Overheating

It could also be a failed fan clutch. The fan is supposed to freewheel when cold and engage when hot. I'm not sure whether they used the bimetallic strip or the fluid-type clutch on a 1990, but both are known to fail.

Bill

Sounds like the fan clutch to me, I've also seen this on mine when the wire came off the switch for the electric aux fan on the front, I'm not sure if the non-turbo cars have that though.

migh be a failed cap on the coolant tank also...drop by your local volvo shop and have them pressure test it w/the proper tool...if it won't hold the pressure...the coolant will "blow" out of the system....The coolant system depends on a certain pressure level......This happined on my s80 last month...$5 cap... replaced...all is well....

Ah. Something else to consider and would be in keeping with having decent cooling at highway speed and poor cooling in stop-and-go traffic.

What would be the effect of this on mileage? I've been getting great mileage lately.

I'd pondered installing an electronic fan conversion a few times in the past, hoping to improve mileage and responsiveness. Is this a worthwhile conversion? If so, I might just go that route if it's the fan clutch that's shot.

No auxiliary fan on my car.

Sticking thermostat?

yeah; bum radiator would be opposite, overheats on the highway, cools off when idling.

My guess is a failed fan clutch.

Well, the mechanic checked it out and flushed the system, checked the radiator, checked for pressure leaks, replaced the thermostat and replaced the temp gauge sender as that's has always functioned rather intermittently. It stayed in the normal range on the highway, in stop and go traffic and while idling for the mechanic. I'll give it a good run tomorrow.

that mystery temp gauge board on back of the instrument panel can be suspect too. sometimes even just the connections to it. when it goes bad, you see huge temp guage swings at unrealistic rates of change but the cooling system doesn't seem overheated.

With the old sender, the gauge tended to not work if the air temperature was above about 55F- the needle just stayed at the bottom. Once in a while it would come to life, such as in the recent overheating scenario. In the winter it seemed to work fine. Today with new sender, I saw some of what you are describing. The car ran like a top.