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Fiesta mk6 1.25lt Engine temp issue...


dannyg1987
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Hi all

New to this forum, and i have a slight issue with my fiesta hoping for some advice. Its a fiesta style 1.25 '56 plate. Last few weeks while driving ive noticed the engine temp gauge needle reaches the middle of the gauge (unsure what temp that is...), and holds the correct temp for a little while. After around 10 minutes or so whilst driving, the needle starts to dip lower until you reach traffic lights or idle when the needle begins to climb to the centre of the gauge again. 

The engine light came on a few days ago and thought it was due to this, it was diagnosed at a o2 lambda sensor (bank 1 exhaust manifold) so had this changed and mot done at the same time as it was due next week anyway, so it would pass easily, (which it did) and hopefully cure my gauge issue. However it hasn't cured this gauge issue and its still continuing to dip after around 10 mins of driving but returning to normal once idle or at traffic lights etc

Any suggestions what this could be?? Google says a bad thermostat. My mechanic who im seeing next week about it says possibly a water pump, or a temp sensor, ive not noticed any leaks but i will do next time im out in the car, and regarding the temp sensor - my car hasnt got an outside temperature reading gauge so didnt think it would be this either.... I will state that once the mot and new lambda sensor was repaired, as i drove home from the garage, the car seemed to be steaming, smelt a bit oily and sounded rough, although once home i lifted the bonnet and noticed the air filter hose hadnt been connected properly again so attached it and went ok. Not noticed any more steam but will continue to look out for that. Hopefully ive explained my issue ok and appreciate any feedback.

Thanks!

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I don't see how a dodgy water pump could cause the temperature guage to go down.  could be dodgy thermostat but I think when they fail they usually get stuck shut, not stuck open, which would mean too high temp, not too low. But I suppose a thermostat could break in such a way that it stayed open. That would cause the car to take a long time to get up to normal temperature

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Thanks, i have this concern too regarding the water pump. If it was faulty or leaking id imagine they would have seen it leaking or atleast wet around it during the MOT. Im just looking at it being hopefully the thermostat after reading through the internet, many are saying the same symptoms. Might just be the thermostat on its last legs. Dont see it being a temp sensor as i didnt think i had one in the car due to it not having that function on the dashboard! Could be wrong though, im not really fluent in car engines...

Forgot to mention aswell, when i lifted the bonnet when the car was running after id been driving for around 20 odd minutes, i felt the coolant tank, it werent as hot as ive noticed in the past. The coolant felt luke warm and could take the cap off with ease, didnt release plumes of hot steam etc. May just be the thermostat, cant figure why it would be anything else but ill check for leaks later on.

Thanks! 

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When he mentioned a temperature sensor he'll have been referring to the coolant temperature sensor,  which you do have (it drives the gauge (and ECU)).

I'd start with the thermostat, either checking it in a pan of boiling water or just replacing it anyway (they are relatively cheap). 

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Cheers guys. I'll have a look for the temp sensor, unsure where that is, but may be something i can replace myself. As for the thermostat, not so much, i'll leave that for a mechanic! 

Thanks all.

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I doubt it'd be the temperature sensor as a failure of that usually means no reading/response at all.

 

 

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Incidentally, one thing I get checked is that there is no air in the cooling system. A rogue bubble might be being pushed round to the sensor when driving (causing the sensor to read low out of the coolant) but then moves back to another location on idle. 

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Thanks for your advice, you're right, could be something simple as an air bubble trapped. Ruling out the temp sensor and waterpump then, just cant possibly be them based on reading around the internet and your advice. Im booked in to the garage on monday for a diagnostic anyway. Hopefully it will be the thermostat as its cheap haha, and a fair few people along with yourselves have said this too.

Cheers!

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Update on todays garage visit. Told them whats been going on with the engine temp needle dropping after driving for about 10 mins or so, so they took it for a drive, noticed it doing what i said its been doing whilst out. Took it back to the garage to plug the car in to look for engine management codes, nothing shown. Continued to look for possible leaks as they said it may be water pump related, again no leaks found, also they disconnected a few pipes to see if there was possible blockage from the coolant hose and blew air down them, still nothing....

Collected the car again after them telling me that just see how it goes as they couldn't diagnose it as it seems fine, apart from the engine temp issue. When i got back i had a look if my air filter needed changing as im due a service, absolutely filthy. Doubt that would have anything to do with it though. Guess ill just have to keep carrying on driving and with any luck the engine light will hopefully come on or ill break down. Someone mentioned coolant temp sensor, where is that located??!! Had a look in the engine and around the internet, genuinely cannot find any info on its location, as if its easy ill try replace that to see if that cures it, if not itll be a thermostat on its way out. 

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I'm sensing deja vu... ;-)

Didn't we already suggest changing the thermostat before the sensor? What's changed to assume the likelihood is the other way round?

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I always thought that thermostats were designed to fail safe in the open position.

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On 10/2/2017 at 7:57 PM, MJNewton said:

I'm sensing deja vu... ;-)

Didn't we already suggest changing the thermostat before the sensor? What's changed to assume the likelihood is the other way round?

You did yes, just asking its location as i dont know where it is, but i know the thermostats location but its far too difficult for me to change that having to remove cambelts, alternators and whatever else to access them 2 parts alone.... Just curious as the mechanic mentioned last week that the coolant temp sensor could be the possible culprit, although it was looked at on monday they couldn't find either thermostat or coolant temp sensor to be faulty as of yet until the engine light comes on.

I have no idea on fixing cars, only what ive seen on guides online, but willing to change simple things if its easy enough to save a bit of £, as ive fixed door parts, wing mirrors, air filters etc pretty easy enough. Im not confident in going far in the car should it break down in the middle of nowhere. It may well be the temp sensor and if its cheap/accessible enough id do that myself to see if that fixes the problem, if its not faulty its a new part regardless that'll last a few more years. 

I'll go back during the week and ask them to just replace the thermostat after all your advice as its annoying now seeing the needle fluctuate whilst driving while waiting to break down! :laugh:

Cheers all! 

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Unfortunately there's no escaping the need to fixing the most likely cause rather than whatever happens to be the easiest! :wink:

I cannot imagine a failure mode of the coolant temperature sensor itself that could cause the symptoms described.

The thermostat, on the other hand, if failing to fully close as suspected would definitely cause the issues you are seeing. With the coolant always (at least partially) passing through the radiator it would cause overcooling of the engine when driving. When stopped, the lack of airflow through the radiator means it would have no (or at least very little) cooling effect hence the temperature reading returning to normal.

Incidentally, if our suspicions are correct then I would not expect you to be at risk of breaking down - you will likely just suffer from poor efficiency due to the engine running at a sub-optimal temperature. You might also find your cabin heater doesn't work as well as it should (when driving at least).

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Bobr says "I always thought that thermostats were designed to fail safe in the open position."

I don't know if modern ones are designed differently but the traditional ones would fail closed.  A spring holds the thermostat closed.  When the engine heats up , the reservoir of  wax in the middle of the thermostat expands with the heat and pushes the thermostat open against the spring. If the thermostat fails by the wax leaking out for example, there is nothing to push against the spring to open it and the spring holds it closed however hot it gets. 

I have not taken one out for years, are they still this style:

SBF%20thermostat.jpg

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