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sunjester · Jul 10th, 2009, 08:19

My 850R suddenly overheated a couple of weeks ago and blew most of coolant out of expansion tank.
AA guy also diagnosed head gasket as his pressure test showed the cooling system was pressurised (coolant systems run pressurised anyway) but his block test did not show a failure. Pump is OK so he was definate it was the head gasket.

I was going to do this myself but speaking to a mechanic at the local Volvo dealer he suggested the thermostat. He said Volvo run the engines quite hot and the slightest problem in the cooling system will trigger this overheating problem. He also said if the car starts OK on all 5 straight away it's unlikely to be head gasket - mine does.

As that was a much cheaper option, I replaced thermostat plus the expansion tank cap and now all seems OK again.
I bought a block test kit, and tested twice (once after a 20 mile run) and engine passed this completely so I'm confident the head gasket is OK.

The hoses still seem to be pressurised, but as I didn't check before the problem started I don't know if it is at a normal level or excessive. I'm a bit suspicious of the small hose from the thermostat housing to the expansion tank and think there may be a partial blockage there.

It seems that diagnosing a head gasket failure is almost the easy option for even experienced people when dealing with these engines.

Jul 10th, 2009, 08:19	#6
sunjester New Member Last Online: Jan 17th, 2017 15:45 Join Date: Feb 2009 Location: Gateshead	Overheating... My 850R suddenly overheated a couple of weeks ago and blew most of coolant out of expansion tank. AA guy also diagnosed head gasket as his pressure test showed the cooling system was pressurised (coolant systems run pressurised anyway) but his block test did not show a failure. Pump is OK so he was definate it was the head gasket. I was going to do this myself but speaking to a mechanic at the local Volvo dealer he suggested the thermostat. He said Volvo run the engines quite hot and the slightest problem in the cooling system will trigger this overheating problem. He also said if the car starts OK on all 5 straight away it's unlikely to be head gasket - mine does. As that was a much cheaper option, I replaced thermostat plus the expansion tank cap and now all seems OK again. I bought a block test kit, and tested twice (once after a 20 mile run) and engine passed this completely so I'm confident the head gasket is OK. The hoses still seem to be pressurised, but as I didn't check before the problem started I don't know if it is at a normal level or excessive. I'm a bit suspicious of the small hose from the thermostat housing to the expansion tank and think there may be a partial blockage there. It seems that diagnosing a head gasket failure is almost the easy option for even experienced people when dealing with these engines.