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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?

UPDATE

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.

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depending on the age of your vehicle the radiator will become corroded with orange/ red rust. This will brake off and clog the thermostat causing intermitent or random overheating problems that quickly go away and come back. Once the car has overheated a few times the thermostat can become damaged and not open until the water in hotter and hotter. This spirals our of control due to a bad thermostat.
I recommend to do a radiator flush and change the thermostat is it is starting to spiral and only open at a hotter temp every time.
Once you have flushed the radiator properly install new antifreeze mix with a radiator conditioner and sealant. This will help your system greatly and keep your coolant system running strong for years to come. Conditioner may take a few weeks to properly lubricate and seal up rusty spots etc.
Last, if you are losing coolant regularly check hoses for cracks and replace bad hoses. If its the radiator try using conditioner/ sealant (follow direction on bottle) or replace radiator.

  • The radiator seems to be in a good shape (according to a mechanic here), but the thermostat will be checked along with the hose tomorrow. – Gergely Bacso Jul 27 '15 at 9:56
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You need to check your return radiator hose. Sometimes when they get weak, they will collapse when they get hot, just like you are suggesting. When the strain is taken off of them, they will return to normal shape and be just fine and the engine will become cool again. The only way you will figure this out is when it starts acting up, stop immediately, raise the hood (bonnet) and observe the hose while you rev the engine. In most cars, the return hose is the bottom hose from the radiator. This may be hard to do by yourself. You may not be able to do it if the car is drive-by-wire (no cable from your gas pedal to the throttle body). Someone will have to press on the gas while you observe (or visa-versa).

  • Monday turned out be a holiday here, new hose will arrive tomorrow. I will update this topic when I have the final answer. – Gergely Bacso Jul 27 '15 at 9:55

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