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Temperature Gauge Erratic


PuddingPuss
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Hi

My 09 tdci Mondeo MK4 has a strange temperature gauge issue.

Sometimes it will appear ok (it never goes past halfway) but most other times not really.

Usually it will warm up fairly slowly and only get up to halfway when at speed (60+), slowly dropping down to 1/4 or just below when encountering towns/traffic and driving at a lower speed, but occasionally it does spend most of the journey at halfway, but will never stay there.

The car drives without fault apart from this minor issue and both heater and aircon work fine - there seems to be no noticeable effect if I have heat/aircon on or not. It has relatively newish coolant (usually at the right level or thereabouts).

Any thoughts? I've had a quick look online and it seems it could be a trial and error job - possibly a faulty sensor/connection, thermostat (less likely) or the instrumnet panel itself.

Thanks, semi-baffled.

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First of all, do you know whether it is the actual temperature that is varying or is it just the reading?

You might have to pull in to a lay by or do some testing at idle on your drive, for example, and it helps of you have, or can borrow, an optical pyrometer.

Something else that you can try is to measure the resistance of the sensor element with a DVM.

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Usually it will warm up fairly slowly and only get up to halfway when at speed (60+), slowly dropping down to 1/4 or just below when encountering towns/traffic and driving at a lower speed, but occasionally it does spend most of the journey at halfway, but will never stay there.

From that statement it sounds like the thermostat is stuck open.

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First of all, do you know whether it is the actual temperature that is varying or is it just the reading?

You might have to pull in to a lay by or do some testing at idle on your drive, for example, and it helps of you have, or can borrow, an optical pyrometer.

Something else that you can try is to measure the resistance of the sensor element with a DVM.

Thanks for that. Not too technically minded I'm afraid, but did diagnostics bit by holding the OK button on the steering wheel down - after a 20 min drive (mixed, half at 50-70mph), the gauge dropped back to about 65, whilst the diagnostic temperature seemed stuck at 56 (I took 3 readings). Gauge sweep test ok. Just wondering If the coolant mixture was slightly out (eg 45/55 either way) could that be having an effect? For info it's a 2.0 TDCI. Thanks.

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Usually it will warm up fairly slowly and only get up to halfway when at speed (60+), slowly dropping down to 1/4 or just below when encountering towns/traffic and driving at a lower speed, but occasionally it does spend most of the journey at halfway, but will never stay there.

From that statement it sounds like the thermostat is stuck open.

Thanks for that. Not too technically minded I'm afraid, but did diagnostics bit by holding the OK button on the steering wheel down - after a 20 min drive (mixed, half at 50-70mph), the gauge dropped back to about 65, whilst the diagnostic temperature seemed stuck at 56 (I took 3 readings). Gauge sweep test ok. Just wondering If the coolant mixture was slightly out (eg 45/55 either way) could that be having an effect? For info it's a 2.0 TDCI. Thanks.

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Just wondering If the coolant mixture was slightly out (eg 45/55 either way) could that be having an effect? For info it's a 2.0 TDCI. Thanks.

Really, No. The coolant mixture is one of the things that I'd be pretty sure that it isn't, unless the concentration is dramatically low (say, 5%) or high (95%), when it could increase the chances of an airlock. I suppose if someone had used a poor antifreeze (eg, one of the ones that is mostly methanol, that evaporates off fairly quickly), that could be a factor, but even then I'd be surprised.

The TDCi can run at low temperature, so it could be normal. When you say it warms up slowly, how long is that in normal driving? If it really is very slow, then probably the thermostat doesn't shut fully, and that could be the problem.

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Really, No. The coolant mixture is one of the things that I'd be pretty sure that it isn't, unless the concentration is dramatically low (say, 5%) or high (95%), when it could increase the chances of an airlock. I suppose if someone had used a poor antifreeze (eg, one of the ones that is mostly methanol, that evaporates off fairly quickly), that could be a factor, but even then I'd be surprised.

The TDCi can run at low temperature, so it could be normal. When you say it warms up slowly, how long is that in normal driving? If it really is very slow, then probably the thermostat doesn't shut fully, and that could be the problem.

Cheers. Thought I'd better check but that's confirmed my belief re mixture. If driving around town or speeds below 50 then don't tend to see the dial much past 75 degrees on the gauge, even after 20 minutes, however the diagnostics (via holding OK button) give a reading of around 60. A journey straight onto dual carriageway motoraway at 65+mph would normally see the gauge reading 90 or just under after 10 mins or so, but would usually drop back to 70/75 when driving in slower traffic. I'm starting to think the thermostat is likely now too.

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  • 7 months later...

Hi, I'm experiencing something similar (and have been for some time) and, although it's not had a noticeable affect on the way the car drives, it's one of those annoying things that I'd like to put right. Did you ever manage to resolve your problem??

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11 hours ago, capegler said:

Hi, I'm experiencing something similar

Those in car temp. gauges are usually very unhelpful.

But thermostat & basic temperature tests can be done on the driveway. With a diesel, it may be best to drive it for a few minutes to warm it up a bit, a petrol car will warm up quick enough on idle. A food / cooking thermometer or something like that would be useful.

The top hose leading from the engine to the radiator should not get more than pleasantly warm until the thermostat opens, usually at about 80C. Then it should heat up fast. Slow warm up, with the top hose warming up too, means a stuck open thermostat.

A suitable contact thermometer can give some idea of engine temperature, ideally on metal parts of the cylinder head, but can be difficult to get at on many modern engines.

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