Car appears to be running hotter in cooler weather

[fbgh2o]

I have noticed during the last few trips around town (mix of highway and surface streets) that my car seems to be running 4C or 5C hotter (93 or 95C vs 90C at highway speeds) than it was during the summer (based on the climate control hack). Yet the ambient temperature is 20F to 30F cooler.

Coming home tonight, the car warmed up to normal operating temperature and seemed good (up to 93C) for about 10mins in traffic 5 to 45 mph traffic. When I finally got out of traffic and into clean air and accelerated up to 65 to 70 mph, the temperature actually rose until it steadied at 95C. It then actually cooled slightly to 92C (with the high speed fans engaged and full heat on) when I exited the highway.

Key facts

• Car is a 1999 C4
• Both fans are operating on both high and low settings
• There is no evidence of a leak anywhere (expansion tank, water pump, visible hoses, passenger footwell for heater core) and the coolant level is normal.
• The expansion tank cap was replaced in August
• Did a full radiator clean in May and pulled everything apart tonight, nothing major is obstructing them
• Oil weight is 5W50 and pressure is good
• Climate control is in the auto mode set at 70F
• I think I get a whiff of coolant smell when the car starts or I shut off the automatic climate control systems. It is really faint disappears quickly and could just be paranoia

I think this leaves a hairline crack in the expansion tank as the final element to be checked. I will pressure test tomorrow.

Any other ideas?

Many thanks.