Why is the temp gauge in my car going up and down , even when engine is completely cold ?

[Fastlife91]

A few days ago, got in my car after it has been parked several hours. As soon as I started the engine the temp gauge instantly hit the red, fell right back to bottom then shot to red then to bottom. Once driving it stayed below half way and behaved normally. 

Got in car the next day and white smoke came out the exhaust for about 5 - 10 seconds and car felt like it was going to stall, revs falling and car juddering. after a few seconds it went back to normal idle and as I was driving it behaved normally staying below 90.

did it again yesterday morning and this morning after parked overnight, smoke out back and unsteady feeling like it was going to cut out for 30 seconds - minute,   and the temp gauge shot right to half way then into red and back to bottom again even though the engine was stone cold.

Usually it's okay after the initial cold start up but while I was actually driving today it was fluctuating again, hit red for a second then dropped below half way and being erratic.

I guess the issue with the smoke on cold start up  and the fluctuating temperature gauge are related to the same problem as they both occurred at the exact same time .

Is it a problem with the coolant temperature sensor do you think and would a faulty coolant temperature sensor cause a hard cold start and smoke on cold start ?  

Many  thanks 

Edited June 26, 2016 by Fastlife91

[jmurray01]

I would suggest the temperature sensor is the problem, as if it is telling the ECU that the engine is warm when it isn't then it will adjust the fuel/air mixture accordingly, therefore making it run badly (like running a carburetted engine cold without the choke).

Other members will be able to give you a more specific diagnosis, but I think that's the crux of the problem.

[Fastlife91]

11 minutes ago, jmurray01 said:

I would suggest the temperature sensor is the problem, as if it is telling the ECU that the engine is warm when it isn't then it will adjust the fuel/air mixture accordingly, therefore making it run badly (like running a carburetted engine cold without the choke).

Other members will be able to give you a more specific diagnosis, but I think that's the crux of the problem.

 

Thank you for replying. Very kind of you indeed. 

 

I hope it is only the temp sensor as it is a relatively easy and inexpensive fix, I have enough on my plate with other things to fix on this car without having to add anything serious so hopefully it is just the sensor 

 

is there any way to test it or should I just replace it ? 

 

 

Many thanks 

[jmurray01]

53 minutes ago, Fastlife91 said:

 

Thank you for replying. Very kind of you indeed. 

 

I hope it is only the temp sensor as it is a relatively easy and inexpensive fix, I have enough on my plate with other things to fix on this car without having to add anything serious so hopefully it is just the sensor 

 

is there any way to test it or should I just replace it ? 

 

 

Many thanks 

The fact is, even if it isn't causing the running problems, it will be causing the temperature gauge issue - so for that alone it is worth replacing. 

[Tdci-Peter]

As Jamie says, the temperature sensor in the cylinder head goes into the ECU (or PCM) & is used to adjust the engine fuel injection (& ignition if petrol). The same information is then transmitted to the instrument cluster to operate the temp. gauge.

If no warning lights are on, then, with a bit of luck, it will just be a faulty sensor, or even just a bad connection in the connector.

The sensor is just a thermistor, that is a resistor where the resistance falls as the temperature rises. You can test it with a multimeter, if you can make contact with the pins in the connector. I am not sure exactly what the resistance should be, betwen 1k ohm & 10kohm at 20C almost certainly, reducing smoothly by a factor 5 to 10 as engine temp rises to normal. And it is that smooth change is the key, any jumping about indicates a fault that would cause the needle to jump about.

But as the sensor is quite cheap, may be best to replace it first.

If it is not the sensor, then the wiring to the pcm, the pcm connector, the pcm (hope not), or the IC (Instrument cluster) are possible culprits. There is a simple gauge test for the IC, google "focus ic gauge sweep".

Most of these faults are likely to light a warning light, so the sensor is suspect no.1

Peter.