# How does the coolant temp sensor affect the way my car would run???



## djshyc (Mar 21, 2007)

What does the coolant temp sensor cause the ecu to do the engine to affect the way it runs??







I was told the coolant temp sensor is a very likely culprit to the way my car is acting..








It idles rough when cold yet accelerates fine.. Once warmed up to normal operating temp it accelerates horribly yet idles fine....















HELP !!!


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## GTijoejoe (Oct 6, 2001)

*Re: How does the coolant temp sensor affect the way my car would run??? (djshyc)*

your CLT sensor provides a fuel enrichment dependent on CLT temperature...
colder = rich


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## ABF Jeff (Nov 16, 2005)

*Re: How does the coolant temp sensor affect the way my car would run??? (GTijoejoe)*

Its there to help with cold running conditions, kinda the same way a choke works on a carb, richens the fuel mixture when the engine is cold to help it run better... the thing is, it's what's called a thermistor... unlike most electronics, the hotter it gets, the lower the resistance is... so when it goes bad, the resistance is high, the ECU thinks the engine temp is like -40 degrees, so it runs pig rich and misses like crazy.
It's a cheap sensor, there's 2 of them... they're on the coolant flange on the side of the head, the 4 wire one is for the ECU, the 2 wire one is for the insturment cluster.
It's as easy as any parts replacement goes... let the pressure out of the system by removing the cap on the coolant res, pull the little spring clip out, unplug the harness, remove the sensor, install the new one, re-install clamp, connect harness, replace resivior cap... I've replaced them with nothing but a VW key in a parts store parking lot in 2 minutes. http://****************.com/smile/emthup.gif


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## JettaspAnk (Mar 25, 2007)

*Re: How does the coolant temp sensor affect the way my car would run??? (ABF Jeff)*

Wow I did'nt know it made you run rich. In fact that sensor caused my lean code to come up again







. When I replaced it it went away.


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## atoson (Dec 29, 2002)

*Re: How does the coolant temp sensor affect the way my car would run??? (JettaspAnk)*

The ECT sensor is one of the main inputs to the engine management system. The signal is used to adapt ignition timing, injection time and idle speed stabilization. In addition, the ECT signal is used to activate the knock control system, the idle speed control system, oxygen sensor operation, fuel tank venting and exhaust gas recirculation.
The ECT sensor sends continues engine temperature information to the Motronic ECM. If there is a break in the signal from the temp sensor, the ECU will simulate a fixed signal based on an engine temp of 176*F. The ECT is combined with the temp gauge sensor in a single housing.


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## djshyc (Mar 21, 2007)

*Re: How does the coolant temp sensor affect the way my car would run??? (atoson)*


_Quote, originally posted by *atoson* »_The ECT sensor is one of the main inputs to the engine management system. The signal is used to adapt ignition timing, injection time and idle speed stabilization. In addition, the ECT signal is used to activate the knock control system, the idle speed control system, oxygen sensor operation, fuel tank venting and exhaust gas recirculation.
The ECT sensor sends continues engine temperature information to the Motronic ECM. If there is a break in the signal from the temp sensor, the ECU will simulate a fixed signal based on an engine temp of 176*F. The ECT is combined with the temp gauge sensor in a single housing.



So WHY AM I NOT GETTING ANY CODE ABOUT THE ECT Sensor ???


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## atoson (Dec 29, 2002)

*Re: How does the coolant temp sensor affect the way my car would run??? (djshyc)*

What scan tool are you using?


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## ABF Jeff (Nov 16, 2005)

*Re: How does the coolant temp sensor affect the way my car would run??? (atoson)*

just because the sensor is out of limits and throwing bogus signals to the ECU doesn't necessarily mean you'd have a code for it... in fact, a lot of ECT issues pop all kinds of other codes... if it's just old and sending goofy signals to the ECU, chances are it's enough within limits that the ECU doesn't know it's the problem.


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## atoson (Dec 29, 2002)

*Re: How does the coolant temp sensor affect the way my car would run??? (ABF Jeff)*

I think he doesn't undrestand that you need a scan tool to read codes and expects it to come from instrument cluster. He hasn't mentioned the words "No Faults Codes Detected", if he did then it would be obvious he used a scan tool or software tool.


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## Thumper1044 (May 8, 2007)

*Re: How does the coolant temp sensor affect the way my car would run??? (djshyc)*

I just replaced the 4 prong coolant sensor in my car and took some resistance reading before I installed it and documented them so I have them in my Bentley Manual. If you want to check yours, here are the readings and position of the terminals:
1. Look at the four prongs of the sensor and have the 2 alignment tabs for the connector at the bottom of the switch.
2. The two terminals on the left side read 1196 ohms
3. The two terminals on the right side read 2220 ohms.
THE AMBIENT TEMPERATURE WAS 74F. This is important as values will change with temperature. Bentleys give you a chart for temperature correction, but does NOT specify the difference between the two terminals. BTW, the removed (old) switch was not too far off from these same values.
Hope this helps some. I got the new switch from NAPA for about $12 but it didn't make my problem of the car NOT idling when cold.
Good luck
Thumper


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## djshyc (Mar 21, 2007)

*Re: How does the coolant temp sensor affect the way my car would run??? (atoson)*


_Quote, originally posted by *atoson* »_What scan tool are you using?

An OBD 2 Code reader from autozone.


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## 97VWJett (May 17, 2003)

*Re: How does the coolant temp sensor affect the way my car would run??? (djshyc)*


_Quote, originally posted by *djshyc* »_
An OBD 2 Code reader from autozone.

That is the problem...you need a VAG-COM


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## atoson (Dec 29, 2002)

*Re: How does the coolant temp sensor affect the way my car would run??? (97VWJett)*


_Quote, originally posted by *97VWJett* »_
That is the problem...you need a VAG-COM


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