Coolant Temperature Sensor Issues!!! (Page 1/2)
82-T/A [At Work] NOV 30, 08:32 PM
Ok, I've got a really weird situation, and looking for some advice. This is my daughter's car... 1985 Fiero 2m4 SE 5-Speed.

Here's my problem... coolant temperature gauge, AND the ECM CTS (Coolant Temperature Sensor) are both showing below 100 degrees while I'm driving the car around. It's affecting my fuel consumption as well.

TLDR: About 4 months ago, I was having issues with coolant gushing out of places. Turns out I had a cracked head that would only exert pressure on the cooling system when the car was driving, otherwise at idle, it was fine. The temperature gauge DID in fact read properly at that point.. but only after replacing the coolant temperature sending unit. The new one I'd installed was giving really bad readings, it would creep up to 240+ when the engine was absolutely not running like that. I replaced the sending unit for the gauge, and it was fine.


CURRENT: I swapped out the cyl head, no longer having coolant issues. Car is driving fantastic, but now both the CTS and the Temperature Sending Unit are reading extremely low temperatures. I've got Paul Romsky's ALDL reader, and it's showing that my car has a CTS of around 50-90 degrees Fahrenheit... and it slowly creeps down and creeps up, but stays in that range. It does this continuously and never gets above 100 degrees. Oddly enough, my actual temperature gauge (which was tested by Jack Gunsett (and working before) refuses to show anything above 100 degrees. When I turn the car on, it literally sits at 100 degrees and stays there, never moves.

I have a 185 degree thermostat in it, and I know coolant is fine. I'm not losing any coolant, and I know it's getting up to operating temperature as I use my laser temperature gauge and the head is around 205-210 degrees, and the temperature at the coolant neck before the thermostat is around 195, and ~180 after it.

Another odd thing is that the radiator fan is always on. It's never not been on. I've been meaning to check the relay, but just haven't gotten around to it.


The car has perfect uncrushed cooling tubes, a brand new 3-core radiator that's oversized, the best water pump design out of like 4 of them, and 50/50 antifreeze mix with Water Wetter additive. Obviously, it's running extremely efficiently... but with it being 70 degrees outside, I know the engine isn't running at 54 degrees.


Not even sure where to check. Everything about the car is great... but with the CTS thinking I'm driving in a snowstorm, it's getting pretty poor fuel economy. How do I even begin? There are no other issues...


Thanks!
cartercarbaficionado NOV 30, 09:32 PM

quote
Originally posted by 82-T/A [At Work]:

Ok, I've got a really weird situation, and looking for some advice. This is my daughter's car... 1985 Fiero 2m4 SE 5-Speed.

Here's my problem... coolant temperature gauge, AND the ECM CTS (Coolant Temperature Sensor) are both showing below 100 degrees while I'm driving the car around. It's affecting my fuel consumption as well.

TLDR: About 4 months ago, I was having issues with coolant gushing out of places. Turns out I had a cracked head that would only exert pressure on the cooling system when the car was driving, otherwise at idle, it was fine. The temperature gauge DID in fact read properly at that point.. but only after replacing the coolant temperature sending unit. The new one I'd installed was giving really bad readings, it would creep up to 240+ when the engine was absolutely not running like that. I replaced the sending unit for the gauge, and it was fine.


CURRENT: I swapped out the cyl head, no longer having coolant issues. Car is driving fantastic, but now both the CTS and the Temperature Sending Unit are reading extremely low temperatures. I've got Paul Romsky's ALDL reader, and it's showing that my car has a CTS of around 50-90 degrees Fahrenheit... and it slowly creeps down and creeps up, but stays in that range. It does this continuously and never gets above 100 degrees. Oddly enough, my actual temperature gauge (which was tested by Jack Gunsett (and working before) refuses to show anything above 100 degrees. When I turn the car on, it literally sits at 100 degrees and stays there, never moves.

I have a 185 degree thermostat in it, and I know coolant is fine. I'm not losing any coolant, and I know it's getting up to operating temperature as I use my laser temperature gauge and the head is around 205-210 degrees, and the temperature at the coolant neck before the thermostat is around 195, and ~180 after it.

Another odd thing is that the radiator fan is always on. It's never not been on. I've been meaning to check the relay, but just haven't gotten around to it.


The car has perfect uncrushed cooling tubes, a brand new 3-core radiator that's oversized, the best water pump design out of like 4 of them, and 50/50 antifreeze mix with Water Wetter additive. Obviously, it's running extremely efficiently... but with it being 70 degrees outside, I know the engine isn't running at 54 degrees.


Not even sure where to check. Everything about the car is great... but with the CTS thinking I'm driving in a snowstorm, it's getting pretty poor fuel economy. How do I even begin? There are no other issues...


Thanks!


radiator fan on at all times huh? did you pinch wires or you got a short you can trace? or did you accidentally cover too many threads in thread sealer
if it aint those then maybe its bad sensors again? new stuff is garbage right now
Vintage-Nut NOV 30, 10:14 PM
Stupid Questions as I'm a V6 Guy:

Did you test the Coolant Temperature Sensor?


{Oversized 3-Core Radiator / 185-Degree Thermostat / Radiator Fan is Always ON and the Coolant Temperature is Too Cold}

It sounds like that the coolant system was designed for the desert.

With an extremely efficient coolant system with a small engine, the coolant flow might be the problem; not the sensor.....

Did you unplug the radiator fan to see if the coolant temperature will raise? {Of course, fix the circuit}

Did you try a standard 195-degree thermostat? {Easy to Change}

[This message has been edited by Vintage-Nut (edited 12-01-2025).]

Patrick DEC 01, 12:13 AM

quote
Originally posted by 82-T/A [At Work]:

I've got Paul Romsky's ALDL reader, and it's showing that my car has a CTS of around 50-90 degrees Fahrenheit... and it slowly creeps down and creeps up, but stays in that range. It does this continuously and never gets above 100 degrees.



Any chance those temperatures are actually in Celsius?
Mickey_Moose DEC 01, 01:21 PM
Test the wiring from the gauge to the sensor - ground out the wire at the sensor - does it affect the gauge/ALDL software?

If it does, you can eliminate the wiring...
GodSend DEC 01, 02:33 PM

quote
Originally posted by Patrick:

Any chance those temperatures are actually in Celsius?



Measuring in beaver pelts instead of eagle feathers is a common mistake...
82-T/A [At Work] DEC 01, 10:18 PM

quote
Originally posted by cartercarbaficionado:

radiator fan on at all times huh? did you pinch wires or you got a short you can trace? or did you accidentally cover too many threads in thread sealer
if it aint those then maybe its bad sensors again? new stuff is garbage right now




That was actually an issue that existed when I got the car. I should take a look at the wiring in the service manual. As long as the wiring isn't shared somehow with the other sensors, then my guess is it's probably a relay or something. I'll probably try to solve these one at a time. The fan runs regardless of whether or not I disconnect the fan switch sensor. So, it might be an HVAC wiring issue since I know that's related.

I should probably try to solve these one by one, I guess I assumed that they might somehow share some wiring or be related.



quote
Originally posted by Vintage-Nut:

Did you test the Coolant Temperature Sensor?

{Oversized 3-Core Radiator / 185-Degree Thermostat / Radiator Fan is Always ON and the Coolant Temperature is Too Cold}

It sounds like that the coolant system was designed for the desert.

With an extremely efficient coolant system with a small engine, the coolant flow might be the problem; not the sensor.....

Did you unplug the radiator fan to see if the coolant temperature will raise? {Of course, fix the circuit}

Did you try a standard 195-degree thermostat? {Easy to Change}





I don't remember if it was an issue before we started to tear down the car. But if you saw my other posts from a while back, we had a cyl head issue after rebuilding the engine (the cyl head had a crack in it). But the brand new sending unit sensor was actually bad. We replaced it, and then it started working.

Now, the temperature gauge is stuck at 100 degrees and doesn't move at all from there. I may try to wire up manually my spare set of 85 gauges to it directly to the sensor.

And yes, haha... I know how to change a thermostat. We just rebuilt the entire engine. Come on man, you know me... I've been on here for 25 years.

Do you know if these sensors share any common wiring... other than maybe the ground?



quote
Originally posted by Patrick:

Any chance those temperatures are actually in Celsius?




Well, I know my gauge cluster isn't in celcius, and it doesn't move. But it's possible I have three completely separate unrelated problems that are all related to temperature. You know... it's seriously unlikely, but anything is possible. I did use the laser temperature reader, and it's reading normal healthy engine temperatures. I'm using Paul Romsky's ALDL scanner... but the CTS says Fahrenheit.



quote
Originally posted by Mickey_Moose:

Test the wiring from the gauge to the sensor - ground out the wire at the sensor - does it affect the gauge/ALDL software?

If it does, you can eliminate the wiring...




Ok, I'll check that this weekend, thank you!



quote
Originally posted by GodSend:

Measuring in beaver pelts instead of eagle feathers is a common mistake...



Lol, this is great... I'm stealing this.
Vintage-Nut DEC 02, 01:12 AM

quote
And yes, haha... I know how to change a thermostat. We just rebuilt the entire engine. Come on man, you know me... I've been on here for 25 years.


Todd, please reread my post...

When I said if you tried a standard 195-degree thermostat; this means "remove the 185-Degree Thermostat and try standard 195-degree thermostat indeed."

Good luck with your issue...
cartercarbaficionado DEC 02, 02:10 AM
hm yeah sounds like you got a hvac wiring issue if you can disconnect the fan switch at the engine unless there is a short to ground somewhere like mine was
honestly i would ohm out your temp sensor and see what happens
82-T/A [At Work] DEC 02, 09:19 AM

quote
Originally posted by Vintage-Nut:

Todd, please reread my post...

When I said if you tried a standard 195-degree thermostat; this means "remove the 185-Degree Thermostat and try standard 195-degree thermostat indeed."

Good luck with your issue...




I guess I’m not seeing how 10 degrees of difference in a thermostat can make up for ~90 degrees of temperature. I have both a new 195 and new 185. Are you saying you think the thermostat is bad? Or are you telling me 10 degrees would make a difference?

The engine was designed for a 180 degrees. 195 degrees was mandated in 1978 to improve catalytic converter efficiency… that’s the only reason why I use a 180 because the engine will be healthier long term.