What's going on here?

at 7;45 I start the car which has been heated overnight by block heater. The outside temp is maybe 30.
Car starts and drives well, nothing eventful.
7:50 Stop at bakery for bagels, leave car running for 5 minutes.
8:00 start to drive at 40 mph, car temp at 60 C or so. Drive 5 miles and get on freeway
8:15 driving on freeway between 60-70 mph, as usual. Engine temp up to 80, as high as it typically gets.

SUDDENLY engine temp gets ABOVE 80 AND SLOWLY moves to the line between 80 and 120, I assume 100~!

I slow down to about 60, temp slowly goes back to normal range, right around 80, still a little higher than usual.

My guesses:

The heating of the block had heated the coolant a lot and with the sudden increase in temp going on the freeway, the car just got warmer than usual.

2. The thermostat may have been stuck, and was no longer sticking and now is operating as it should. In other words, I was perhaps running too cold for a while and didn't realize it.

Your thoughts please. Especially interested in assessments from 616 owner/drivers.

Thanks

__________________
Strelnik
Invest in America: Buy a Congressman!

1950 170SD
1951 Citroen 11BN
1953 Citroen 11BNF limo
1953 220a project
1959 180D
1960 190D
1960 Borgward Isabella TS 2dr
1983 240D daily driver
1983 380SL
1990 350SDL daily driver alt
3 x Citroen DS21M, down from 5
3 x Citroen 2CV, down from 6

Check your coolant. The anti freeze component if there is any chance it is weak or non existant. The coolant may have frozen in the radiator overnite a little. More likely heavy slush formed.

The block heater does not do anything for the radiator. I have never checked that but still suspect the heat of a block heater will not reach circulation temperature. If that powerful it probably would be a waste or overkill.

Just a long shot but you would not be the first.

Or low coolant.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Bad wire connection to the sensors on the block?