Cooling system and ECT reading question

[MechaZao]

Hi all, first post. I have a 1996 Mustang 3.8 that I've driven for close to 10 years now. the heat used to work, but now does not (I'm not sure when it stopped working, living in FL I don't use it much at all).

I got a P0125 code the other day which was a first for me, so I set off on trying to track the issue down. I checked my coolant level, and everything was nice and topped off. my thermostat opens after a couple minutes of idling, which is visible on the dash gauge (it drops a bit from "R" to "O" on NORMAL.) however, according to my OBD2 reader, the ECT is only reporting a range of 140-160 degrees F. and it takes a LONG time to get there, close to 10-20 minutes of running.

I tried turning my AC off (its always on) and the fan never comes on by itself, even when it was threatening to overheat (boiling coolant and gauge at M/A). Turning the AC on makes the fan come on again and it normalizes.

I borrowed an IR thermometer and the temp by the thermostat was in the 200 range.

now for some stuff I did.

as a just in case fix, I went ahead and changed the thermostat out with a new one (both 195 degree) and also changed the ECT sensor with a new one in case it was reading low and faulty. the new one behaves exactly the same. I did notice quite a bit of debris and gunk in the piping under the ECT sensor, which i cleaned out with my finger as best I could - and my coolant appears to be fairly rust colored when I was burping with my funnel.

no matter what I do, I don't have heat and the ECT won't read higher than 170 (after an hour idling) maximum, so the fan never comes on and I imagine its impacting my fuel ratio as well.

the heater core hoses seem equally hot.

could this be something as simple as some clogged passageways, or do I have a bigger issue occurring? anyone battled something like this before?

 

[07GtS197]

I’m not as familiar with the 3.8 but I know that year gt has two sensors. One is a sensor for the ecu and the other one is a sender for the gauge cluster. The sensor for the ecu may be faulty causing a low reading.

 

[MechaZao]

I’m not as familiar with the 3.8 but I know that year gt has two sensors. One is a sensor for the ecu and the other one is a sender for the gauge cluster. The sensor for the ecu may be faulty causing a low reading.

I've put a new sensor in for the ecu already, though - no change in the reading

 

[95opal]

Are you sure you replaced the ECT sensor and not just the temp sensor.
To clarify, the 96-98 MY's, the dash temperature gauge colors are R/W and Y/R. The PCM ECT wire colors are LG/R and G/R.
If your coolant looks rust color it way be a couple of fins on your water pump have decintigrated. It may move enough to keep the car from over heating but not enough to move coolant through the core. Start the car warm it up, turn on the heat and bring the RPMS up. See if you get heat.

 

[cobrajeff96]

You said it yourself: the only time the fan kicks on is when A/C forces it on.

Could be one or more of three things:

- The temp sensor for the ECU is faulty
- The CCRM is malfunctioning
- The ECU is malfunctioning (unlikely)

Or any combination thereof.

 

[MechaZao]

Are you sure you replaced the ECT sensor and not just the temp sensor.
To clarify, the 96-98 MY's, the dash temperature gauge colors are R/W and Y/R. The PCM ECT wire colors are LG/R and G/R.
If your coolant looks rust color it way be a couple of fins on your water pump have decintigrated. It may move enough to keep the car from over heating but not enough to move coolant through the core. Start the car warm it up, turn on the heat and bring the RPMS up. See if you get heat.

yea - my motor has a single wire temp sensor to the dash, directly above the thermostat

the ECT sensor is to the left of that, more on the passenger side on the piping heading to the the heater core. thats the one I replaced. when I unplug it, my scanner shows -40F for engine coolant temperature. I noted that its supposed to be a bigger pigtail with 2 wires to the dash for a 96-98, maybe my motor is a slightly older version? either way, reading real low. I'll see if I get heat at higher RPMs - at idle and normal driving when I shift the dial from cold to hot the amount of airflow lessens considerably but no temperature change happens

 

[MechaZao]

You said it yourself: the only time the fan kicks on is when A/C forces it on.

Could be one or more of three things:

- The temp sensor for the ECU is faulty
- The CCRM is malfunctioning
- The ECU is malfunctioning (unlikely)

Or any combination thereof.

I'm thinking its the first one, since its reading so low. but replacing it changed nothing. the fan wouldn't run if the computer didn't know it was hot enough, right? lol

 

[95opal]

I'm thinking its the first one, since its reading so low. but replacing it changed nothing. the fan wouldn't run if the computer didn't know it was hot enough, right? lol

You are correct

 

[MechaZao]

Are you sure you replaced the ECT sensor and not just the temp sensor.
To clarify, the 96-98 MY's, the dash temperature gauge colors are R/W and Y/R. The PCM ECT wire colors are LG/R and G/R.
If your coolant looks rust color it way be a couple of fins on your water pump have decintigrated. It may move enough to keep the car from over heating but not enough to move coolant through the core. Start the car warm it up, turn on the heat and bring the RPMS up. See if you get heat.

gave this a shot tonight, don't feel any difference in heat at higher rpms vs lower. i think the water pump is working well because when I rev it with a no-spill funnel attached, the water in it sucks down real quick then goes back up when i let off. that may not be a good indicator though lol. I've not ever had any overheating issues in the time i've owned it. its always been rock solid on the gauge and I've never even had to top coolant off really.

now my normally ice cold AC is for some reason blowing only kind of cool..

i'm going to try replacing the connector for the ECT in case there's some kind of issue with it causing a bad reading - and if that doesn't help, I suppose I'll do a coolant flush and see if that works. I'm honestly not terribly concerned about the heat not working, since itll be hot again in a month or two anyway, but I would like my ECU to be reading my temps correctly at least lol

anybody know why the temp sending unit for my 1996 car is the single wire kind? every time i look it up online, they want me to get one with a more modern connector on it. is it just a model year discrepancy or something? for that matter, the ECT sensor i put in had a differently shaped brass end on it. the one that came out was cylindrical with a lip, and the new one is more of a cone - rockauto has both for my M/Y so idk.

on a side note, what should IAT temperatures look like an an OBD2 scanner? I noticed today that mine was reading about 100-105F, which seemed weirdly high considering it was only like 75 out.

 

[MightBuyMustang]

Blend door and relay issues. Check fuse block engine bay and under/dash blend door operation

 

[cobrajeff96]

Now the ECT for the ECU should be a 2-wire. I know a lot of aftermarket sensors (pretty much the only ones you can find these days) are not color-coded and often just have the same color for both wires. Maybe try switching them around temporarily. If I remember correctly, one of those wires coming from the engine harness should be energized. You can probe for voltage and if that's actually the case, then if you accidentally reverse polarity by connecting it incorrectly then this would 100% cause the ECU not to read actual coolant temp.

The other single wire, for the gauge cluster, is just a resistive measurement across ground as the gauge itself is already powered and only needs to read a variable resistance from the sensor.

And also check your engine bay grounds, near the headlamps. No rust.

 

[MechaZao]

an update today, i replaced the ECT sensor once again with one in the same style that i took out. I also cut and spliced in a brand new connector since my old one was brittle and not secured to the sensor. unfortunately, this didn't make a difference. I ran the car with my scanner hooked up again and made some notes on how it behaved with regards to the ECT reading.

Started up - AC off, no fan running, sensor reads 84F (close enough to ambient air temp, its 80 today and sun is directly on the motor)

2 minutes - no fan, sensor reads 92F, thermostat closed

3 minutes - no fan, sensor reads 110F, thermostat open

4 minutes - no fan, sensor reads 114F

8 minutes - no fan, sensor reads 152F

10 minutes - no fan, sensor reads 173.8F, dash gauge now exactly exactly half (normally sits on "O")

13 minutes - no fan, sensor reads 188F, dash gauge past half now (more on the M)

16 minutes - no fan, sensor reads 197F, dash gauge now halfway between "M" and "A" and I can smell hot engine/coolant so its close to overheating

19 minutes - fan finally comes on. sensor reads 208F, gauge is pretty much on "A" when this occurs.

at this point I turn on the AC to keep the fan on and cool it down slowly to normal temps before shutting off. with the AC on again it stabilized around 175-180 on the sensor.


my ECT appears to be working, but why the hell is it clearly reading much lower than the actual engine temp? i've checked coolant levels several times, followed the air clearing procedures, etc. when i squeeze any hoses with the sensor out, coolant flows easily visibly in that elbow so its not blocked up. i'm so puzzled.

at least I know the fan does properly come on when the "right" temps are seen, the issue seems to be getting it to read those temps in the first place.

the heat issue is probably a blend door problem at this point so not going to worry about that. I just want my car to see the correct engine temps, cause it was running noticably better when it was seeing 200ish (though i'm sure it was actually closer to 220)

 

[95opal]

an update today, i replaced the ECT sensor once again with one in the same style that i took out. I also cut and spliced in a brand new connector since my old one was brittle and not secured to the sensor. unfortunately, this didn't make a difference. I ran the car with my scanner hooked up again and made some notes on how it behaved with regards to the ECT reading.

Started up - AC off, no fan running, sensor reads 84F (close enough to ambient air temp, its 80 today and sun is directly on the motor)

2 minutes - no fan, sensor reads 92F, thermostat closed

3 minutes - no fan, sensor reads 110F, thermostat open

4 minutes - no fan, sensor reads 114F

8 minutes - no fan, sensor reads 152F

10 minutes - no fan, sensor reads 173.8F, dash gauge now exactly exactly half (normally sits on "O")

13 minutes - no fan, sensor reads 188F, dash gauge past half now (more on the M)

16 minutes - no fan, sensor reads 197F, dash gauge now halfway between "M" and "A" and I can smell hot engine/coolant so its close to overheating

19 minutes - fan finally comes on. sensor reads 208F, gauge is pretty much on "A" when this occurs.

at this point I turn on the AC to keep the fan on and cool it down slowly to normal temps before shutting off. with the AC on again it stabilized around 175-180 on the sensor.


my ECT appears to be working, but why the hell is it clearly reading much lower than the actual engine temp? i've checked coolant levels several times, followed the air clearing procedures, etc. when i squeeze any hoses with the sensor out, coolant flows easily visibly in that elbow so its not blocked up. i'm so puzzled.

at least I know the fan does properly come on when the "right" temps are seen, the issue seems to be getting it to read those temps in the first place.

the heat issue is probably a blend door problem at this point so not going to worry about that. I just want my car to see the correct engine temps, cause it was running noticably better when it was seeing 200ish (though i'm sure it was actually closer to 220)

From what you just posted all seems normal.
Your temp gauge being on the A is meaningless. The ECM appears to be sending the fan signal at the correct temp.

 

[MechaZao]

its normal for the motor to take almost 20 minutes to reach operating temperature with no fan running?

my concern is that its a 195F thermostat, so if that opens at 3 minutes in, would that not imply the engine is in that temperature range at that point? yet the ECT shows significantly lower at 110. I know the dash gauge isn't the most accurate thing, but the thermostat opens at "R" and then it settles at "O" when the AC is on (and thus fan running)

for it to be on "A" almost makes me think its closer to 230F when the ECT finally thinks its hot enough lol.

I admit I have not changed the sensor to the gauge, so I don't know if that's actually perhaps incorrect or something.

I would love to just be overthinking it and for there to not actually be any issue. the whole thing started because I got the P0125 code last week, I had never even checked my ECT before that.

 

[95opal]

its normal for the motor to take almost 20 minutes to reach operating temperature with no fan running?

my concern is that its a 195F thermostat, so if that opens at 3 minutes in, would that not imply the engine is in that temperature range at that point? yet the ECT shows significantly lower at 110. I know the dash gauge isn't the most accurate thing, but the thermostat opens at "R" and then it settles at "O" when the AC is on (and thus fan running)

for it to be on "A" almost makes me think its closer to 230F when the ECT finally thinks its hot enough lol.

I would love to just be overthinking it and for there to not actually be any issue. the whole thing started because I got the P0125 code last week, I had never even checked my ECT before that.

According to your post

3 minutes - no fan, sensor reads 110F, thermostat open

How are you verifying that the tstat is open at 3 min?

 

[MechaZao]

According to your post

3 minutes - no fan, sensor reads 110F, thermostat open

How are you verifying that the tstat is open at 3 min

with my hand, honestly. I see the dash gauge do its usual thing of going up to "R" and then back down to "O", and around that same moment the upper hose started getting hot. its possible I could be wrong about the timing there.

 

[95opal]

Check your ECT values with a volt meter. You can use a pot of hot water and any type of thermometer

Volts Degrees
5.1 -40
4.64 -40
4.48 -10
4.18 20
3.680 46
1.46 134
.74 180
.4 222
.26 254

 

[cobrajeff96]

208 is quite normal for the fan to operate. With a pressurized system, typically at 16 - 19psi, coolant's boiling point is well past ambient boiling point: somewhere around 230 - 240 depending on exact pressurization. My car runs happily at 220 all day no matter the weather. If you smell coolant even below 220, replace the cap. It goes without saying that hoses need periodic replacement over the years. And coolant, just in my opinion, should be changed every 2 - 4 years no matter the mileage.

I don't think there's anything wrong here.

 

[MechaZao]

the ECT values seemed consistent with the expected results when i tested them.

208 is quite normal for the fan to operate. With a pressurized system, typically at 16 - 19psi, coolant's boiling point is well past ambient boiling point: somewhere around 230 - 240 depending on exact pressurization. My car runs happily at 220 all day no matter the weather. If you smell coolant even below 220, replace the cap. It goes without saying that hoses need periodic replacement over the years. And coolant, just in my opinion, should be changed every 2 - 4 years no matter the mileage.

I don't think there's anything wrong here.

the cap is brand new FWIW, and i replaced all the hoses a year or so ago when one was cracking.

yeah, I understand that honestly but the main thing for me is that my car *doesn't* run at 208 or 200 or even 190. if I run the AC at all, it stays at 175 or lower (typically 155-160) even after long drives.

if I don't run the AC, it gets really hot (on the gauge) before it even attempts to cool itself lol.

I wish I had some way to see inside and confirm the sensor has coolant flowing on it and not an air bubble or something.

I tested my dad's car (admittedly it is newer), an 03 grand marquis with a 4.6L. watching the ECT reading on that, it went from ambient to right around 195-200 within 5 minutes very smoothly and stayed there, even with the AC running the entire time. its just weird behavior seeing mine be so slow to warm up allegedly and never hitting those sweet spots

I do appreciate everyone's assistance though, even though I'm feeling frustrated cause it feels like a "non-problem" problem

 

[cobrajeff96]

FWIW, my motor in the winter just idling won't get to temp until at least 20m. I had to do this often last winter when the diff blew up and I wanted to start the car and run it at least once a week. I would give it three temp cycles before shutting it down. It would take a real long time for it to reach op temp but once it did, it only took about 5 - 10m to run through three back to back heat cycles (fan on and off again).

Yea, you might want to burp the thing if coolant has been changed before all this happened.