Sounds to me like your thermostat is stuck open. It is a quick and easy do it yourself repair. Does you guage on the dash read 80 to 90 degrees? If it does then it could be in the climate control system.

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