quick question temp sensor/ thermostat

[stevelfc]

on a mk4 golf 1.8.t we all know if a temp sensor is playing up or faulty it can increase fuel compustion and give running problems but is this the same if a thermostat is faulty ? As it's not doing it's job .
Why i ask is my gauge goes up to 90 if car stationry but soon as you drive it starts to drop back down ( need new thermostat ) so one minute the gauge will be normally 90 next minute back down to cold .
so if a car engine is cold the car adjusts fuel mixture.e.t.c. because it thinks it got to get to running temp but if its always running cold due to knacked thermostat surley the car adding more fuel e.t.c

[Granta Gti]

That does sound like thermo. It will use a bit more fuel, not a huge amount more but will run richer.

[barrym381]

change both they are cheap an easy to do

[stevelfc]

change both they are cheap an easy to do

done temp sensor , water pump so next thermostat

[Wayne]

change both they are cheap an easy to do

done temp sensor , water pump so next thermostat

Did you use a genuine temp sender.