I had the same problem with my 190E 2.6. It turned out to be the thermostat. The thermostat would open only a crack and would cause poor circulation at low speeds but the temp would be normal on the highway. It is very easy to check the thermostat on these cars about 5 minutes. I would recommend replacing it as the first thing you do. It cost about $25.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T