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Ross, Diagnostic and repair technician
Category: Car
Satisfied Customers: 1183
Experience:  Licensed diagnostic and repair technician
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I have a Mazda mx5 only a few months old. Heater works when

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I have a Mazda mx5 only a few months old. Heater works when on the open road, but when I stop, at traffic lights etc, starts to blow cold - any ideas?
Ross :

and thanks me to help you today. I am a licensed diagnostic and repair technician, I look forward to working with you.

Customer: Thanks
Customer: What do you suggest is wrong with my car?
Customer: Get in touch when you're back on line
Ross :

Sorry delay getting back to you!

This problem is very strange, it's not something I would consider to be normal operation. Some vehicles will shut down certain systems reasons so if your model is fitted with an economy button. Try running with this switched off.

If the problem still occurs I suspect there may be a problem within the climate control system which the dealer will need to look into. I understand they are not always that helpful so my best advice would be to ask them again if this is normal and ask them from documentation explaining this.

Ross :

If they cannot give you this. Accept that it is not normal and ask them to investigate the problem under the manufacture warranty. You can also put in a phone call to Mazda customer services, they will be helpful and can work between you and the main dealer to get the problem corrected.

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Customer: replied 3 years ago.
Ross. I do not find your answer satisfactory. You have told me nothing I did not already know. I'm looking expert answer to a technical question. ie, my MX5, which is 6 months old, has a heater problem. When revs are high as when driving fast, the heater works fine. When revs are low as when stopped at traffic lights, the heater then stops giving hot air, and pushes out cold air. High revs, hot air. No revs, cold air. You're the expert. - why does this happen? There must be an explanation. There's only a finite number of things can go wrong with a new car cooling system. If you don't know the answer, you're not the expert I need.
David, sorry my initial answer was not satisfactory, people tend not to want technical advice so much with newer cars due to the warranty but it's no problem!

If this is dependant on the engine RPM it will be related to the flow of coolant through to the heater matrix. The water pump is the only RPM dependent part of the cooling system so a problem with the water pump is a possibility. Of course without having the car to investigate the fault it's impossible to say what is going on but the only cause I have ever come across to cause this is would be the water pump impeller broken or spinning on the shaft. I have known the impeller to break apart and still pump but not efficiently, but as they tend to use plastic now they can also spin on the pump shaft which makes it either stop or not pump efficiently. When its revving higher the pump will catch up. I would never expect to see this on such a new car but having worked with main dealers years, these things do happen and being a fairly new design of engine layout this may be a one off failure or may become a common issue.

The next possibility would be low coolant level, this is an obvious one which im sure would have been checked but low coolant will certainly do this as will an air lock. Air locks can cause real problems and strange problems like this, I have had to raise some vehicles several feet of the ground at the front to get air out in the past so it can easily get trapped and go un noticed until it causes problems like this.

Some engines also use a secondary coolant pump, this is an electric pump which will help circulate the coolant at lower RPM if the designed of the system gives poor circulation to the matrix, If this is fitted with a secondary pump it could be the pump, control unit or electrical faults although I would expect the engine ECU To detect a fault with the electrical side of things and this should bring the engine management light on.
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Customer: replied 3 years ago.
Thank you Ross. That's the diagnosis I was seeking. I can now visit the Maazda dealer, who is playing dumb. Could it be a thermostat problem?
Its highly unlikely to be the thermostat, If it was blowing hot when stopped and cold when driving, the thermostat would be a suspect causig overcooling but i don't think that is the cause in this case.
Customer: replied 3 years ago.
Thanks Ross, you've been very helpful.