Topic: Temperature fluctuating

[Eucalypta]

Hi guys,

My car seems to be more of a puzzle than I ever thought....

The temperature of my coolant is fluctuating.
Accoring to my temperature gauge, it varies from 100 degC to 50degC and back up again within just few seconds.
The fluctuation seems to be engine load driven: with hot engine (100degC) pulling up, the temperature drops to 70degC (sometimes lower), when coasting, the temperture rises again to the 100degC.
Swapped the sensor; no difference.
I have no coolant loss, no 'mayonaise' on radiator cap or oilcap, no white smoke out of tailpipe,...

Any ideas on this weird problem?

John

[sharpxmen]

yeah, this is a combination of how the thermostat works and some other factor(s) that can make it that much more obvious in your case

in the winter you thermostat will open and close in a repeated cycle, will be closed until the coolant reaches the opening temp and then the engine gets filled with cold coolant, thermostat closes and the cycle repeats - this is normal (or somewhat normal if we look at your scenario)

why your temps are so far apart can due to a bad thermostat, a bad temp sender or a bad gauge (or maybe a combination of these).

[Eucalypta]

Tempsensor has been replaced so I can rule that one out.
Temp gauge: the temperature doesnt go from 100 degC to 0degC as I would expect with a faulty gauge or shorting of the wires. Also, it seems to be engine load driven...

Leaves the thermostat: the temperature moves very quickly from 100degC to 50 degC; would a broken thermostat cause this behaviour? Isn't it just a cilinder with wax that slowly opens en closes the valve?

Ofcourse I can go and replace the thermostat but rather would be fairly sure the thermostat is the problem.
And if I replace it, should I go for a lower opening temperature and why would that be better?

John

[grumpygy]

Where did you get the temp Sensor.  I finally had to go to the dealer to get one that read right.

[aw12345]

I would start with the thermostat, easy and cheap to replace it controls the minimum engine temperature and since that is what wildly fluctuate, it seems that would be a good starting point. If there where electrical problems with the gauges most likely all of them would fluctuate unless the sender wire would have a intermittent somewhat short to ground, which I deem highly unlikely.

[sharpxmen]

Leaves the thermostat: the temperature moves very quickly from 100degC to 50 degC; would a broken thermostat cause this behaviour? Isn't it just a cilinder with wax that slowly opens en closes the valve?

it doesn't open and closes slowly, it opens at the set temp rather quickly (which is 195F stock or 90C) so that's your first problem, if you're sure your gauge reads correctly then i would start with that, if you never replaced it then it's a good precautionary task.
It can also get stuck on open so that could be the reason for the low temp (and again i am going to mention that this is only the case if your gauge indication is correct) and would be more obvious compared to a thermostat that operates properly.

It is load driven since there is more heat dissipated at higher load your low temp would be higher.

as an exaple on cold days my temp fluctuates between 80C and 87C (180 to 190F).

[Eucalypta]

Installed a new thermostat and problem seems to be solved.