a bit of help would be good before I tear my hair out. My cars a 2006 Vectra Sri 1.8VVT (45000 miles and full VXSH). Late last year it developed what I thought was a thermostat problem as the car temperature gauge wasn't moving from rest even at 60mph speeds for a 25 mile journey. The car was booked in at a garage who replaced what they termed the 'heater switch' (I think it was the Engine Coolant Sensor) this cured the problem and engine temperature returned to smack bang in the middle of the gauge (90 degrees) and was warm within 10 minutes of a cold start.
Over the last few months the same problem has started to occur only this time its intermittent - the car sometimes warms up as normal but then drops to cold if I take my foot off the accelerator and coast down a hill for example or if i'm lightly accelerating due to traffic flow. I expect a slight drop but not to go from 90 deg to registering nothing in the space of a few hundred yards when going down hill. It then will take an age to rise again if at all and is tending to float more between nothing and 80 degrees.
I took it back to the garage but the day I booked it in the car started to run normal again! 90 deg and all well with a constant readout. Garage said they could find no fault and off I went again.
Well now the problems back and if anything its worse fluctuating between no reading and 80 degrees, the car seems to have a very slight flatspot at about 3000rpm as well, more noticable in 3rd gear. No warning lights are on though.
I did a cold start today, leaving the coolant expansion cap off and the car did start to warm up after about 10 minutes with both the top hose and bottom hose feeling warmish, I squeezed the top hose to try and expell air but all it did was make the fluid in the expansion tank bubble as I squeezed. Replaced cap and left it at that.
Does anyone think it could be a sticky thermostat? I'm unsure but I hope to get it resolved as I look after my car and this issue is spoiling an otherwise relatively good car.
Sorry for the long post.