I have a 1993 Volvo 940. Recently, I have had to add a very small amount of coolant once per week presumably due to a very small leak within the cooling system. I have maintained the coolant level at just below the max. line on the Coolant Expansion Tank and have not had any problems with the engine temperature control... Today, I noted, after a 20 minute drive, that the level in the Coolant Expansion Tank was at the very top of the tank at the same level as the filler cap... well above the max. line. I had never seen this before. Normally, the coolant expands a bit but not that much. I checked, the thermostat which was working. The radiator is new and unclogged. all hoses were fine. What could have caused the coolant to expand as much as it did? Could it possibly be a malfunctioning auto cooling fan?... Any advice would be greatly appreciated as I am intending to go on a three hour trip shortly... BTW, the engine temperature remained normal and when I let the engine completely cool down, the coolant level returned to its normal level, just slightly below the max. line.

Any insight will be greatly appreciated. Where is the coolant sensor switch that presumably activates the fan on a Volvo 940? Is there a fuse for this?

Many Thanks,

OnlyVolvo

Take the car to a good mechanic and have them test the coolant for the presence of exhaust gases, indicating a head gasket leak. How many miles on the car? What coolants have you been using, btw?
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See the 700/900 "FAQ" at the menu bar top screen left side.

One explanation for your symptoms might be a blown head gasket.

At first, a small amount of coolant leaks into the break, then eventually compression leaking out causes gasses in your cooling system, causing the high level.

I would pull the plugs and take a good look at then. If any look different, look into the plug holes to see if any cylinders look clean. This would be proof that coolant has entered.

Of course, a compression check is in order.

I would not drive the car until I found the cause of the rise in coolant.

A small leak is less of a worry, but should be found.