Hi all,

I've got a 2007 Mk 2 Focus 1.8TDCI.

Have been having some problems with cold starting for a while then a couple of weeks back the temperature dipped and it refused to start. I checked all the glow plugs which were fine but noticed in the diagnostic mode that the cylinder head temperature reading was well off. I presume this is affecting the fuelling and when its too cold its not giving it enough to start.

For example when the temperature outside is 12C it will read the CHT as 40C

See the picture below for the discrepancy between the sensor reading and the gauge reading.

Would this explain the cold starting difficulties and therefore changing it fix the issues?

I can get one from eurocarparts for £8 but looks like it might be a pain to change so wanted to check that these slightly strange readings were abnormal first!!

Any help greatly appreciated.

Cheers,

Ben

I believe it's common for there to be a difference between the 2 temperature readings.

Had figure in my head of 30 degrees different and these links seem to back that up

http://www.focusowners-social.com/forum/viewtopic.php?f=45&t=1151

http://www.cmaxownersclub.co.uk/forum/technical-39/temperature-sensor-10421/

The coolant temp on the gauge is 30 below head temp so 90 degree coolant makes 120 head temp

Thanks guys, I may be barking up the wrong tree then. Can someone check whether when their engine is cold whether it gives a much higher reading? I can understand there being bit of a difference between coolant temp and head temperature when its running and a bit of a difference between sensors but being 30 degrees out when its cold not been run for a week seems odd to me!! Unless its like that on everyones!

Thanks

its correct the inside of the head has nothing to do with outside temp its inside the engine inside the engine bay away from the elements the coolant and head temps are 30 degrees difference however if its 12 degrees outside that will not effect the head temp

What I'm saying is if its 12C outside and the engine hasn't been run then the head should be at 12C give or take a bit, hence why I don't get why its saying it 39 or 40

why would the head be 12 degrees it has nothing to do with the outside temperature if you look at it its very simple the head temp is 30 degrees higher than the coolant so the head temp will start at 30 degrees even when cold thats how the difference is calculated and they are all like that all will say 30 degrees or a bit more on cold so yes youre barking up the wrong tree as its a diesel battery is a good start ie correct type and a free test to check for a faulty cell and then the glow plugs may be old and need renewed they may look fine but that doesnt mean much with glow plugs you answered youre own question it got hard to start when the temp dropped suggesting battery or glow plugs

Ok, that's great thanks, I've put a new battery on (as when my focus wasn't working the battery on my girlfriends car broke so i used the newish one off mine on hers) and I stupidly managed to snap off the top of one of the glow plugs after I checked them so have replaced just one glow plug at the moment! However she is now running, and all appears to be fine. I'll have to see what happens when the temperature drops off, if I have issues I'll replace the other three plugs and go from there I think.

I've got some DTC codes on the dash from when I had the sensor disconnected when i was initially looking at removing it. will they clear themselves, and if not will they cause any problems?

Thanks for all your help and advice

some codes will clear themselves after 20 successfull starts just make sure its a calcium battery you replaed it with as this will cause issues if its not a silver calcium or double calcium battery make sure all the wiring to the glow plugs is unbroken and that the glow plug light on the dash goes off before you try to start it

faulty head sensors usually display a minus so it would say minus 20 when its 10 degrees outside

Yea, it's a calcium battery, all the wirings sorted and glow plug light went off before I started it, hopefully its just a matter of time before it clears.

Thanks again for your help

Was suggested to me that the difference in temps between the gauge and the digital value was potentially a software bug which Ford were never bothered about fixing.

Plus, can't say for certain with the Focus but I know with the Jag X-Type they changed the calibration of temperature gauge so it wasn't a linear reading across the scale - was due to the normal running temp putting the needle fairly low on the gauge, so owners thought it wasn't getting up to temperature. The update put the needle nearer to a middle position for normal running temps.

That's interesting, I've heard about the Jag temp gauge as you say, so could be the same.

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