Coolant temp sensor.

[Brent Hunter]

Could a bad coolant temp sensor cause my car to run like poo?

[Phil Maki]

yes, when it fails, it fails and gives a "cold" reading to the ecu.  when I say cold, i mean around -40*C i think...

[Brent Hunter]

Any chances you've hit a spare Phil?

[Jason Harwood]

Go to lordco.

Buy this part new.

[Brent Hunter]

Okay thanks.

[Ryan Coft]

Okay thanks.

If you have a 1g I have a brand new one installed in a coolant neck. Even comes with a trust rad cap! $10 come grab it before Sunday. I'm out of town all week starting Monday.

[Chad Giffen]

Can you describe how the car is running, I'm curious 

Like barely stays running? Low RPM's ? 

[Brent Hunter]

Very hard starting, hard to keep running while cold, spuratic idle, crappy fuel milage, poor performance. And a check engine code :p

[Chad Giffen]

So what is the purpose of both coolant sensors?

1) The one on the thermostat neck?
2) The one at the bottom of the radiator?

Would both give shitty performance?

[daniel Dee]

Very hard starting, hard to keep running while cold, spuratic idle, crappy fuel milage, poor performance. And a check engine code :p

Had all these problems on my Mazda....Went away when I replaced the ECU coolant sensor.

On my DSM AEM EMS the ECU was reading -40F and running 40% rich causing similar issues.
I was able to adjust the A/F fuel versus Temp map until the problem was fix (wiring issue)

The coolant sensor on the water neck (thermostat housing) is the ECU coolant sensor.

[Brent Hunter]

The lower sensor is the ecu one right? Not the one on top

[John Hartman]

The one in the rad is to turn the fan on.

one near the thermostat is for the gauge on the dash, and the other one is for the ECU to base its fuelling calculations on.

If you say its running poorly when cool, its not a typical coolant temp sensor issue.  Normally the car will run ok cold but very poorly when warm.  However, they are not much money, so its not a bad gamble to change it.

That said, pull the code for the check engine light and find out what the ecu is telling you the problem is.

Also, you can datalog the engine and read all the various sensors.  If any of them are obviously out of whack, there's your culprit.  IE, the coolant temp sensor reading -65 and its 20 degrees outside(before the car has started up, coolant will be the same temp as the outside air)

[Brent Hunter]

Ya it's throwing a temp sensor code, just trying to figure out which sensor it is lol. The big circular metal thing in the lower rad hose is the block heater?

[John Hartman]

gotta be something aftermarket.  Nothing in the stock lower rad hose.

The coolant temp sensor in question is in the water neck, below the pressure cap

[Chad Giffen]

Maybe a 1gb thing John?

I have one and it also has a wiring harness plug.