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[philsteward]

Hi all, my car was running a bit cold on a long run, and got worse when the air con was put on, so decided to change the thermostat.  Has improved, it now sits perfectly at 85C on the motorway, and goes up to 90C when around town, assume that is normal?

When warming up however, it warms up much quicker than before, the needle goes quickly up to 90C within a mile, but all of a sudden then drops right down to about 75C and takes a while to then get back up to 85C or 90C.  I assume this is because the stat is opening and the cold water in the rad is sent round the engine, but is it normal?

I would have expected the temp gauge on a big car like this to be more stable!  Just wondering if the thermostat might be faulty, you never know 

Thanks in advance

[danzigfan]

Mine does the same and sits at 92/93 normaly. In this heat sits at 95/96 sometimes

[Shallow Al]

Mine does exactly the same too. A lot of the cars I have had have done the same. 

[philsteward]

Thanks for the replies. Just been on another long run, and the gauge did not move off 85c, surely if the thermostat is set to open at 92c, the temp should be more like 90c which would also make it sit in the middle of the gauge?

Maybe I am just being picky!

[Andy H]

IIRC the temperature sender is at the opposite end of the engine to the thermostat. The coolant temperature at the thermostat outlet is probably 95deg+ while the coolant temperature at the sensor is cooler because it is near where the coolant from the radiator re-enters the engine :-/

(that would also explain why the sensor reading plummets when the thermostat first opens as a slug of cold coolant washes over the sensor)

[Shallow Al]

A while ago, I thought that mine was running a bit cool and changed the thermostat. This was even worse by 5 degrees on the display, so eventually put the old one back, and it shows the same as yours.
I will take this as confirmation that it is ok.