Thermostat problems? coolant leak and overheating


I have finally completed a water pump replacement on a 2007 Mazda 6 3.0. I also replaced the heater hoses that goes to the lower part of the water pump and all the hoses and a t connector above the thermostat housing.

I read everywhere that you need to burp the air in the cooling system and I tried but I don’t think I did it long enough. I didn’t drained or flushed the coolant out but I am going to do it soon at Jiffys since I don’t have a way to work under the car as it is always parked in a slope.

I let the car warm up with heat and fan speed at full blast while squeezing the radiator hoses occasionally but other than a gurgling noise I didn’t see the coolant level drop. I also didn’t have any heat.

I drove the vehicle to O’Reilly’s the next day to get some coolant, the whole time the engine coolant temp according to the gauge and my phone’s OBD app was fine.

As soon as I got out of the parking lot the engine starts to overheat. I immediately stopped the vehicle to let it cool down. I popped the hood and heard a hissing noise and water coming out from what it seems to be the water pump area.

The coolant in the reservoir or (expansion tank I think it’s called?) was low. I opened the cap slowly, but the hissing stopped and the fumes comes up to the reservoir.

I added coolant till it stopped going low and drove again. Temp gauge was on cold and I also had the heat on but it was blowing cold air still. I couldn’t drive more than 20 seconds because it was overheating. Found the coolant was low again, there were leaks but no hissing this time. Again I let it cool.

I repeated this till I got come. I lived about 10 mins away from Oreilley’s. It took me about an eternity to get home like that. The tow truck driver never arrived.

A couple of blocks on my way home I had stopped twice and noticed the coolant was not going low as fast. I still didn’t have heat but I made it.

I took the Haynes manual and read that if there’s leak it might be because of loose hoses or the thermostat. Admittedly, I threw away most of the original clamp that were in the hoses and bought the ones that you have to tighten with a screwdriver. I was afraid of overtightening the hoses but I was also afraid it might be too loose.

The thermostat was also not replaced. The car lost heat before the water pump replacement but I thought it was because of the leaks and water pump.

The leak seems to be difficult to spot as it dries up pretty quick when the engine is hot. I also don’t want to take the whole thing apart if the problem is not the thermostat but it seems like it could be shucked. How do I go about testing these two things? I

Hope I don’t have anything more serious that that. There are no check engine lights and no fumes of any kind coming from the tailpipe.


First, why did you replace the water pump? Leaking from the shaft? Noisy?

“The car lost heat before the water pump replacement but I thought it was because of the leaks and water pump.”

So there was a leak before changing the water pump?
Sounds like the original problem is still there.

“I popped the hood and heard a hissing noise and water coming out from what it seems to be the water pump area.”

Could also be that you didn’t properly install the gasket on your new water pump.

I suggest you get it towed to a good independent shop, not “Jiffys”, so an experienced mechanic can pinpoint where it’s leaking

Temperature sensors will not show overheating conditions if they are not submerged in circulating coolant. It’s not clear, did you replace ANY coolant after the repairs but before the next day when you went to get some coolant to put in it? I’d think a fair amount of coolant drained out when you replaced the water pump and hoses…

Did you add coolant after the pump was replaced. You need a full system to burp the air out.

It sounds to me like you didn’t get the gasket on the water pump correctly.


Thanks for the replies guys!

I replaced the water pump because it was my mechanic who told me that it was on its way out, though he never told me anything about leaks elsewhere. I also bought the whole unit with the housing, not the water pump alone.

Yes I added coolant but didn’t understood how the whole burping concept work. I tried again today after checking out some videos and I finally got heat! It looks like I have successfully burped the system. Coolant level doesn’t drop anymore.

I also found where it was leaking. I didn’t tight the hoses enough because I was afraid of damaging them. I used the worm type clamps instead of the OEM because I hated them when I was removing them.

I have one problem though. I just noticed my radiator fans won’t spin even when the coolant temp has gone past 200 F. Is the sensor broken? Wiring problems? Broken fans? How do I find out?

I am having trouble finding anythung about this in the Haynes manual.


It is probably your cooling fan control module. if it was mine I would run battery voltage directly to the fan and see if the fan runs. If the fan runs, it is probably the controlled.

Caution, your cooling fans should only run if the temp reading is high enough to call for them to run. In order for that to happen you cooling system has to be full. The temp sensor can’t read if it is sitting in an air pocket.

it should be easy to get your car hot enough with some cardboard in front of the rediator.

Make sure to check the coolant level inside the radiator, not just in the overflow plastic bottle. Do that when the coolant isn’t hot of course. It should be almost at the top of the radiator, within half an inch of the top or less. Usually you can eliminate air bubbles in the cooling system on most cars by idling the engine while the front wheels are elevated on ramps, the heater control set to max heat, and the radiator cap removed. As the engine heats up from a cold start, you’ll see at some point (usually about 5 minutes after starting) coolant start pouring into the top of the radiator. It will sometimes overflow a little. If that happens just pour more coolant in to keep it topped off. After about 5 minutes of this all the air bubbles should be gone. Shut the engine off, let it cool, then put the radiator cap back on.

It’s critical to get the radiator fan working correctly, otherwise you’ll soon be back under the car with dripping coolant. Or worse. Suggest to not use the car until that is fixed. I can’t speak to your Mazda, but I’ve had that problem on my Corolla a few times and each time the problem was the coolant temperature switch which controls the fan. It’s a small gadget about the size of half a cigarette , and screws into the water jacket, with an electrical connector on the dry end. It’s normally closed. When the engine coolant gets above 200 degrees, it opens. That causes the fan relay to turn on. The way I test it, I disconnect the wiring harness from the switch, and if that turns fan on, I know the problem is probably the coolant temp switch for the fan . So I remove the switch and test it to make sure it opens at the correct temperature in a pot of hot water on the stove.

The thing with this Mazda 6 is that the 3.0L engine does not have the radiator cap like most cars so I am not sure how to check the level other than the using the expansion tank.

I parked the car facing uphill and tried perform the usual steps to burp the cooling system, leaving the expansion tank cap open and refilled the coolant to the max level as it drops. The radiator hoses were warm to touch and the heat is finally working now.

I also drove around while checking coolant temp on my OBD app and the temps were fine. The coolant level hasn’t dropped since the last time I filled it, but yeah the radiator fans never came on.

I will try to find that switch and perform that test too see if the fans work. In the meantime I will just leave the car there till I can find what’s going on with the fans.


The procedure to fill the cooling system from what I can see is, after turning heater to max …

Fill the radiator and the reserve tank . Replace cap. Start & idle engine until radiator fan turns on. Next, 2,500 rpm for 5 min. 3,000 rpm for 5 s. Idle. Repeat those 3 steps a few times. Idle 1 min . Turn engine off and wait until cool. Check coolant level. If low, fill as required and repeat above steps. Monitor the coolant temp gauge at all times during the bleed procedure, and turn the engine off and let it cool if it overheats.

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Hey George thanks! So it looks like I must find that switch and fix my fan problem first.

Got a question… How do I fill the radiator on this vehicle. I don’t see a radiator cap anywhere. It is a 2007 Mazda 6 with the 3.0L engine. The “cap” in the front of the engine is actually the power steering cap.

Allow the engine to reach normal operating temperature before condemning the fan operation. The radiator fans won’t switch on until the coolant reaches 212 F, then the fans will turn off when the coolant temperature drops to 203 F. The fans shouldn’t operate below 203 F unless the A/C is on.

I found the coolant temp sensor on this car but decided not to touch it yet. The fans doesn’t spin when the engine reaches normal operating temps. I also turned the A/C on and still it would not come on. Could it be the “relays”? The computer?

I had this vehicle with the battery disconnected for about two weeks or so. The battery died… I read somewhere that doing this could damage the computer but it was already too late.

I replaced the battery two days ago. That day the left directional light was blinking twice as fast and the front passager window would not roll down if controlled only on my side. The next day, everything works normal.

I am going to autozone and get some cables to connect stuff directly to the battery as suggested.

200 degrees F might not be hot enough for the fans to come on.


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I tried this but the fan isn’t spinning either. Am I doing it wrong?

That doesn’t look like the radiator fan connector. Are you sending 12 volts to the PCM though the coolant temperature sensor connector? This could damage the PCM.

If the radiator fans do not switch on at 220 F or when the A/C compressor is engaged you should test the radiator fan control module.

I followed the wire from this connector and it led directly to the fan in the passenger side.

When it didn’t turn on I gave up, and connected everything back, started the car and warmed it up till it reached 196 F and then turned the A/C on. The fans are spinning now?!

Yesterday when I turned the A/C on and it wasn’t spinning… I am puzzled. Could it be because the engine was still cold?

I read some forums people are saying the fans should come on at 200 F… Others are saying 200F is not enough. I will just keep an eye on the temps via OBD since I have confirmed but fans are spinning now.

I have one last question. I am using the worm type clamps, the ones that you screw to tighten the hose. How tight should it be? I have used my fingers to find wet spot around the hoses and tighten them, but one is still leaking. I don’t want to overtighten it because it goes to the thermostat and I have bad experience with plastic parts!

it’s possible that the connector wasn’t seated fully or developed some crud in it, and you disconnecting and reconnecting it helped
I would think the fan should come on when the a/c is on.

tight enough that it doesn’t leak, lol
I’m not sure of a torque figure for your car, but if it is seeping a bit, tighten it a bit more. clean it off and check it tomorrow, or later today. Worm type clamps work just fine.

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Alright thanks!

Heres that unit. It had 3 connectors. The one to the right leads to the passenger fan.

I just finished tightening the clamps and will check again for leaks. I think it should be good now.

Hope it doesn’t overheat. Fingers crossed.