My wife was driving our 05 Tribute home a few weeks ago (she was running the defroster) she noticed that the engine light had turned on and that the temp gage was all the way down on cold. She stopped at the store, came out, went to start the it, it wouldn't start. Waited for 15 minutes then it started. Engine light still on, was ok coming home. Had it toed to the dealer the next day. Scan said bad ECT Sensor, was telling the truck it was 40 below. Repaired and everything seemed fine. Friday I left work, used the air conditioned on my way home (7 miles) dropped a fellow worker off then the engine started idling very high and the temp gage went all the way down again. Immediately turned off the AC pulled in my drive, let it idle and the temp gage went normal and the truck was idling fine. Engine light never came on this time. It runs just fine as long as AC compressor isn't running. Dealer is checking with Mazda looking for answers. Anyone have any ideas????

 

First time into dealer you only noted "repaired". What was repaired? The ECT registering -40 (actually -46'F) would lead me to first believe that there was a poor connection in the ECT circuit (ie ECT sensor completly disconnected gives a -46'F signal and would set a P0118 code).
The second instance of this occuring where temp went back to normal after idling could also point to a poor ECT connection that is intermittent but could also be a thermostat stuck open. Driving and A/C fans running would get coolant temperature cooled off considerably and then allowing engine to idle with no fans from A/C and no air flow thru radiator would allow coolant to warm back up.

 

Thanks for the reply. The Water outlet was replaced. I believe the thermostat was also replaced. Without the use of the AC or defrost, it runs perfectly fine, temp gage in normal range.

 

Where is the ECT sensor located?

 

Does the Trib have the 4 cylinder engine or V6?

 

V6.

 

Couple of pics below should help you locate the ECT. First is from farther away and second close up.
"water outlet" ( OE part # AJC1-15-17Z) that you said was replaced should have included the ECT as the water outlet is that black plastic housing that ECT is mounted into. Still thinking you have a thermostat getting stuck open. There are dozens of different problems that can cause and engine to run to hot or overheat but only one (thermostat) that could make engine run too cold, especially in the manner that you are describing. Would not be surprised if you get a CEL on with a P0125 or P0128 for excessive time for coolant to reach operating temp.

[IMG]local://upfiles/1377/A18B417C2C4943F7B60B373C5C0B8605.jpg[/IMG]

[IMG]local://upfiles/1377/66C22F6C2D5047D4BA720F95EAD9D9FC.jpg[/IMG]

 

Thanks for the pics.

So the ECT sensor responds to the coolant temp. With the thermostat open, the temp readings are going to be lower, the sensor sends this info to the ECM and the engine adjust accordingly? i.e. high idle, CEL on, etc.

The other part they replaced (0000-77-507E-02) is that the thermostat?

So I guess the question is, why is the thermostat getting stuck open, and why only when the AC compressor is running?

 

That part number is for a gallon of Mazda Gold Coolant that was drained out in order to replace the water outlet/ECT sensor.
ECT signal voltage changes with coolant temperature and sends this signal to the engine computer or PCM. PCM then sends this signal to instrument cluster and registers on the gauge. PCM also uses this signal to determine if engine is warmed up to operating temp to control idle speed, fuel injection amount etc. and also if engine is getting too hot to turn on the cooling fans. Thermostat being stuck openis not related to A/C being on.
Quick look at how it all works and then will understand better. The thermostat is simply a gate that either allows coolant to flow thru the radiator or not. When coolant temperature reaches approx 180'F the thermostat begins to open slightly and allows a small amount off coolant to circulate thru the raditor and get cooled off. As coolant temperature continues to rise the thermostat opens up more and more untill fully open at around 212'F. At this point the maximum amount of coolant is being allowed to flow thru the radiator to be cooled. If you are driving down the freeway then you have a 65 mph air flow thru the radiator to provide the cooling. If you are stuck in traffic then the cooling fan will come on to provide the air flow thru the radiator for cooling. Now, say you are driving down the freeway and coolant is being cooled too much (dropping down to 180'F) The thermostat will start closing off and reduce coolant flow thru the radiator in order to maintain an ideal operating engine/coolant temperature of 210'F. This is what the thermostat does. When you turn on your A/C or use the defrost mode, the A/C condenser fan automaticly turns on to cool off the A/C condenser which is located just in front of the engine radiator. This also blows air over the radiator and is cooling it down as well. If the thermostat is mechanically stuck open then it cannot close off to maintain an ideal engine temp and cooling system temperatue begins to fall untill you see the gauge needle dropping.

 

babyhuey, thanks. you have been a big help.

I thought the service manager said they replaced the thermostat, but there is no other part listed on the invoice.
So if the thermostat is stuck open, and the fan from using the AC is running, that is going to cool it down even more. So all this could just be a bad thermostat?