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.

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?

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.

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.