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K. Wiggins
K. Wiggins, ASE Certified Technician
Category: Car
Satisfied Customers: 2874
Experience:  ASE Certified Technician, Associates Degree in Automotive Technology, 15+ years in the trade
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Mazda MX-5 Miata: 2007 mazda mx5 6-sp automatic bought the

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2007 mazda mx5 6-sp automatic bought the car last month 52000 miles appears radiator has been replaced
heater does not heat. blower fan works, but no heat.
so far:
inspected water pump - appears ok
replaced thermostat unit - was leaking, replacement stopped the leak
flushed heater core - all clear
after warming engine to operating temp, hold at 2500 rpm for approx 5 min. then 3500 rpm for 5 sec and back to idle ( repeat 3500 to idle 4 times )
still no heat
engine temp stays in normal range
With the car at operating temperature are both lines for the heater core hot to the touch?
Customer: replied 3 years ago.

The engine was at normal operating temperature. The "in" line to the heater core is warmer than the "out" line. The out line is cool to the touch. The "in" line is warm, but not as hot as the radiator hose.


After the car was turned off for about 30 min, both lines were very warm. Restarted the car and the heater worked for about 3 or 4 minutes then started blowing cool air. The heater control is set to knee/feet position. The a/c compressor is not running as it would be with a defrost setting.

Thanks for that information. Could you tell me how you flushed the heater core? As in did you use a "T" adapter in the hose or did you remove the hoses and flush the core directly?
Customer: replied 3 years ago.

Removed the hoses and flushed directly, so I'm sure I left air in the core.

From what you describe here is what I would suggest. Go ahead and purchase a good quality flush along with one of these kits. Install the "T" in the heater core inlet. Remove the lower radiator hose or open the radiator drain kit and do a reverse flush. It sounds like you may have some stuff gunking up the water supply to the core.
After flushing top off with coolant and begin the "air burping" procedure. You want to top of the coolant reservoir and leave the radiator cap off. Crank the vehicle and allow it to run until the thermostat opens. (this may take 10-15 minutes at idle) When you see the coolant level drop go ahead and fill it back to the full mark and install the cap and shut the vehicle off. I know this is somewhat different then the factory method but I've used this method for years with positive results.
LEt me know how the reverse flushing goes and if I may be of further assistance just reply back here at any time.
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Customer: replied 3 years ago.

The car got back from about a 10 min drive about 15 mins ago. After being parked ( engine off ) for about 10 min the heater hoses are both 140 deg. The lower radiator hose is 170 deg. Just additional info.

Did you have heat during the drives end?
Customer: replied 3 years ago.

Sorry, she didn't try the heat. It was 78 here today. I'm hoping the reverse flush will clear something from the system and get it working again. Won't get to try it til Tuesday, though.

I'll be around all week so please let me know how it does after the flush.
Customer: replied 3 years ago.

Picked up the flush kit today, but I wanted to be sure. Which line is the inlet for the heater core?

It'll be the line that runs to the thermostat housing.
Customer: replied 3 years ago.

Still haven't resolved this. Took the car to Mazda dealership in Memphis. They determined the aftermarket thermostat was causing a restriction. Replaced the (new) aftermarket thermostat with oem, also replaced the (new) rediator cap with oem cap. Didn't fix the heater problem. They determined the flush adapter installed in the heater hose was causing the problem, replaced the heater hose. Heater worked for about 10 minutes, then back to no heat. On the road at 70 mph in 6th gear ( 2000 rpm ) no heat, in 5th gear (3300 rpm) mild heat, 4th gear (4500 rpm) much more heat. Driving at 1000 to 2000 rpm the engine temp guage will occasionaly climb close to H then move back to normal temp. Also losing some coolant from vent tube at catch tank.

Hello, sorry for the delay in getting back to you. I appreciate your info regarding heat output and engine RPM. When you first posed your question we surmised it was a flow issue. Now that everything has been flushed and thermostat replaced we know it is not a sediment or "gunk" issue.

The heat working at higher RPMS and the fact that the vehicle will go over to hot at idle supports a water flow issue. With everything being clean now we are most likely looking at a water pump issue, mainly worn and corded impellers.

I don't know if they did any checks at the dealer put that is where I would say things are headed next. You're looking at about 100 for the pump and 1.5 hours shop labor to install. If this is something you want to attempt yourself let me know and I can send you the procedure.
Customer: replied 3 years ago.

I appologize, I lost track of what I had told you. The water pump was examined while replacing the thermostat, it appeared to be good and was reinstalled. When the thermostat didn't fix the problem, I replaced the water pump. Still no heat.

Thank you for the additional information. I'm going to open this back up to see if any of the other experts have any thoughts on your issue. Someone should be with you shortly.
Customer: replied 3 years ago.

ok, thanks. It's got me stumped.