I have a Vauxhall Vectra 1.6 (2007). The cooling system operated perfectly (a few kms after starting the engine the needle was pointing to optimum temperature without the slightest waver). 2 weeks ago I had to drive around 600 kms in an extremely hot weather (42 C), and a blinking light alerted me on the dashboard. I stopped for an hour, then slowly drove home.
Next morning I checked the coolant level, it was a bit low so I topped it off. Since then it keeps recurring if I drive during the hottest hours. The coolant level is constant, no problem with that. I can drive hours uphill with full load in the morning, but on the same route at 3 pm it overheats again. When it overheats, I stop for a few minutes, and it gets back to normal like a snap. Both going from normal to maximum temperature and then getting back to normal takes less than half a minute. It is unrealisticly quick. When it started acting up once, it will do it every few kms until I stop for hours. Next morning it is all fine again.
Attempted fix so far:
Checked with two services, none of them found anything. The closer one also refused to replace cheap parts (like thermostat, water pump) or to flush the radiator, because "they do not think that this is the problem".
Any suggestion on what could cause this odd behavior?
Just to save the sanity of somebody as desparate as I was (reaching this page and not finding an answer) this was the "solution" in short:
Take it to Mechanic #3 (electric engineer): "It is surely the ECU (the main computer). Needs to be replaced for an awful lot of money." - I did not believe him, went to another one.
Take it to Mechanic #4 (regular mechanic): "It is the thermostat." - He fixed it, but strangely enough, the problem remained exactly the same...
Take it to Mechanic #5 (Vauxhall service): two days of investigation, no result.
Take it to Mechanic #6 (kind of a hobbyist, with plenty of time): "It is a blown head gasket. I am going to fix it quick'." - He "fixed" it, but strangely enough, the problem remained exactly the same...
Take it to Mechanic #7 (Independent Vauxhall-specialist garage): "It is a system-wide problem. Something caused the original problem, and that later put extra strain on every other part of the cooling system. You need a full investigation." - ... And they were right. They found the exhaust manifold cracked, leading to the blown head gasket (which was also not properly fixed earlier). Expansion tank was also cracked, lambda-sensor being burnt into its socket, and thermostat got killed again as well (so replaced it one more time).
In the end I paid a lot and more importantly spent an awful lot of time (two months in total) on getting this fixed. My advice to anybody in a similar situation: find a good professional and have him to do a full check.