temperature gauge is fluctuating, goes from zero to red to zero. what is wrong ?

[Fastlife91]

I drive a 2006 mk2 Ford Focus Sport 1.8 TDCI

 

Yesterday I got in my car and started the engine, instantly the temperature gauge shot to red despite the engine being off for 2 hours, stayed in the red for 2 seconds then dropped to minimum then shot right back to red and then back to minimum. Once I began driving it went back to normal staying below the half way line.

 

Got in the car this morning after it was parked overnight, soon as I started the engine it shot straight into the red but then straight back to minimum line. It drives fine when driving it, it's just when you first start the engine from cold, it jumps right to the red zone for 1 or 2 seconds before falling right back down.

 

All the hoses get hot, one hose takes a little longer to heat up before the other hoses, the hose to the radiator to the thermostat takes a little longer than the others to get hot. The thermostat was replaced 20,000 miles ago ( last year ) 

 

Once the car moves from idle the temp gauge is fine with no fluctuating whatsoever , it is even fine at idle apart from the first few seconds you start the engine that it fluctuates .

any ideas why the temp gauge in my car is fluctuating ?  

 

My car is blowing hot air and has plenty of water in the expansion tank 

[Tdci-Peter]

7 minutes ago, Fastlife91 said:

soon as I started the engine it shot straight into the red but then straight back to minimum line

It can only be an electrical fault. Sensor, connectors, wiring. The sensor is in the end of the cylinder head.

The sensor is connected to the PCM, then the signal goes over the HS-CAN bus to the IC where the dial is. But PCM, bus or IC faults are unlikely unless some warning lights or other symptoms are present.

It is worth sorting, since the PCM may use this signal to optimise engine operation, and probably control the rad. fan.

These sensors are usually just thermistors whose resistance drops as temperature rises, so could be tested with a multi-meter if you can make contact inside the connector. But if the connector seems ok, may be best to just replace the sensor anyway.

 

The hose from thermostat to rad should stay cool (or just warm) for several miles after a cold start if the thermostat is working fully. My car takes up to 10 miles in Winter before the thermostat opens, and that is on main (60 limit) roads.

Peter.