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Discussion Starter #1
I left my car at the MOT station this morning and they just called to say that they left the car running for 10 minutes and the water boiled out of the cooling system. The temp gauge was at 90 so it hadn't overheated? The emissions is the first part of the test so it has to be left running for the engine to get warm. They're going to refill the system and try again but otherwise the test is cancelled :(

Any ideas whats caused this? I left it running for 5 minutes or so when bleeding the PS system and there were no problems, saying that the weather was a bit cooler than it is now...
 

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Discussion Starter #3
Didnt think of that. Would this only happen when its been sitting running? Other suggestions have been a blocked system, trapped air and sticking thermostat. I shouldnt think the system is blocked because a new radiator was fitted before I bought it and coolant was changed.
 

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Only reason I say it's the expansion cap is my brother's old [email protected] (read: rust-bucket with dodgy clutch) had the same prob and it turned out to be the tank cap. Cos the cap was crapped the system wasn't getting pressurised properly, therefore the boiling temp of the coolant wasn't raised and if I recall it did start to bubble at about 90 degrees.
If you've checked all the obvious it can't hurt to rule it out for a few quid :)
 

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the only reason it can boil over is if the system loses pressure. When the system is fully pressurised it raises the boiling temp to 110-120 deg C.

loss of pressure can come from a leak, air lock or duff impeller on the pump.

dodgy fan switch would also not help.
 

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The expansion tank cap went on my mates cally, it didn't over heat, just had loads of steam p1ssing out of it!
 

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Discussion Starter #8
Davo, I'll check the switch but if the thermostat isnt opening the water in the radiator wont be getting hot so it will think the fan isnt needed.

The cooling system doesnt seem to have a leak and there is a receipt for a new water pump. I spoke to the MOT guys again and they are going to refill the system and give it another go at about 3pm. Maybe I didnt tighten the coolant cap properly last time I had it off...

Thanks for the replies.
 

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Yeah maybe you didn't tighten it, mine seems to get to a tight spot but then loosens off a bit and can be done up another half-turn. Hope that's all it was, let us know how they get on with it ;)
 

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Discussion Starter #10
Just spoke to them again and they reckon its the thermostat. They just filled the system and left it so I'll need to sort it before taking it for an MOT again. Vauxhall do the thermostat for £18ish + VAT and wont order me one unless I pay for it so I'm going to make sure this is the problem before buying it.
 

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i left mine running once - then noticed a trickle of coolent running down the drive - thought summit serious had happened - was nothing more than the cap not on tight enough - this happened after a 30 min drive - idled on drive for 5 mins and out it came - give the system a refill then put cap on tight and try this first - always a bonus when you prove a mechanic wrong in front of his face lol
 

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Discussion Starter #12
I spoke to the mechanic when I picked it up. He's got a black cavalier saloon with various body mods and a couple of small Regal graphics - maybe a member on here. Anyway, he just said to try a thermostat and that the coolant tank lid was tight enough.

I took the car home and then left it idling. After about 5 minutes I gave up and held it at 2000rpm. It rose to 90 so I let it idle again. It crept up above 90, the fan came on and cooled it back down below 90, so its safe to assume the thermostat and fan are ok.

Next I took it for a run up and down a mile straight and then home, left it idling and still no water coming out of the header tank. Its booked in again on wednesday but I cant recreate the fault.

:confused:
 

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i thought my cap was tight too - was only a small movement but enough to stop it happnin again. if it aint doin it now and someone has had the cap on and off to check id lay my money on it bein not tight enough - other one is to replace the coolent tank cap in case the seal has gone (if it does go that is)
 
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