Temp gauge up and down

Richard53

Quick query for the knowledgeable: 2006 Mondeo Tdci 130, ~65k miles.


Temperature gauge has always been rock steady at just below half-way, whether at -12 in a British winter or 30+ on a French motorway. Recently I have noticed the gauge moving between about quarter and half-way. It's definitely related to engine load - coasting down a long hill it drops, pulling up the other side it rises again. It's never been over half, so I am assuming it's not overheating. I'm guessing a stuck thermostat, but I thought I would post it here to see if there were any other ideas before I have a proper look at the weekend. No loss of coolant, no signs of leaks, no gunge under the oil filler cap. Starts and runs fine in every way.


I had the thermostat housing replaced earlier in the year as it developed a crack, but not sure if a new thermostat was fitted at the time. Thanks for any suggestions.

Iceweasel

I would agree - sounds very like a duff thermostat.

forgotmyname

2 thermostats on the Mondeo diesel, replace both of them.

Richard53

Thanks chaps! Thermostat(s) first port of call. Didn't realise there were two, though. Must read the manual more thoroughly.

EdGasket

I had this and it wasn't the thermostat but a worn radiator where most of the cooling fins had rusted and fallen out leaving a still watertight matrix that just wasn't dissipating heat fast enough.

reeac

From the behaviour it sounds as if the thermostat is stuck in the open position. If you have noticed that the gauge shows an abnormally slow warm up from cold then this would support this theory. Best to change it soon as running cold will accelerate engine wear.

reeac

EdGasket wrote: »

I had this and it wasn't the thermostat but a worn radiator where most of the cooling fins had rusted and fallen out leaving a still watertight matrix that just wasn't dissipating heat fast enough.

But that would cause overheating not over cooling.

Richard53

The radiator is fine. The engine tends to take a long time to get to working temperature anyway, as it runs quite cool, but I will do some checks against known landmarks on my regular journey (basically, I start to get heat from the heater between the roundabout and the petrol station) and see if it is taking any longer. A thermostat stuck open was my first guess, and I appreciate having it confirmed.

arcon5

The second one is for the oil cooler.

EdGasket

reeac wrote: »

But that would cause overheating not over cooling.

Yes it did overheat going uphill but cool down again going down. It was hot enough that the thermostat just stayed open all the time. On a hot day it would boil and steam would come out! I changed the thermostat first which made no difference and it took me a while to figure out the problem as the radiator had no leaks and I assumed it was OK.

Richard53

arcon5 wrote: »

The second one is for the oil cooler.

Thank you for that!