I have two M103 engined cars which recently suffered with this problem. On the freeway they would run right down to 40 degrees C.

I have discovered that the erzatz el cheapo Thermostats are supplied with a poxy gasket that doesn't let the thermostat sit in the housing correctly therefore causing bypass leaks .... resulting in too cold running.

If the engine runs too cold, the temp sensor will send the wrong information to the ECU which results in too rich a fuel mixture and consequential lousy economy