over time drains your coolant.
If you've got enough coolant, the next thing to check is the temp of the heater hoses. There are two side-by-side heater hoses that go through the firewall just to the right of the engine as you're looking at the engine bay. With the engine at operating temperature and the heater on full blast, feel the hoses and make sure they are BOTH hot (It doesn't take a thermometer to figure this one out). If one is hot and the other is cool or just warm, then that means there is an obstruction in the heater core that won't allow the hot coolant to pass through. If that's the case, try back-flushing the heater core to see if you can blow whatever the obstruction is back out of the core. It could be flakes of mineral deposits or rust that were getting pumped through the engine cooling system and got hung up inside the heater core.
If both hoses are hot, check the heater and fan controls. Make sure that every time you move the selector **** (defrost, blowing out the main vents, blowing out the floor vents, etc.) the air comes out where it's supposed to. When you move the heat control from cool to hot, listen for a change in the sound of the air flow. If any of that isn't happening, then you may have a loose vacuum line.
Also, check the engine temp guage, and make sure the engine is getting to temperature. The engine temp guage should be pointed straight up, more or less, when at operating temperature. If it is running cool, it is likely the theromostat has failed open and needs replacing. If the coolant's not going to get hot, the heater's not going to get hot. Check the two big hoses connected to the radiator: The upper hose (top-passenger-side-corner of the radiator) should be hot, and the lower hose (lower, driver-side corner of the radiator) should be warm. If they are both the same, lukewarm temperature, that's another indication that the thermostat is stuck open.
If everything checks out, then there is a possibility that the air mixer flap inside the HVAC box under the dash has a problem. The actuator motor could be broken, or the mixer flap could have broken off the "axle" that moves it.
And although it may be obvious, I'll just mention it for completeness: The heater only applies a "delta" temperature change to outside, ambient air. If the outside air is -13 degrees, you can't expect the heater to put out as much heat as it does when it is 50 degrees outside. I don't know what the delta's are for the Miata, but I know on my Jeep the heater output is supposed to be 147 degrees F when it's 70 degrees outside. When ambient is about 20 degrees F, the heater output (for the Jeep) should be about 132 degrees F. I found that the best way to measure the output temperature was to get my wife's candy-cooking-thermometer and clip it to the floor heater output vent. You can get one from WalMart if your wife won't let you use hers!
How to fix the heat problem depends on the results of the tests named above. And if you guessed that I just went though a heater nightmare on a Jeep - you guessed right!
Hope this helps. In the mean time, keep your Snuggie in the car!
First, check your coolant reservoir. If your coolant level is very low, your heater won't get hot. You may have enough coolant to keep the engine from overheating (especially in the winter while you're driving), and still not have enough coolant to keep the heater hot. If you have a sweet smell inside the car, or you find the carpet is wet, you could be leaking coolant from the heater core. Also, older NB's are notorious for developing cracks in the plastic header tank of the radiator. Look around the top of the radiator and see if there is any buildup of greenish, powdery residue around the top seam of the radiator header. Pull the foam "weatherstriping" up on the front top of the radiator, and check the seam under that for the residue. If you are low on fluid, you could have a small leak that over time drains your coolant.
If you've got enough coolant, the next thing to check is the temp of the heater hoses. There are two side-by-side heater hoses that go through the firewall just to the right of the engine as you're looking at the engine bay. With the engine at operating temperature and the heater on full blast, feel the hoses and make sure they are BOTH hot (It doesn't take a thermometer to figure this one out). If one is hot and the other is cool or just warm, then that means there is an obstruction in the heater core that won't allow the hot coolant to pass through. If that's the case, try back-flushing the heater core to see if you can blow whatever the obstruction is back out of the core. It could be flakes of mineral deposits or rust that were getting pumped through the engine cooling system and got hung up inside the heater core.
If both hoses are hot, check the heater and fan controls. Make sure that every time you move the selector **** (defrost, blowing out the main vents, blowing out the floor vents, etc.) the air comes out where it's supposed to. When you move the heat control from cool to hot, listen for a change in the sound of the air flow. If any of that isn't happening, then you may have a loose vacuum line.
Also, check the engine temp guage, and make sure the engine is getting to temperature. The engine temp guage should be pointed straight up, more or less, when at operating temperature. If it is running cool, it is likely the theromostat has failed open and needs replacing. If the coolant's not going to get hot, the heater's not going to get hot. Check the two big hoses connected to the radiator: The upper hose (top-passenger-side-corner of the radiator) should be hot, and the lower hose (lower, driver-side corner of the radiator) should be warm. If they are both the same, lukewarm temperature, that's another indication that the thermostat is stuck open.
If everything checks out, then there is a possibility that the air mixer flap inside the HVAC box under the dash has a problem. The actuator motor could be broken, or the mixer flap could have broken off the "axle" that moves it.
And although it may be obvious, I'll just mention it for completeness: The heater only applies a "delta" temperature change to outside, ambient air. If the outside air is -13 degrees, you can't expect the heater to put out as much heat as it does when it is 50 degrees outside. I don't know what the delta's are for the Miata, but I know on my Jeep the heater output is supposed to be 147 degrees F when it's 70 degrees outside. When ambient is about 20 degrees F, the heater output (for the Jeep) should be about 132 degrees F. I found that the best way to measure the output temperature was to get my wife's candy-cooking-thermometer and clip it to the floor heater output vent. You can get one from WalMart if your wife won't let you use hers!
How to fix the heat problem depends on the results of the tests named above. And if you guessed that I just went though a heater nightmare on a Jeep - you guessed right!
Hope this helps. In the mean time, keep your Snuggie in the car!
Last edited by RX8driver; Jan 20, 2010 at 10:42 AM.
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