- but only intermittently...and I’m at a loss as to what to ‘fix’ next.

A few months ago it overheated in stop-n-go traffic, it was towed to the local dealer, but they could not find anything wrong. They inserted a new thermostat as their best guess for a fix.

A month ago in traffic again, the temp needle spiked again - and let me tell you it goes up very fast. Off to my independent mechanic. He checked to see that the thermostat was new and working and installed a new fan switch - again as a best guess repair.

Now it has done it again driving in hot weather in the city. The temp needle started to go up fast, I found some open road and got up to 40 mph for a few minutes and the needle dropped just as fast as it went up. Though, sometimes the gauge goes up fast and then comes down fast without any change in driving speed.

I might think the gauge has gone nuts but it actually boiled over on me the first time. There just isn’t that much to the cooling system - is there? So what's next - replace the radiator...I hate chasing a problem by just throwing money at it...

it is the speed of the rise and fall that tells me [but surely not!!] it is a design problem - so does anybody else have same problem?

in 1970s Leyland introd the Triumph Stag, a V8 convert aimed at USA market, a great car but the engineers were forced to "cheat" on the proper design of cooling to allow for really sleek "non brick" lines for engine compart

result was similar to yours and roads were littered with Stags with bonnet up and steam everywhere and car got called a lemon and failed

I owned one but as an engineer I modified mine back to proper design [its all to do with the expansion tank design and I wont try to explain] - but moral is it gradually sucks in air but cant VENT the air and first warning is temp sensor no longer sitting in water so up she goes then there is air lock in rad and it boils

so I would say that is the problem - where do you add the fluid? at expan tank only? - is the exp tank ABOVE level of rad top?

if you can add fluid at rad itself then top it up daily - like the main prob with this design is the exp tank is full so you assume the rad is too, but theres a lousy blob of air up there

When it starts getting hot do you hear the fan come on?
If not then the fan may be bad.

This my third 850 and I have never had any overheating problem before and I have not heard of any design issue so I kinda doubt that it is a 'bubble'....but I'm interested if anyone else has had a similar problem...

The old fan kicks on and is trying to do its job but the temp just keeps rising.

The radiator might be clogged then.If the fan is coming on and the thermostat is good then it is the radiator. Did they use a good quality thermostat?

The main problems on these cooling systems are thermostats and the radiators.
The water pump rarely even goes bad.

OK so maybe I am wrong but could you please answer

"so I would say that is the problem - where do you add the fluid? at expan tank only? - is the exp tank ABOVE level of rad top?"

I am still wondering re sudden rise, but could simply be the point where rad cap decides to let her blow [ie from blocked rad or whatever] and THAT gives sudden loss of fluid

the REAL problem is if you keep going and pump is running dry - then you have a very expensive engine repair

Volvo's use a closed system with the resivior.You fill the resivior only.The tanks are about the rad. top.The system is constantly moving including the resivior.

so that is still same as my 1976 model, ie the expansion/makeup tank had a main connection at the bottom to the pump suction and a small vent tube to the high point of the rad - such a system is self venting

the other system has an overflow bottle also with filler cap but only one connection to the system - as engine warms level in bottle rises and IN THEORY when it cools it sucks fluid back - works fine as long as no air at all in rad