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Discussion Starter #1 (Edited)
Out of no where (at 154,000 miles), I noticed that in 100 degree temperatures, my engine temperature gauge that normally hits the halfway mark and stays there rather quickly (like within a couple minutes of running), started fluctuating and showing cool and sometimes nearly on cold. I was getting erratic (never hot) temperature gauge readings over the weekend. Once I would drive on the highway or have it running for a good while, it would rise to the midway mark like normal but if I were to go into say the convenience store (keeping in mind it's 100 degrees out) and then come back out after 5 minutes and start it back up, the gauge is back down there near the damn cold line. Obviously, after having the car for 154,000 miles, I know this isn't the case, the engine is blazing hot like always.

Ok so things to most likely rule out:

-Air bubble in coolant. Yes, I had changed two coolant hoses a couple weekends ago as preventative maintenence. Of course I purged a lot of old coolant in the process but filled up the cooling system and burped the car like I have done before. Additionally, the car has been driven about 1,000 miles since then and checked for air in the radiator and topped off like I have done numerous times previously.

-Bad Engine Coolant temperature sensor. This part is brand-new as a month ago. Keeping in mind that my old ECT sensor was completely busted (as in the top connector part spun freely independently of the metal nut part that screws into the head). Even when my old ECT sensor was bad, I was still getting the engine temperature to read correctly. The engine temp gauge must get a signal from some other temperature sensor.

-Thermostat. The engine had a new thermostat installed when I had the new engine installed at 109,000 miles. It's a factory Mazda thermostat. Additionally, if a thermostat was stuck open, I don't see how the coolant could be at the midway point (according to the gauge) and then after being shut off for 5 minutes, show COLD.

Now, one piece of additional info. is that my check engine light did come on once during this little mystery and the code was for "insufficient coolant temperature for stable operation" which makes sense. So the question becomes, is there another sensor somewhere that sends the engine temperature to the gauge? It must not be the ECT sensor because mine was completely broken in the past and my temperature gauge still functioned correctly.

I would definitely appreciate some help guys. I'll also confirm, with my AP, that my ECT is around 200 like it normally is. I would expect the ECT will be 200 even though the gauge will read cool or even COLD.
 

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Fluctuating temp gauge is usually a sign of bubbles in the coolant.

Since it's the easiest thing, try burping the system again. Sometimes it's easier to open the radiator cap (with the car cold of course), jack up the front end as high as possible (so the fill location is the highest point), start the car and let it idle until the rad fans kick on 2-3x, filling when necessary.
 

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Discussion Starter #3
Fluctuating temp gauge is usually a sign of bubbles in the coolant.

Since it's the easiest thing, try burping the system again. Sometimes it's easier to open the radiator cap (with the car cold of course), jack up the front end as high as possible (so the fill location is the highest point), start the car and let it idle until the rad fans kick on 2-3x, filling when necessary.
Thanks and yes that does make sense. The thing that makes me wonder though is that I did the coolant hoses a couple weeks ago and have driven the car at all speeds for about 1,000 miles since then and now it has just started doing this.

Do we know where else the engine temp is read? There has to be another place other than the ECT sensor that's screwed into the head near the side of the battery area.
 

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I'm fairly certain there's the temp sensor from which the ECU receives input, and a type of sending unit from which the gauge in the cluster operates...
Don't you have a scangauge or Dash Hawk or AP? Have you checked the temps there?
 

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Discussion Starter #5
I'm fairly certain there's the temp sensor from which the ECU receives input, and a type of sending unit from which the gauge in the cluster operates...
Don't you have a scangauge or Dash Hawk or AP? Have you checked the temps there?
I've got the AP, I can check the actual temperatures with that. I just hadn't had a chance to do it yet. This happened while I was out of town over the weekend and just got back yesterday evening. At work at the moment.
 

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While I've read on 6 club multiple times in the past about a sending unit, I could not verify this in the WSM. Was your error code P0126/P0128?
I would speculate that if IAT remains below a threshold (normal), but ECT signal is erratic or indicates high temp, the ECU does not "see" a high ECT, but instead determines a mechanical malfunction, so your gauge does not display a high reading.

WSM indicates the following:

"DTC P0126:
If the ECT signal never exceeds 160 degrees F. after engine is started for a specified period, the PCM determines that the coolant thermostat is stuck open.
DTC P0128:
PCM monitors MAF, IAT, VSS, and ECT signals and calculate radiator's heat the radiation ratio when the following monitoring conditions are met. If the calculated value exceeds the threshold, the PCM determines that the cooling thermostat is stuck open."
 

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Discussion Starter #8
Ok so here is what I noticed today. I plugged in the AP and brought up the coolant temperature. The temperature gauge came up to the middle range and was holding and then coolant was at about 180 degrees. Keep in mind this was in the morning from cold start and I was just loafing down the street to the gym so it hadn't climbed to the usual high 190s or over 200 like it does after some agressive highway driving. As I cruised down the street I noticed very erratic readings on the AP. It was jumping all around from 136 to 175 then to 160 then to 150 then back up to 180. I noticed my engine temp needle was dipping down and then came back up and stabilized again. Clearly something is giving/receiving an erratic signal.

Now just a recap, when my ECT (engine coolant temperature) sensor was completely broken (I discovered the top connector part broken free from the actual sensor in the head), I was still having full operation of my temperature guage on my dash. This tells me it has to be receiving it's signal from something else.

Additionally, the ECT sensor is brand-new as of about a month ago. It's straight from Mazda, OEM and all. I thought I had heard a while back that there was another sensor from which a temperature reading is taken but who knows if it was accurate.

On the flipside, if the ECT sensor that is in the head isn't used to send the engine coolant temperature to the gauge, then what is it used for?! LOL Frustrating little mystery.
 

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Many cars have 2 sensors: One for the crappy analog in-dash gauge, and one that's much more accurate for the ECU to use, which is what the AP would read.

If it's jumping around like that it still sounds like air bubbles. I've seen thermostats be flaky & stick open/closed only sometimes, but your symptoms don't really match that behavior.
 

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Agreed alot have a sending unit for gauge and separate ECT sensor for ECU input. But I could not verify this in this case in the WSM.
Here's my theory:
With the connector broken and the ECU receiving no input from the sensor, the ECU then utilizes input from IAT. With this input and perhaps input from others, a calculation is made and the gauge displays accordingly.
With a bad or faulty ECT sensor, or a problem with connection or circuit, the erratic readings are causing your gauge to fluctuate.

The ECT sensor is just resistance based like most other temp sensors on the car. You can test it yourself easily...
 

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Discussion Starter #11
Ok just got off the phone with Mazda and they don't know for SURE from which sensor the gauge and ECU pull the signal but there is a second sensor. It apparently screws into the side of the heatercore up under the dash. It's apparently

Part number : GK3M61762

Now I almost remember hearing something about how there are two sensors in the speed 6 that measure coolant temperature, before, and one of the purposes is to measure to make sure the coolant has a drop in temperature when compared to the engine and after the radiator. In otherwords, to keep a reading on whether the radiator/themostat are working correctly.

I wish I knew if the heatercore sensor was where the gauge gets it's reading. Also keeping in mind that the AP is also apparently getting its reading from the sensor that affects the gauge.

Are any of the other Madaspeed 6 veterans around that might be able to offer some insight? From what I would think, getting to the heatercore is no simple/quick task. Really don't want to delv into that if it is pointless.
 

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Aha! They are right!

There is indeed a water temperature sensor which does appear to land in the heater core. But what's weird is the WSM specifies that only vehicles equipped with climate control have the sensor in this location. Hmmmm....
I didn't find it before because I figured it'd be in the cooling system, like in the rad core or something. But there is a "water temperature sensor" identified in the WSM.

Anyway it appears to be a bitch to remove, as it's listed in the manual. Remove glove compartment, console, meter hood, inst cluster, column cover, lower panel, steering shaft, A pillar trim, front scuff plate, front side trim, side panel and dashboard. WOW.
But, what are the chances this sensor is bad? Maybe you have air bubbles trapped in the core. Have you run the heater at all? You're suppose to run the heater anyway if you want to try to burp the system...
 

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Discussion Starter #13
Aha! They are right!

There is indeed a water temperature sensor which does appear to land in the heater core. But what's weird is the WSM specifies that only vehicles equipped with climate control have the sensor in this location. Hmmmm....
I didn't find it before because I figured it'd be in the cooling system, like in the rad core or something. But there is a "water temperature sensor" identified in the WSM.

Anyway it appears to be a bitch to remove, as it's listed in the manual. Remove glove compartment, console, meter hood, inst cluster, column cover, lower panel, steering shaft, A pillar trim, front scuff plate, front side trim, side panel and dashboard. WOW.
But, what are the chances this sensor is bad? Maybe you have air bubbles trapped in the core. Have you run the heater at all? You're suppose to run the heater anyway if you want to try to burp the system...
What the hell??? All that stuff comes off for the heater core?

I ran the heater for a couple minutes. Doesn't coolant circulate through heater core regardless though? I thought turning on the heat just redirected air through the heater core instead going around it.
 

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I don't see how it could be done any easier. The entire A/C unit/assembly is resting behind/under the dash. Removing all that shit isn't difficult, just time consuming.
As for the heater core, I'm thinking there is a variable control valve which varies the flow through the core...
This is how the climate/temp control stuff works.

Do you have access to an online WSM? I know some people have the whole thing hosted on the web. I've seen it available to VIP members on MSF, and I think I saw Justin once post a link to it for free.
Or, you can barrow mine, if you want. It has in detail everything that needs to be removed to access the core and the sensor itself.
 

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Discussion Starter #15
I don't see how it could be done any easier. The entire A/C unit/assembly is resting behind/under the dash. Removing all that shit isn't difficult, just time consuming.
As for the heater core, I'm thinking there is a variable control valve which varies the flow through the core...
This is how the climate/temp control stuff works.

Do you have access to an online WSM? I know some people have the whole thing hosted on the web. I've seen it available to VIP members on MSF, and I think I saw Justin once post a link to it for free.
Or, you can barrow mine, if you want. It has in detail everything that needs to be removed to access the core and the sensor itself.
Hey thanks, I really appreciate that. Before I tear all that apart though I would like to see if I can narrow it down. Check this out.

I went out to blast the heater as miserable as that was in this heat, and I drove around for about 20 minutes up and down entrance ramps, on the highway slowing down, speeding up aggressively. This was all done with the heater on max temp and fan speed. I had my AP plugged in and monitoring the coolant temperature the whole time. I saw the damn temp bounce around from 133, to 185, then briefly it would flicker to 223 and then hover around the 188 mark for a while. EVEN STRANGER I noticed that whenever I would press the throttle, the temp would click over to a believable temperature (like 200 or so) and it would do it instantly and the instant I would let off the pedal, the temp would drop back to 156 or so. It's almost like the temp readout was connected to my throttle positioning sensor!!!

I am at a loss here. I can't imagine what could be going on there.
 

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I'm kinda thrown off as well...
It's still unlcear to me where the gauge gets its input.
Why would 6's that are not equipped with "full auto" HVAC not utilize this "water temp sensor"? Don't those cars also have temp gauges in the inst. cluster?
And it seems like an odd location as well, being on the heater core. Seems like that'd be a good location for an HVAC related operation sensor.
What doesn't make sense it how you're seeing the inst. cluster gauge fluctuate in addition to the temp displayed on your AP/DH.
I think it's typical that when or if there's air in the system, the car indicates a high temp condition, not a drop in temp. And I would suggest an electrical problem or an open, short or interrupted circuit, but if there's indeed a sensor for both inputs, and BOTH inputs are fluctuating, how can that be?

Finding out what the water temp censor does for absolute certain will assist here.

I wish somebody else would chime in!
 

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Discussion Starter #17
I'm kinda thrown off as well...
It's still unlcear to me where the gauge gets its input.
Why would 6's that are not equipped with "full auto" HVAC not utilize this "water temp sensor"? Don't those cars also have temp gauges in the inst. cluster?
And it seems like an odd location as well, being on the heater core. Seems like that'd be a good location for an HVAC related operation sensor.
What doesn't make sense it how you're seeing the inst. cluster gauge fluctuate in addition to the temp displayed on your AP/DH.
I think it's typical that when or if there's air in the system, the car indicates a high temp condition, not a drop in temp. And I would suggest an electrical problem or an open, short or interrupted circuit, but if there's indeed a sensor for both inputs, and BOTH inputs are fluctuating, how can that be?

Finding out what the water temp censor does for absolute certain will assist here.

I wish somebody else would chime in!
Thanks, yah I really need to pinpoint where the temperature gauge and ECU read the temperature since the AP is also showing the fluctuation. I might start by calling some more Mazda Service Departments.
 

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you really need to look through the WSM. theres all sorts of information on the HVAC/cooling system in the back and it talks about all the logic and where everything is connected. the wiring manual is also hosted on MSF as well.
 

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Discussion Starter #19
Thanks, yah I need to look through it and see if I can figure it out. I did call around and talked to Jason at onlinemazdaparts.com and he was saying that the ECT sensor that screws into the heater core is strictly for the HVAC control so I guess I can rule that one out. Odd thing is it seemed that my temperature fluctuations were connected to my throttle position like I was saying. I pushed on the connector on my throttle boddy and coincidentally, the fluctuations have stopped. I know that is random and wild shit but maybe it's just a coincidence. Maybe an airbubble moved out also. I know our coolant moves through the throttle body so I began to wonder if the temperature was measure there also possibly but then again, some people have done the coolant bypass from the throttle body and haven't heard any reports of their temp gauges not working. It's like chasing a ghost sometimes.
 

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Can't contribute much to the conversation here, but I have done the throttle body coolant bypass, and can report no changes whatsoever to my coolant temp readings. I haven't even replaced the ounce or two of coolant that was lost rearranging the hoses.
 
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