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Low Engine Temp

16K views 20 replies 5 participants last post by  aerospaced 
#1 ·
So, I am getting the following CEL's: P0126 and P0128. I've checked the fluid temp and that is fine and the fan has been running. I pulled an engine diagnostic stat after the car had been running for a little bit and the engine temp is around 130 F. I am thinking the thermostat has gone bad and needs replaced but I wanted to get other opinions before I swap it out.

Thanks
 
#3 ·
So I replaced the t-stat this past weekend and it seemed to run fine on Sunday. Well the wife came home yesterday and said the CEL came back on. I pulled the code and it was P0128, Coolant Thermostat. I'm thinking maybe this time it is the Engine Coolant Temperature Sensor. Do anyone know where that is or could send me a pic from the service manual?

Thanks
 
#5 ·
Thanks flipper. Is it possible to get to that without taking the 2 coolant tubes off? I am guessing not based on how tight it is in there. Is there anything else that would cause that CEL?
 
#6 ·
What did you use for a thermostat? If it doesn't have the secondary spring and shut-off valve, it won't work right. Doesn't allow coolant to mix or flow properly.

Same goes with O'Reilly's thermostat that has the secondary spring and shut-off valve; it doesn't work correctly either. It's too high of a temp- 192. Factory is 188.

I went rounds with both situations 4 weeks ago. I then put in the correct Motorcraft thermostat (RockAuto.com part RT1175) and everything has been fine since.

One interesting thing I discovered when I had the incorrect thermostat with no secondary spring and valve- the coolant DOES NOT completely/properly flow through the block. It was plain of day because of the different in color of the coolant.
 
#8 ·
What did you use for a thermostat? If it doesn't have the secondary spring and shut-off valve, it won't work right. Doesn't allow coolant to mix or flow properly.

Same goes with O'Reilly's thermostat that has the secondary spring and shut-off valve; it doesn't work correctly either. It's too high of a temp- 192. Factory is 188.

I went rounds with both situations 4 weeks ago. I then put in the correct Motorcraft thermostat (RockAuto.com part RT1175) and everything has been fine since.

One interesting thing I discovered when I had the incorrect thermostat with no secondary spring and valve- the coolant DOES NOT completely/properly flow through the block. It was plain of day because of the different in color of the coolant.
I used the thermostat from O'Reilly's; Murray brand. The temperature is only getting to about 140 degrees with the new thermostat in there, so there is still something else going on. That's why I am thinking it is the CTS. Which I did notice that on Rockauto, there are two of them listed, and one in the wrong part. The CTS doesn't screw into the block like the cheaper one shows, it just slides into the hole.
 
#11 ·
I haven't been following that exact procedure, but basically doing that to get any air bubbles out of the system. I'm going to replace the CTS this weekend and see of that fixes it.
 
#12 ·
Erratic Temps

Got CEL code for low temp coolant which was not surprising as the temp gauge in the car reads colder than normal (about the 1/4 position instead of mid-position). I replaced the thermostat with an aftermarket one from NAPA with no change on temperature gauge and got another CEL code for low temp coolant. I replaced the thermostat once again this time with a motorcraft RT1157 as recommended on this forum. Now when the car idles the temp is at mid-position however when I drive the temp drops down to the 1/4 position as before. Heater blows very hot air and hoses seem very hot to the touch. I have purged the system of air as evidenced by the need to add coolant multiple times until the level stabilizes. Was wondering if it could be a faulty CTS - I cleaned the contacts on both male and female connectors with no change in results. Was wondering if replacing the CTS solved your temp problems - greatly appreciate any info or suggestions. Thanks.
 
#13 ·
^^ more often than not; both the thermostat and the CTS are wrongly accused. Supposedly (i have not verified this) the CTS wires are wht/red & Brn/red and they go into the cabin to the BCM. Because this is a "resistance" based measurement, any degradation in a connection between the CTS itself and the BCM and multiple systems are impacted;

A few things that come to mind are Fuel trims, warm up time, Open loop/closed loop operation, shift points, idle speed, and lord knows what else but you can bet it impact the emmsions when the run time doesn't correlate to all the other data. OH - FOR the SAKE OF CONFUSION; the WHITE/RED goes into a connected and comes out as LT Green/Black at the BCM!!

What I don't see is the values for COLD resistance 32F and resistance at 212F. With this, we stand a way better chance of knowing who is telling the truth.

I am all in favor of going with common sense. The engine is hot, its blowing hot air, and when you grab the hoses it burns. In short - data is being lost. Perhaps as a good faith effort I may measure mine at cold and hot temps and see what it is. Please don't wait up for me tho. I have no reason to tear it apart at this moment but will be doing spark plugs some time soon so I'll try and do it then.

For your own sanity; get an ohm meter and measure the sensor directly. Now crawl under the dash and measure those same leads at the BCM. 2H and 1M are the locations.

Good luck! and add tag me or PM so I keep up to date on this.
 
#15 ·
PM from OP moved to forum. Hope thats OK!

Many thanks Final Impact - wow never considered that many variables! What bothers me is the fact that when stationary the temp gauge is right at the mid point. Even if the CTS is providing the wrong resistance while the car is stationary there is a definite change in the reading versus when the car is moving. What i dont understand is why there would be a change in resistance of the CTS and associated wiring when moving vs stationary? It is definitely not intermittent as it has been very repeatable. Any thoughts/suggestions greatly appreciated.


A couple things come to mind:
1) the system has air trapped. Try pulling the right front reservoir side up on block of wood to elevate it 3 to 5 inches and run it. Add more fluid if it will take it.

2) a bad connection + heat increases resistance. So, any bad connection could propagate this. Look for those near heat sources like the back of the engine. Follow TCS wires to every point including those inside the cabin. Unplug them and inspect them for corrosion. Clean anything found.

3) a bad ground. Inspect all engine grounds. Again, if the resistance changes it will impact this. Excessive heat changes resistance more than anything especially if joint or socket has corrosion in it heat will impact even more!Most senders of this type are changing about a 100 ohms from hot to cold (WAG).

Thats where I would begin.
 
#17 ·
FYI - Found the following values for the Engine Coolant Temperature Sensor:

Water temp Resistance (kohms)

68 F 35.48 to 39.20

158 F 5.07 to 5.60

176 F 3.65 to 4.02

Will be checking resistance on my sensor tonight.
Where did you find that? Me wants to know! Also, what did you find?
 
#18 ·
ECT Sensor Values

Got the values from a Mazda factory troubleshooting guide - it contained similar data for many other sensors as well. Unfortunately got sidetracked - will try to take measurements this w/e and advise. Anyway to easily measure coolant temp with a thermometer without having to remove coolant temp sensor? Appreciate your thoughts.
 
#19 ·
Fixed Low Temp Problem - Coolant Temp Sensor

Got the values from a Mazda factory troubleshooting guide - it contained similar data for many other sensors as well. Unfortunately got sidetracked - will try to take measurements this w/e and advise. Anyway to easily measure coolant temp with a thermometer without having to remove coolant temp sensor? Appreciate your thoughts.
Got a replacement coolant temp sensor on ebay for $34 which fixed the low temp coolant problem on my car. Temp gauge now consistently reads mid scale once engine is up to temp. Hard to believe the CTS could cause erratic readings as I would have thought the CTS would have failed as either an open or short circuit but not in between.
 
#20 ·
[QOTE=aerospaced;3496391]Got a replacement coolant temp sensor on ebay for $34 which fixed the low temp coolant problem on my car. Temp gauge now consistently reads mid scale once engine is up to temp. Hard to believe the CTS could cause erratic readings as I would have thought the CTS would have failed as either an open or short circuit but not in between.[/QUOTE]

Good news! Thanks for sharing!

Sad that basic thermocouple can't be made to last. They run everything around us. Anything that has temperature control (even the hard drive in your computer reports the temperate) has them. Glad that fixed it.

I would like to find the print of info having the limits. Could you recite page number and PN of the manual? Thanks! Was it a Mazda Manual?

JJD952
 
#21 ·
ECT Sensor Values

After a little more homework I found out the values for the ECT previously provided are for the 4 cyl L3 engine (see page 01-40A-36). The voltage at the PCM terminal 1M for the L3 should be as follows:
68F - 3.04 to 3.14 Volts
140F - 1.29 -1.39 Volts
Per table on pg 01-40A-9


The correct values for AJ 6cyl:

The voltage at PCM terminal 38 are:
68F 2.9-3.1 Volts
"After warm-up" - 0.2 to 1 Volts
See page 01-40B-8

With respect to sensor resistance:
68F 2.2 to 2.6 Kohms
176F 0.29 - 0.40 Kohms

Per page 01-40B-28

The above info came from 2004 Mazda6 sport sedan workshop manual part number 9999-95-063B-04R
 
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